AN INTRODUCTION TO BEES
What Are Bees?
Anyone who has spent a lovely warm morning in a garden in spring has shared company with a bee. From a farm in the Central Valley to a community garden in the center of Los Angeles, bees are busy buzzing around, visiting flowers, gathering resources for their off spring, and in the process, transferring pollen from flower to flower.
Bees are flying insects that first emerged about 100 million years ago, during the Cretaceous period, as part of the radiation of insects. The earliest record of bees is from fossilized amber in Myanmar (formerly Burma).
Bees are part of a larger group of insects, the order Hymenoptera that includes wasps, sawflies (a primitive and less well-known group), and ants. Bees belong to a large group within the Hymenoptera that are distinguished, in part, by having females with stingers. This group is called the Aculeata and contains the bees, ants, and stinging wasps. Recent work on the relationship between bees and wasps shows that bees are really just a very specialized group of wasps. Formally, we say that bees are in the superfamily Apoidea along with the sphecid wasps and make up the group Apiformes. No wonder bees and wasps can be so hard to tell apart!
While beetles were probably the first pollinators, bees were the first group of insects to really show a diversity of adaptations for pollination, such as specialized hairs and leg modifications, both for carrying pollen. This close relationship between bees and flowers is thought to have aided in the rapid evolution of different types of flowering plants and has strongly influenced both the body plans and lifestyles of bees and flowers.
When you look at most bees, you see a fairly hearty and hairy flying insect. Although there are bees that are slender and sleek, the most well-known, like the bumble bees and honey bees, are robust and hairy. Their hairs are an essential part of the service that bees are renowned for, transferring pollen from plant to plant. The hairs hold pollen onto the bee body. However, while it may seem bees are providing this service to plants free of charge—almost as if the flowers were taking advantage of them—the bees are actually getting great benefits from the interaction. What the bees are really doing is searching out food for their off spring and themselves. Bees rely on plant pollen for protein and nectar for energy. In fact, the sole source of protein for most bees is pollen. These bees could not exist without flowers to feed them.
When you see a bee visiting a flower, it is probably either collecting pollen or drinking nectar. If the bee is collecting pollen at flowers, she is a female gathering food to take back to her nest for her larvae. She might stop to fill up on some nectar, but her primary aim is to gather pollen for her off-spring. This behavior highlights one big difference between bees and the closest wasp relatives of bees, the sphecoid wasps: the larvae of sphecoids are carnivorous, eating spiders and insects, whereas bee larvae are vegetarians, relying on pollen for protein. Of course, as with everything in nature, there is an exception to this rule: a group of tropical bees that feed their off spring carrion. If you see a bee simply drinking nectar, it is more likely to be a male bee. Male bees do visit flowers to tank up on nectar, but they do not collect pollen, as they do not provision nests.
At first glance, it can be difficult to tell bees, flies, and wasps apart because they have similar sizes and colors. Flies and wasps are confusing because they often look like bees. Some striped flies may actually be bee mimics, trying to fool predators! To tell bees and wasps from flies, there are three main distinguishing features: the number of wings that there are on each side, the shape of the antennae, and where the eyes are placed on the face. Let’s start by distinguishing flies from bees. First, the number of wings on a side makes it easy to distinguish flies from bees and wasps. Flies have a single wing on each side. Bees and wasps have two per side.
It can be hard to tell that bees have a pair of wings on each side, because the wings on bees and wasps can be hooked together with special hooks called hamuli. When looking for bees on flowers, you can often tell that an insect is a fly because its wings are not neatly folded over its back; instead they point out at an angle. Second, you can look at the insect’s antennae. Flies generally have short, thick antennae, whereas bees have longer, thinner antennae. Finally, bees have large eyes that are on the side of the head. Flies have large eyes on the front of the head. Other general rules of thumb are that bees tend to be hairy, and they carry loads of pollen, whereas flies have fewer hairs and do not generally carry pollen. This does not work when distinguishing male bees from flies, as male bees do not generally carry pollen and do not have elaborate structures like scopae, the pollen-carrying structures on a bee, for doing so. Behaviorally, bees do not hover whereas many but not all flies do.
Wasps are much more difficult to distinguish from bees. Given that bees and wasps are much more closely related than bees and flies, this is not too surprising. Bees and wasps both have two pairs of wings per side and are often the same size, shape, and color. In general, wasps look “meaner” than bees. This may be because wasps seem to have more armoring in their exoskeleton, the protective covering over the body, than bees. Bees tend to have a broader body and wider abdomen than wasps. Bees’ bodies are usually hairy, and in general, their exoskeleton is a single color, except for stripes on the abdomen. Wasps are less hairy and often have patterns or designs in their exoskeleton. One unique thing about wasps is that the hairs on their faces are often shiny or metallic, often silvery, whereas bees have duller hairs or no hairs. Under magnification, bees can be identified by the presence of plumose or branched hairs on some parts of their body and their legs and by the hind basitarsus (basal segment of the tarsus), which is often more flattened (broader in one dimension and narrower in another) than the next segments of the tarsus.
MIMICRY
Many distasteful or poisonous species that are trying to signal a warning use combinations of the colors yellow, red, and black. From snakes to butterflies to bees, those colors make them very apparent and protect them from attack. Predators quickly learn to avoid prey with particular color patterns. The color patterns of bumble bees are a good example of warning coloration. When two or more poisonous species share a predator and have a similar color pattern that cannot be attributed to relatedness, scientists call it Müllerian mimicry. This might explain the similarity in color patterns between bees and wasps. Another explanation for the similarity of color patterns might be that they all inherited the pattern from a common ancestor. We also see a different kind of mimicry of bees. When a harmless species mimics the color pattern of a dangerous species, it is called Batesian mimicry. In this situation, the dangerous species is called the model and the common species is the mimic. A number of species of flies have color patterns very similar to those of bees, especially bumble bees. These are probably examples of Batesian mimicry. For Batesian mimicry to be effective and maintained, the model must be more frequently encountered than the mimic. Otherwise, the predator may learn that the mimic is a good prey item and periodically consume some of the model.
Importance of Bees
The conservation of bees is central to biodiversity and life on Earth, to food security, and to the global economy. While bees are well known for their honey production, it is pollination that makes them critical to life on Earth. Bees are involved in pollinating about 70 percent of the world’s flowering plants. For flowering plants, which cannot move themselves, having a bee move pollen dramatically increases the distance pollen can travel to fertilize seeds and thereby increases the potential number of mates. There are many advantages to expanding the number of potential mates. In particular, mating with more individuals increases genetic variation, which provides the raw material for evolution and adaptation, and those populations with greater genetic variation are going to have a higher probability of surviving as environments change.
There are several examples of plants that seem to be declining more than they would if they had a healthy pollinator community. For example, in the Antioch Dunes of the San Francisco Bay Area, there is a primrose (Oenothera deltoides subsp. howellii) that is pollinated both by hawk moths and by bees. In fact, it has a specialist bee that visits it, Sphecodogastra antiochensis, that flies in the early morning and evening when the flowers are open, and this bee is now rare. This primrose is producing only 35 percent of the seeds that it might with a healthy pollinator community. Both the specialist bee and the primrose are considered rare or endangered and are only found within the 75 remaining acres of the Antioch Dunes.
If we think about the benefits of bees to people, the pollination of food plants is equally important. Bees pollinate about 75 percent of the fruit, nuts, and vegetables grown in California. Amazingly, with this much of the food supply at stake, agriculture relies almost exclusively on a single pollinator, the Western Honey Bee, Apis mellifera. In the United States alone, the value to crop production of pollination by this single species is estimated to be $14 billion. Our reliance on this species is still increasing. Demand for Honey Bee colonies increased approximately 25 percent from 1989 to 1998 and has continued to increase since that time.
Pollinator service to plants depends on both Honey Bees and other, native pollinators. We know native bees can have a significant role, because increases in the number and species of native bees have been associated with increases in crop production. The actual value of native pollinators for crop production is much more difficult to estimate, because the values of specific pollinators differ for each crop and are dependent on geographic location, availability of natural habitat, and use of pesticides. Studies in agricultural systems have shown that native pollinators can provide significant pollinator service directly, by pollinating the plants themselves, and also indirectly because the efficiency of Honey Bees is improved by the presence of native bees. For example, Sarah Greenleaf and Claire Kremen found that when native bees were foraging in sunflower fields in the Central Valley, Honey Bee efficiency at pollination increased up to fivefold. This increase in efficiency is because Honey Bees move more often when in the presence of native bees, perhaps because of competition.
In addition to their role as pollinators of crops, animals pollinate approximately three-fourths of the flowering plants (angiosperms). Declines in pollinators may increase the risk of extinction of many native plant species, as we are seeing for the Antioch Dunes Evening Primrose, particularly in biodiversity hotspots like the Bay Area. In these systems, bee pollinators other than Honey Bees are of primary importance.
Bee Diversity
When I talk about bees, most people immediately assume that I am talking about Western Honey Bees. In fact, Honey Bees are only one of the approximately 20,000 species of bees in the world. Here in North America, scientists estimate there are about 4,000 species, and almost 1,500 species, or almost 40 percent of those North American bee species, are found in California. The bees in California range in size from a tiny sweat bee (Perdita), not much larger than the head of a pin, to a large carpenter bee (Xylocopa), the size of a man’s thumb. There are 82 genera of bees known from California. Many are represented by a small number of species. I have chosen to profile the most common genera across California. See Appendix 1 for a complete list of California bee families and genera.
Bees come in a variety of shapes, sizes, and colors. They nest in a variety of places, including holes in trees and underground. Bees can be social, living in large family groups with different roles for different bees, or they can be solitary, with a single queen working and nesting alone. The surprising diversity in bees is part of their fascination to humans. Over the next pages, I will try to share some of that wonder.
Bee Morphology
To understand bees, it helps to know how they are put together. There are three main parts to a bee. These are usually called the head, the thorax, and the abdomen (Figure 1). The word thorax is a bit misleading for bees because the final segment attached to this part of the bee’s body is actually part of the abdomen in other insects. This structure is the propodeum. The eight segments of the abdomen are each encased in two plates: the upper plate of a segment is called a tergum (plural terga), and the bottom plate is the sternum (plural sterna). A lot of bees have pale bands of hair across their terga. The number of terga and sterna are important for distinguishing male and female bees. Male bees have seven and female bees have six. If you look at the end of the abdomen, you may see a fringe of hairs and a small plate called the pygidial plate. This plate is used to tamp down soil within burrows and nest cells. When you are looking at head a female bee, you may also see an ovipositor. The ovipositor is the structure that a female uses to lay her eggs. The stinger is derived from the ovipositor and has a less pleasant function in defense. In many species, the female bee is larger than the male.
Figure 1
In many California native bees, the stinger is not strong enough to penetrate human skin. However, some bees pack a wallop. When stinging, the stinger is pushed out from the abdomen, and then it locks into position. In a situation where a bee is stinging, it will use the muscular plates on the abdomen to push the stinger into your skin. The stinger has a very sharp tip, and at the top of the stinger there is a venom bulb. In the Western Honey Bee, the stinger breaks off from the abdomen, yet the venom bulb will continue to pump venom for 30–60 seconds. While Honey Bee workers die after stinging, this is not true of all bees.
The abdomen and thorax of bees house most of their internal organs. The heart, the stomach, air sacs for breathing, and the sex organs are all contained in these parts of the body. Bees breathe through small holes in their exoskeleton, called spiracles. These spiracles connect to air sacs and allow for the free passage of air. In addition, the thorax of the bees is where the legs and wings connect. The thorax is a very complex structure with numerous fused plates and sites where muscles attach.
Bees and other insects have six legs: two fore legs, two mid legs, and two hind legs. Each leg is made up of a coxa, trochanter, femur, and tibia tarsus and ends with a pre-tarsus (with claws). Legs are important for grooming, for courtship, for pollen collection and carrying, for constructing nests, and of course, for locomotion. When a female bee collects pollen, she usually gathers it with the fore legs, transfers it to the mid legs, and then stores in on the hind legs.
There are some unique functions that legs perform that correspond to their shapes. If you look at the apex of the tibia, you can often see a small spike called a tibial spur. The spurs on the fore leg are modified into a tool that bees use to clean antennae. We do not know the function of the tibial spurs on the other legs. You can actually watch bees groom their antennae with their fore legs. They run the antennae along the intersection of the tibial spur and the leg itself. On the hind legs of many bees, you can see scopae, or pollencollecting hairs where pollen is concentrated before the trip back to the nest. The scopa is a complex structure made of modified hairs on the coxa, trochanter, femur, tibia, or underside of the abdomen (in the family Megachilidae) and even the sides of the propodeum (in some members of the family Halictidae and the genus Andrena). There is more about pollen-carrying structures below.
Bee wings are composed of a transparent wing membrane supported by a stiff network of round wing veins (Figure 2). The patterns of wing veins are very variable and are really important for identifying groups of bees. The veins add strength to the wings. There are two wings on each side of a bee: a fore wing and a hind wing. The leading edge of the hind wing has little hooks on it called hamuli (Figure 3). These hooks hold the fore wing and hind wing together in flight. Hamuli stabilize the wings and influence the bee’s ability to fly. The number of hamuli on a wing is related to the size of the bee and how far it flies. Wings beat about 400–500 times per minute. Amazingly, it has been documented that there are some tropical bees that fly over 14 miles in a single trip.
You may have heard that from an engineering standpoint, bumble bees should not be able to fly. While the originator of the calculation that shows that it should be impossible for a bumble bee to get off the ground remains in dispute (attributions range from a French entomologist to a German physicist), the same error is attributed to each. Whoever it was, he estimated the amount of lift that a bumble bee would have, given its body and wing sizes. To keep the calculations simple, he used a model for a rigid, smooth, fixed-wing craft . From these calculations, it appeared that a bumble bee would have insufficient lift to take flight. In fact, bumble bee wings are very flexible and have considerable movement and would be better modeled using something more similar to a helicopter.
Figure 2
The most obvious features on the head of a bee are the large compound eyes, the three simple eyes called ocelli, and the antennae. The compound eyes are made up of over 6,000 tiny lenses. Each lens is connected to a tube that has receptors that respond to polarized light. Interestingly, research on Western Honey Bees has shown that when a bee leaves the nest, she will fly around in a circle until her eyes receive maximum stimulation from the polarized light. This signal tells her that she is facing away from the sun and allows her to orient her flight. However, a bee’s vision is believed to be sharp for a distance of only about 3 ft . Bees that fly in the early morning, late evening, or night have enlarged ocelli. For these bees flying in low-light conditions, the ocelli might function as light receptors that help them keep their bodies level while flying. The antennae are used basal vein both to touch and to sense the environment in other ways. When you look at the face of a bee, using a microscope or a powerful hand lens, notice there are two sutures running down from the antennae. These are the subantennal sutures (see Figure 1). They are important features for identification because if there are two subantennal sutures, you can know it is a bee in the family Andrenidae. Most other bees have just one suture below each eye. With a microscope, you can also see depressions in the face called facial foveae that indicate the site of some glandular structures. They are found along the inner margins of the compound eyes in many bees, particularly in the families Andrenidae and Colletidae.
Figure 3
Now, if you look underneath the face, you will see the lower face and mouthparts (Figure 4). The mouthparts sit within a deep groove in the head. The lower face and mouthparts are made up of a pair of mandibles plus the labrum. Mandibles are used to bite, work wax and pollen, and carry objects such as resins and mud. The clypeus is the plate on the front of the head of a bee, below the antennae. The labrum is attached to the bottom of the clypeus. Bees are often divided into two groups: the long-tongued bees and the short-tongued bees. The short-tongued bees (the families Colletidae, Andrenidae, Halictidae, and Melittidae) have labial palpi that are all similar in size and shape, whereas in the long-tongued bees (the families Apidae and Megachilidae) the first two labial palpi are long, flat, and almost like a sheath (see Figure 4).
If you do not have a bee under magnification, the most obvious way to tell a male bee from a female bee is by whether it is carrying pollen or has pollen-carrying features like scopae (Figure 5). Females do carry pollen and males generally do not. This is generally true, though in some bees, such as the genus Hylaeus and the parasitic bees, females do not have external pollen-carrying features, and males sometimes inadvertently get pollen on them. There are some other characteristics that are less reliable. Males often have relatively longer, narrower bodies; less-hairy legs; and white markings or hairs on their faces. There are some groups, particularly the families Apidae and Halictidae, where males have much longer antennae. In a few species, notably the carpenter bees (genus Xylocopa) and some bumble bees (genus Bombus), males and females have different color patterns.
Using a microscope, it is fairly simple to tell male and female bees apart. Male bees have antennae with 13 subunits (see Figure 1); females have 12 subunits. Also, males have seven exposed metasomal terga; females have six. Even without a microscope, you can often see the stinger on a female bee.
Figure 4
Figure 5
Bee Life Cycles
All bees go through a series of developmental stages and follow a pattern of metamorphosis. Bees are considered holometabolous, which means that they follow a set of stages of development similar to those of butterflies and beetles. A holometabolous insect starts as an egg. The egg hatches into a larva (e.g., a caterpillar, grub, maggot; plural is larvae), which goes through an inactive, pupal stage (e.g., wrapped up like a cocoon; singular is pupa, plural is pupae) before emerging as an adult (e.g., a butterfly, beetle, wasp, or bee). In most of our solitary bees, an adult female emerges in spring or summer. Her emergence is usually tied to the timing of the plants she needs in order to provision the nest cells for her off spring. The actual cues for emergence are not well understood, though they are suspected to be related to air temperatures or carbon dioxide levels in the soil. This female bee will spend all of her time building and provisioning a nest or nests filled with brood chambers. In each brood chamber, she will lay a single egg. Most bees are active as adults only for 2–5 weeks. They spend the other 47–50 weeks of the year in their nests in cavities or underground.
The female bee lays her egg on provisions. These provisions are made up of nectar and pollen. Many provisions look like a round ball but there are some that are very soupy. The quality and quantity of pollen and the amount of sugar in the nectar in those provisions affect the growth of larvae. Larvae with provisions made of higher-quality pollen and with higher sugar content grow larger and faster. Interestingly, there is no evidence that specialist bees use pollen with higher protein contents. Specialist bees also do not develop well on pollens from plants they do not normally use.
There are several different interesting ways that male bees find females. Some male bees set up territories by marking flowers with pheromones, which are scents for attracting females. Males in the genera Andrena, Nomada, and Triepeolus do not just mark flowers; they also set up “patrol routes” by marking nonflowering plants. Other males are often attracted to those scents and join the patrol. When a female bee enters the patrol route, a first male will attempt to mate with her. If there is more than one male in the area, the female will be mobbed. Larger mobs can actually tumble to the ground in the excitement. In some bees, the males patrol nest sites before females emerge, so they can be the first to mate. Males in other genera set up territories that they defend from other males.
The males of some species are aggressive. These males either patrol their territory (as some Megachile do) or defend a single site (as with Xylocopa). The size of male territories can be as small as a single flower clump or much larger (up to 20 sq. ft .). Territories probably change owners several times during breeding season, and the desirability of the territory probably changes as the floral resources and amount of competition change with the season.
Mating can take place in flight over the nesting site, inside a ground nest, or at or in a flower or simply on the ground (Figure 6). Males of many species can be found patrolling quickly through patches of flowers searching for females. Oft en, the mating pair is mobbed by other males.
Bumble bees actually use a queen-attracting scent to find mates. A male bee will fly in a circuit, putting scent (pheromone) in suitable places such as tree trunks or rocks. Different species of bumble bees have been shown to have different preferences for the height of the scent, ranging from low on bushes to high in tree tops. The scents themselves are different. There are even some scents that are detectable by human noses. New bumble bee queens emerge about a week after the males. When the new queen is ready to mate, she will locate a site with pheromone and wait for a suitable mate. Mating usually takes place on the ground or on vegetation. Periodically you may see a large queen flying with a small male attached to her—still mating.
Figure 6
Matings usually last 10–80 minutes. The sperm transfer happens right at the start, often within 2 minutes. So why does the mating take so long? After the sperm has been transferred, the male transfers some sticky material that will harden in the female’s genital opening. This genital plug blocks the entry of sperm from other males. Therefore, it is in the best interest of the male to continue mating so the plug has time to harden, thereby increasing the probability of his genes getting passed on to the next generation. Most bumble bee queens have only one mate, though some species do mate multiple times.
After a bumble bee queen has mated, she will find a spot to overwinter. Usually these spots are in shallow holes in the ground, often under a root or on a gentle slope. This overwintering site is called a hibernaculum (plural hibernacula). During the time from her emergence to her entry into the hibernaculum, the queen will visit many flowers to drink nectar and build up fat supplies in her body. She also fills her honey stomach. Bumble bees have a large honey stomach. This stomach is located in the abdomen and can hold 0.06–0.20 ml, depending on the size of the bumble bee. When full, the honey stomach can take up as much as 95 percent of the abdominal space and account for as much as 90 percent of the body weight. The body fat and honey stomach resources allow the queen to hibernate. In areas where it freezes, if the temperature in the ground gets very cold, the queen’s body will start to produce glycerol, which prevents ice crystals from forming in her cells. If she didn’t produce glycerol, when the fluids froze, they could expand to the point that they might burst the cells. After the queen emerges in spring, she will not reuse the hibernaculum for her nest site.
Evolution of Social Behavior: Social versus Solitary Bees
When most people think of bee societies, they think of Western Honey Bees with their queen and caste system of workers. Most North American bees do not have castes of workers like Honey Bees. Rather than being social, they are solitary, meaning that a single female works on her nest by herself. However, several other groups of bees are social. There is a set of formal criteria that defines sociality for animals. To be considered social in the scientific sense of the term, a bee must share the work of caring for the brood, there must be an overlap in generations so that off spring can assist parents, and there must be a reproductive division of labor, which means that not every female produces off spring. Among the social bees, there are different degrees of sociality. The most complex relationships are the eusocial bees like Honey Bees. These bees share a nest, have cooperative brood care and worker castes, and have overlapping generations. Between the eusocial bees and the solitary bees, there are several intermediate levels of interaction. Semisocial bees have small colonies of bees that are of the same generation. These bees share a nest and have worker castes and cooperative brood care. Usually one of the sisters becomes the queen and does most of the egg laying. Some of our bees in the family Halictidae exhibit this type of sociality. Communal bees nest together but do not cooperate. We see this in some of our Megachile and species of other genera.
Bumble bees are considered primitively eusocial. Bumble bee queens emerge from hibernating during winter. The queen finds a new nest hole, often an old rodent hole, and lays her eggs. She then forages for nectar and pollen to provide food for the larvae once they hatch. She does all the work on the colony until those first larvae, her daughters, become adults. Then, the queen devotes herself to egg laying, and the daughters take over the work.
Scientists think that the evolution of sociality in bees may have been made easier by their unusual sex determination system. Unlike humans, bees do not have two sex chromosomes that determine gender. Female bees develop from fertilized eggs, so each female has two sets of chromosomes and is diploid. Male bees develop from unfertilized eggs and have only a single set of chromosomes. This type of sex determination system is called haplodiploidy. After a queen bee has mated, she is able to determine whether or not to fertilize an egg using the sperm she has stored. In social colonies, both worker bees and queens are females. The diet that the bee is fed as a larva determines whether she becomes a queen or a worker. The implication of this for the development of sociality is that workers are more closely related to their sisters than they would be to their off spring so, because workers share more genes with their sisters who may become queens, more of the workers’ genes may get passed on to the next generation than they would if the workers reproduced.
Parasites and Robbers
Not all bees forage for themselves. There are bees that actually rob the food stores of other bees. Within the bees, there are three types of bees that behave “badly.” First, there are bees that will enter a nest, fight with the owner, and, if they win, take over the nest. This was found in one of the invasive bees in California, Megachile apicalis. Some of the social bees actually post guards at the entrances to their nests. These guards probably defend the nest from intruders from the same species intent on taking over the nest, as well as from other predators and parasites. Second, among the social bees there are bees that are called social parasites. These social parasites enter a nest, replace the queen (often by killing her), and then use the workers in that nest to rear their own off spring rather than the off spring originally in the nest. This is very common in the bumble bees (Bombus). Bumble bees have a whole subgenus called Psithyrus that are all social parasites. These parasitic bumble bees have even lost their corbicula for carrying pollen. Interestingly, the host bumble bee queen is not always killed when a parasitic bumble bee takes over. She sometimes remains alive. The third and most common type of food-robbing behavior is called cleptoparasitism. Cleptoparasites enter the nests of other bees and lay eggs in the cells of the host bees and then depart. Some of these parasitic adult bees destroy the eggs of the hosts before leaving the cell, but many do not. In some cases, the parasitic egg is hidden in the cell wall of an unfinished cell, but in other cases, the parasitic queen uses her ovipositor to insert her egg in an already closed cell. When the parasitic egg hatches, the larva feeds on the food that had been stored for the host bee. Many of these parasitic larvae hatch with sharp mandibles that they use to kill their host eggs or larvae.
Parasitic bees generally have reduced pollen-carrying structures (scopae, corbiculae) and often resemble wasps. This is because they do not need to have as many hairs for carrying pollen. They also often develop strong cuticles, spines, and stings that are used for defending themselves. Table 1 lists a number of parasitic bees and their hosts.
TABLE 1Parasitic Bees and Their Hosts
Other Parasites and Predators
There are numerous nonbee predators and parasites that attack solitary bees. For example, here in California, a study of Anthophora busleyi, a bee common to central California, found 22 different organisms in the nests of the bees. Eighteen of those visitors were known to kill bee larvae. Some of the other invertebrates that attack native bees include bee flies, velvet ants, wasps in the families Chrysididae and Ichneumonidae, and oil beetles. Oil beetles and bee flies lay their eggs near bee nest sites and the flowers that bees forage on, and when their larvae develop, they find their way to a bee nest or to a flower. When a bee visits the flower, the quick larvae jump on to her and catch a ride back to the nest. Once at the nest, the larvae eat some of the bee’s eggs as well as the nectar and pollen stores. Bee wolves (Philanthus) are wasps of the family Sphecidae that prey almost exclusively on bees. The bee wolf carries the prey back to a tunnel but usually stores it only temporarily until it is later used to provision a cell burrow, where an egg is laid. A single bee wolf cell can contain multiple bees. In Massachusetts, a single species of bee wolf, Philanthus sanbornii, was documented attacking over 100 different species of bees and wasps.
Some of you have probably noticed small spiders sitting in flowers. These crab spiders ambush bees (Figure 7). The crab spider (Misumenia vatia), which is known to catch bumble bees, can change its coloration to match a range of yellow to white backgrounds, effectively camouflaging itself. These are not web-spinning spiders; they are sit and wait predators.
Bees also battle a variety of other invertebrates. These include wax moths that feed on food stores in bumble bee and honey bee nests, flies that consume or parasitize larvae, mites that infest the trachea and feed on hemolymph, parasitic wasps, and nematodes.
There are also larger animals that eat bees. As you might guess, given Pooh’s fondness for honey, black bears are well known for raiding beehives. A bear will eat either honey or brood. Other mammals, including badgers, foxes, raccoons, opossums, weasels, house mice, skunks, meadow voles, and other rodents, also attack beehives. They eat pollen, honey, and the bees.
There are also many birds that consume bees. For example, the European Bee-eater, Merops apiaster, a gorgeous species found in southern Europe, northern Africa, and western Asia, is a voracious consumer of bees. These birds will catch a bee on the wing and bang the insect on a hard surface to remove the sting. One pair of European Beeeaters can consume 30,000 bees in a season! Bees are often eaten by woodpeckers, tanagers, kingbirds, jays, and other native species here in California.
Figure 7. Crab spider
Nesting
The nests of the bees are where they lay their eggs and their off spring develop. For most California bees, the nest is created by a mother. However, for a few of our social bees, like the bumble bees, workers can help with nest construction. Native California bees are primarily solitary, which means that a single female constructs a nest and lays eggs in it. There are some bees that nest in aggregations where you can find tens or even hundreds of individual nests near each other. Every summer, I get calls about Anthophora bomboides, a solitary bee that nests near the beach. Beachcombers are very surprised when they suddenly come upon a nest aggregation and see a lot of good-size bees emerging from the sand and creating small turrets of sand where they have excavated nests.
The basic unit of a nest is a cell or brood chamber. The nest cell protects the developing bee and its food stores, usually a ball of pollen. This nest cell will serve as a nursery for the bee as it transitions from egg to larva to pupa and for its final transition to adult. These cells are often lined. The lining can vary from a cellophane-like substance, to cell walls made of secreted materials that impregnate the matrix, to small pieces cut from leaves. Most nests have more than one cell.
Bees nest in a range of materials, from cement walls to trees to cavities in hollow stems. There are even bees that nest in snail shells. We divide cavity-nesting bees into two main groups; miners or ground nesters, and masons or cavity nesters.
Male bees usually do not sleep in the nest. They sleep outside in places like flowers, tree branches, and grass stems. You can sometimes find clusters of hundreds of male bees sleeping together on a tree branch.
Miners
Bees that nest in mines usually choose a sunny spot with fine soil. The female digs a long shaft that can vary from inches to several feet long. Some nests have a central shaft with cells opening off of it. Other nests have several lateral branches off a main shaft . At the end of the shaft , the bee creates a chamber that will house her pollen ball and egg. She smoothes the surface of the cell or brood chamber using the pygidial plate at the tip of her abdomen and sometimes secretes a substance to line the cell. Once the chamber is built, she will gather enough pollen and nectar to provide for her baby. She often rolls the pollen and nectar into a ball and then lays her egg on the ball. Once the egg is laid, she seals the chamber. Many bees create multiple chambers in the same nest, creating an elaborate network of brood chambers off the central shaft .
Cavity Nesters
Cavity-nesting bees usually use a hole created by other species such as beetles. They are called secondary cavity nesters because they are the second users of that cavity. Bees nest in everything from hollow stems to beetle holes to snail shells. In these holes, the female bee creates brood chambers filled with pollen balls just as the mining bees do. Brood cells are in a linear array, with the first cell filled being the deepest in the cavity. Interestingly, the bees often do not emerge from back to front. Usually the innermost cells contain eggs that will develop into females and the outermost cells contain males. The females take longer to develop, so the bees in cells closest to the cavity entrance usually emerge first.
The bee family Megachilidae is called the leaf-cutter family. Those of you who grow rose bushes may have wondered who created the perfectly round holes in your rose leaves. The culprit may be a member of the family Megachilidae. This group generally nests in existing aboveground cavities, but a few either dig holes or use existing holes in the ground. They are called leaf cutters because they often use leaves to both line the inner walls of their nest and cap the end of the nest. If you look closely at a fence, you can sometimes detect a small round hole that has a piece of leaf placed on the end; these are the nests of some leaf cutters. These bees also use mud to line their cells and to construct dividers between the brood chambers. These bees get called masons. A queen mason bee, such as Osmia nemoris, will find an area with mud. She will dig until she has enough mud to roll into a ball. She will carry the mud back to her nest in her mandibles. She will then build the dividers for each of her cells, using the mud (much like a mason!), and then cap the nest with a mud plug.
Carpenters
In California, we have only one group that carves its own nests in wood, the carpenter bees (Xylocopa). These bees usually start by digging a tunnel in an overhanging branch (or deck). Once they have bored into the wood, they dig a tunnel to the left and right, creating a T-shaped nest. These bees carve out their tunnels by vibrating their bodies as they rasp their mandibles against the wood. These bees do not eat the wood, though they sometimes reuse the wood particles to create dividers between the nest cells.
Pollination Basics
For plants, bees are a welcome partner. Because plants do not move, they rely on wind, water, or animals to transfer their pollen from the male plant to the female plant. This is called pollination.
Bees go to flowers in your garden to find pollen, a primary protein source, and nectar, which is a sweet liquid that provides energy. Animal-pollinated flowers are usually adapted to attract a particular type of visitor. Most animal-pollinated plants rely on bees. However, bats, birds, moths, butterflies, mammals, and even a lizard have been documented as pollinators. To attract pollinators, plants use color, shape, scents, and various rewards such as pollen and nectar. The flower itself is like a big sign advertising to bees that there is pollen or nectar available—though sometimes a flower will cheat and have nothing! Most bee-pollinated flowers are blue, yellow, or white. This is because bees have blue, green, and ultraviolet receptors in their eyes that do not distinguish reds and pinks well. Bee flowers often also have nectar guides. These guides are usually markings on flowers that guide the bee right into where the pollen or nectar is. They are commonly produced from ultraviolet-absorbing pigments called flavonoids so, when you apply ultraviolet light to a flower or look through eyes like those of bees, these nectar guides show up like targets, directing the bees right to the nectar and placing them in the flower in a way to enhance pollination.
Most flowers have pollen. Bees gather pollen to feed their babies, which start as eggs and then grow into larvae. While some adult bees like bumble bees eat pollen, most pollen goes to the larvae. Bees use the nectar for energy. When bees go to a flower in your garden to get nectar or pollen, they usually pick up pollen from the male part of the flower, which is called an anther. When they travel to the next flower looking for food, they move some of that pollen to the female part of the next plant, which is called a stigma. Most flowers need pollen to make seeds and fruits.
After landing on the flower, the bee moves around. As it moves, some of the pollen on its body gets transferred to the flower. Female and hermaphroditic flowers have a structure called a stigma that serves as a sticky landing pad for pollen (Figure 8). When pollen is transferred to the stigma, the pollen tube grows down the stigma and into the ovary where the unfertilized seeds or ovules are found. Inside the ovule, cells from the pollen join up with cells from the ovary and a seed is created. For many of our garden plants, the only way to start a new plant is by growing from a seed. Fruits are the parts of the plants that have the seeds. While we use the term fruit to define a specific group of things in the grocery store, many vegetables such as tomatoes, cucumbers, and peppers are also fruits.
These dustlike pollen grains, the male gametophytes, are produced in an anther, the swollen tip of a stalk-like structure called a stamen. If you look at a lily, the yellow dust that falls from it is made up of pollen grains, and the long, thin stalks with the pollen on the tops are the stamens. For a flower to be fertilized and seed to be produced, pollen (which contains the male gametes) must be transferred from an anther to the structure that contains the female gametes, called the carpel. Pollen lands on the sticky tip of the carpel, the stigma, which is connected to the ovary by a stalk-like extension called the style. Seeds are produced in the ovary, found in the base of the carpel. A simple flower may have only one carpel; morecomplex flowers may have many carpels. Collectively, the female structure of a flower is known as the pistil, regardless of whether there is only one carpel or several.
Figure 8
The androecium, or male part of the plant, is made up of the stamens. Different species of plants have different numbers of stamens, ranging from zero to dozens. Stamens are put together in different ways. Some resemble a ring of fringe surrounding the pistil. This you can see in most open cup-shaped flowers like poppies. Other plants have their stamens fused together. You can see this in a typical tomato flower. Each stamen contains pollen-holding chambers that are fused into an anther. The anther is usually supported on a stalk-like structure called the filament. Each of the chambers in the anther produce the microspores that become pollen.
Adaptations of Bees to Flowers
While there are several other types of pollinators such as bats, birds, and beetles, bees are truly professionals. They have a stunning number of adaptations that really make them good at transferring pollen. Probably the most important trait is their branched hairs. Unlike wasps, which have unbranched hairs (like you might see on a human), bees have hairs that are barbed or plumose, resembling feathers or split ends (see Figure 5).
Bees have specialized brushes or tuft s of hairs called scopae that are thought to specifically be designed to transport pollen back to the nest. Different bees carry pollen in different places. The most common body parts for carrying pollen are the hind legs. Some bees have a corbicula—a concave, smooth area on the hind tibia or thorax that is surrounded by long hairs—used to collect and hold pollen that is carried to the nest. This is sometimes called the pollen basket. The term scopa usually refers to a brush of hairs used to collect pollen. When bees carry pollen on their legs, they can either carry it dry or mix it with a little nectar. When you look at a bee, it is usually easy to see whether it is carrying dry or wet pollen. Bees in the family Megachilidae carry their pollen on scopae on the underside of their abdomen. You can sometimes be fooled into thinking that a bee has a golden-yellow or even purple underside to its abdomen when, in fact, that color is the color of the pollen the bee has been collecting. When a bumble bee’s pollen basket is full, it can contain as many as one million pollen grains. Bees in the genus Hylaeus do not carry their pollen externally; they carry pollen internally in their gut.
Bees have other adaptations that hold pollen to their bodies, in addition to hair. A second factor, an electrostatic charge, enables the pollen to stay attached to the insect. The electrostatic charge helps pollen stay on a bee, and recent research has shown that the accumulated charges on bees are sufficient for pollen to be transferred to a flower by electrostatic forces. As a pollen-covered bee enters a flower, the pollen is preferentially attracted to the stigma because it is better grounded than the other parts of the flower. Interestingly, even if the bee does not actually touch the stigma, pollen grains can jump from the bee to the stigma.
Certain bees are able to vibrate or buzz their bodies. For example, bumble bees are very good buzzers, while honey bees and members of the family Megachilidae cannot do this. When bumble bees buzz, they vibrate at a frequency that causes the pores in the anthers of certain plants to open, causing pollen to explode out of the pores, covering the bee with pollen. You can mimic the buzzing of a bee by using a tuning fork. An A440 or middle C tuning fork will work.
Many members of the family Solanaceae, which includes tomatoes, require buzz pollination. Because bumble bees are semisocial and can have a number of worker bees in a single nest and are able to pollinate tomatoes, a commercial industry has developed supplying bumble bees to tomato growers for pollinating hothouse and—in some states (but not California)—field tomatoes.
Adaptations of Flowers to Bees
If you have ever marveled at the diversity of flower shapes, colors, sizes, and scents, much of that variation can be attributed to the plant’s need to transfer pollen. Individual flowers can have both male and female parts, in which case they are called hermaphroditic or perfect, or they can have only male or female parts. Individual plants can have only hermaphroditic flowers, or a combination of male and female flowers, or a combination of hermaphroditic and male or female flowers, or only male or only female flowers.
So why do bees visit plants? In exchange for having their pollen transferred to a new flower, flowers have evolved many rewards to attract pollinators. The most common attraction is food: either nectar, which is a sugar solution, or pollen, which is high in protein. In many plants, nectar is produced in special glands called nectaries. Nectaries are commonly found in the flowers but can be located on leaves or other parts of the plant (extrafloral nectaries). Regardless of where nectar is produced, it is usually protected from evaporation into the atmosphere and from dilution by rainwater. The nectar is also protected from “robbing” by other animals that do not aid in pollination. The concentration of sugar in nectar has evolved to match the energy requirements of the specific animal pollinator. Nectar for bees is typically 10–30 percent sugar, which is similar though slightly lower in sugar content to that produced by hummingbird-pollinated plants and quite a bit lower than that in bat-pollinated flowers. Bees need dilute nectar because their mouthparts cannot suck up thick, syrupy nectar. Sugars are costly to produce, so plants tend to mete out nectar. Some flowers refill when drained; others refill on a daily basis.
Flowers vary in the volume of nectar they produce. Some produce as little as 0.001 ml. If you imagined a worker bumble bee that had a honey stomach volume of 0.1 ml visiting a plant that produced only 0.001 ml of nectar in each flower, you would see that the number of flower visits required to bring back nectar to make a teaspoon (5 ml) of honey is staggering. If half the flowers that she visits have nectar, she would have to visit 200 flowers to fill her honey stomach. Since honey is about half nectar and half water, if her nest is a mile away, she would have to make 100 foraging trips, traveling 200 miles and sucking 20,000 flowers to create a teaspoon of honey! You can see why bees might prefer flowers with higher volumes of nectar.
Plants encourage bees to visit by providing a wide variety of rewards. Pollen, which produces the male gametes of flowers, is also the sole source of protein for bees and is required for larvae to develop. While bees are interested in gathering pollen for their larvae, plants are interested in having their pollen go to the next plant rather than back to the bee nest. Therefore, plants often mete out their pollen in different ways. Some ripen only a part of the pollen in a flower each day; some open only a certain number of flowers each day. Others have elaborate morphologies that allow only certain bees to access the pollen. In addition to nectar and pollen, some of the more unusual rewards provided by flowers to bees are oil, scent, and warmth.
Plants can also be deceptive. Flowers without nectar or pollen often have structures that look just like anthers or mimic a very rewarding flower. Some flowers are shaped like female bees and trick naive males into mating with the flower, although none of these are found in California.
Plants get pollinators’ attention primarily through colors and aromas. You can think of many parts of a flower as mechanisms for advertising nectar and pollen rewards. The colors deployed by plants vary depending on the pollinators. Different animals perceive light differently. If you recall, the visible spectrum ranges from 380 to 760 nm (violet–blue–green–yellow–orange–red). Bees see best at the lower end of the visible spectrum and into ultraviolet radiation. This means that a bee’s visual acuity is much greater for blues, yellows, and whites than for reds and pinks. Therefore, flowers pollinated by bees are most often violet, blue, or yellow and may have ultraviolet markings (invisible to the human eye), whereas birds are particularly attracted to red flowers and bats’ flowers are a dusky white.
Plants also use visual guides (see Figure 8) to direct pollinators to rewards. These are called nectar guides or honey guides. Once the pollinator comes closer to the flower, it sees a visual guide that pinpoints the location of the reward. Some examples of this are the dots on the throat of a foxglove (Digitalis) or the lines on the petals of a violet (Viola) or a contrasting color pattern like the yellow center and blue petals of baby blue eyes (Nemophila menziesii).
Plants also use aromas to lure in pollinators. Aroma is more important for insect-pollinated than bird-pollinated plants, as most birds do not have a strong sense of smell. Bee mouthparts are covered in tiny hairs that have pores in them. Molecules pass through these pores and stick to receptor sites on sensory cells inside the exoskeleton. This is how the bee tastes and smells. There are also similar hairs on the antennae that pass scent molecules to sensory cells. With two antennae, the bees can monitor the concentration of the odor and locate the source.
A number of compounds have been identified as being associated with bee pollination in particular. Sometimes scent can combine with morphology. For example, the nectar guides of daffodils have a fragrance that is stronger than that of the rest of the flower.
Aromas do not always have to be pleasant. Some plants actually produce scents that smell like rotting flesh. These odors tend to be more associated with fly and beetle pollination than bees, fortunately.
Bee flowers come in a variety of shapes. An open cupshaped flower is the easiest for most bees to access. More complicated shapes, like that of a lupine or orchid, serve to limit which bees can access the nectar or pollen in that plant. It takes a strong or heavy bee to open a lupine flower.
TOXIC FLOWERS
Bees are affected by some other compounds that they find in flowers. Bees are sensitive to ethanol, which causes them to lose their balance and have trouble moving around. It has been reported that very inebriated bees will lie on their backs and wiggle their legs. Bees are also sensitive to alkaloids, coumarins, and saponins, and some compounds like cardiac glycosides are toxic to bees at some levels. Several California native plants are toxic to bees, such as death camas (Zigadenus). The California buckeye (Aesculus californica) produces pollen and nectar that are reported to be toxic to Western or European Honey Bees but not to native bees.
Ecosystem Services and Bees
As you sit at the table today, do you know where the water you are drinking came from? In San Francisco, 85 percent of the drinking water comes from the Sierra Nevada. How about the last prescription medicine you took? It probably originated from a natural source. Of the top 150 prescription drugs used in the United States, 118 originate from natural sources: 74 percent from plants that may depend on pollinators, 18 percent from fungi, 5 percent from bacteria, and 3 percent from a species of snake! And where did the ingredients for your lunch and dinner come from? One of every three bites you took probably came from a plant pollinated by wild pollinators, primarily bees. This is just the beginning of a list of the services provided by healthy natural ecosystems.
Economists and ecologists have started working together to find a way to place a financial value on the contribution of natural ecosystems to human existence. The estimates are eye-opening. For example, the value of pollination services to agriculture from wild pollinators in the United States alone is estimated at $4 to $6 billion per year. While these ecosystem services are currently produced for “free,” replacing the natural ecosystem would cost many trillions of dollars. Unless human activities are carefully planned and managed, valuable ecosystems will continue to be impaired or destroyed.
Bees and Agriculture
About one-third of the food supply in the United States depends on animal pollinators. While other species like hummingbirds and beetles are involved in pollination, bees are the most important pollinators. Western Honey Bees (Apis mellifera) are used extensively as managed pollinators, and native species provide significant pollinator service also. Actually, Honey Bees are not always the best pollinator of a particular crop. Tomatoes are a good example of this. Tomatoes require buzz pollination to release their pollen. Since Honey Bees do not buzz, they are not good pollinators for tomatoes. Table 2 lists some bee-dependent crop plants in California and the bees that pollinate them.
If you are going to talk about pollination and agriculture in California, it is best to start with almonds. Approximately 80 percent of the almonds produced in the world are grown in California. The 2009 estimate of the value of the almond crop to California’s economy was $1.9 billion. That is a lot of almonds! Almonds are primarily pollinated by Honey Bees. They bloom in February, which is early enough that the majority of native bees have not emerged. However, when there are native bees in the almond orchard, Honey Bees change their behavior and become much more effective pollinators because they are more likely to move among orchard rows.
TABLE 2Some Bee-dependent Crop Plants in California
Each winter, virtually every commercial Honey Bee hive west of the Mississippi River (about one million Honey Bee hives) is shipped to California. Hives are then placed out in the almond orchards waiting for the flowers to open. Farmers put out about two hives per acre, and the cost of Honey Bee hive rental in 2011 was about $50 per hive for almond pollination. Almonds are a major source of income for both the almond farmer and the beekeeper.
Most of the bees that are managed for agriculture are cavity-nesting bees like Honey Bees and leaf-cutter bees. This in part is because they can be moved around by farmers who need their services. The only managed ground-nesting bee is the Alkali Bee, Nomia melanderi. This bee has been used to pollinate alfalfa (Medicago sativa) for over 50 years. In some areas where alfalfa is grown for seed, notably the Touchet Valley in Washington State, which grows about 25 percent of the US seed, soil-nesting beds for N. melanderi are actively managed. Growers subirrigate and surface-salt the areas where the bees nest. And, it is wildly successful. On a single 3-acre (1.5-hectare) nesting bed, there is an aggregation of over five million bee nests with up to 1,000 nests in a square meter. This is the largest bee nesting aggregation ever recorded.
Bees in Urban Environments
To maintain biodiversity and to meet the increasing demands for ecosystem services, we must move conservation science into cities. Cities are important for conservation for two reasons. First, 80 percent of the US population already lives in urban areas. Second, cities are growing. They already encompass about 3 percent of land (59.6 million acres) in the United States, and 230,000 additional acres become urban each year. Because of their large human populations, cities are the places where many ecosystem services, such as environmental quality of life, are delivered. Given the growth of the urban population, it is clear that we need to develop the knowledge necessary for maintaining natural habitats in the urban setting and find a way to give urban dwellers access to nature.
We know that pollinators are declining in certain wild and many agricultural landscapes. However, little is known about urban pollinators. Our recent data on bumble bees in San Francisco suggest that urban bees may also be declining. While the loss of these pollinators is important, it is also important to understand what effect these losses have had on pollinator services. A recent study on cherry tomatoes in San Francisco suggests that without pollinators, there would be lower yield from each plant. In other words, the plants’ tomato production seems to be determined by the supply of pollen.
We do not know much about how healthy bee populations are maintained in an urban environment. Because natural habitats are less common in urban landscapes, they may not provide enough resources to support viable pollinator communities. However, if other habitats, such as urban gardens and restored areas, are sufficiently connected to natural habitat, then native populations may thrive. Work in seven California cities (Ukiah, Sacramento, Berkeley, Santa Cruz, San Luis Obispo, Santa Barbara, and La Canada Flintridge) by Gordon Frankie suggests that in California, urban environments can provide habitat for bees. He found that urban environments support approximately 60–80 species from all five families found in the state. In these settings, the bees often used nonnative ornamental plants as their food sources. While 60–80 species is much less than the 100–300 you might expect to find in the surrounding rural habitats, they are still good pollinator communities.
Conservation of Bees and Other Pollinators
Bees have complex life cycles and specific habitat needs. As such, bee populations are threatened by habitat destruction and fragmentation, pesticides, climate change, and a host of incompatible land management practices. Here are some things you can do to help bees.
PLANT A POLLINATOR GARDEN One of the best things to do for bees is to plant a pollinator garden. The Xerces Society, an invertebrate conservation organization, has developed guidelines for improving habitat in situations ranging from a backyard to a golf course to a farm. The guidelines here are adapted from their recommendations.
Flowers are food resources. Patches of habitat can be created in many different locations, such as field edges, stream corridors, backyards, school grounds, golf courses, street medians, and city parks. Even a small area planted with the right flowers will be beneficial because each patch will add to the mosaic of habitat available to bees.
INCORPORATE A SUCCESSION OF FLOWERS Doing this will provide bloom throughout the entire growing season. A diverse selection of flowering plants that bloom in succession will support a more diverse community of pollinators.
CHOOSE SPECIES THAT ARE EASY TO PLANT AND ESTABLISH Your local chapter of the California Native Plant Society and native plant nurseries can give advice on local plant species. For California, there is a list of urban bee plants.
USE LOCAL NATIVE PLANT SPECIES Native plants are more likely to attract native bee pollinators than are exotic flowers. You may supplement natives with heirloom varieties of herbs and perennials. Do not plant invasive species.
TABLE 3Excellent Plants for Native Bees
CHOOSE FLOWERS WITH A VARIETY OF SHAPES AND COLORS This will attract a wide variety of bees. Different bee species have mouthparts adapted to different shapes of flowers; short-tongued bees can drink only from open flowers such as asters or daisies, while long-tongued bees can reach the high-energy nectar in deep flowers such as bluebells or lupines. Flower colors known to attract bees are blue, purple, violet, white, and yellow. We recommend that you use native plants wherever possible. We have listed some of the native plant genera most attractive to bees in Table 3.
PLANT FLOWERS IN CLUMPS Flowers clustered into clumps of one species will attract more pollinators than individual plants scattered through the habitat. Where space allows, make the clumps 4 ft or more in diameter.
Land Management Considerations
You can help maintain healthy populations of bees by making slight adjustments in land management practices to ensure habitat is provided throughout the year. In order to maximize pollination opportunities, avoid maintenance treatments while plants are in flower, and allow plants to bolt before tilling. Never burn, graze, or mow an entire area at once. This will allow for recolonization of the treated area from unaffected areas, an important factor in the recovery of pollinator populations.
AVOID PESTICIDES Do not use pesticides unless absolutely necessary; insecticides can kill bees, and herbicides can kill their food sources. If they must be used, pesticides should be applied with careful timing and targeted spraying methods in order to minimize impacts on pollinators.
PROTECT NEST SITES Solitary ground-nesting bees need direct access to the soil surface to excavate and access their nests. Where possible, keep some bare or partially vegetated ground. Clear the vegetation from small patches of level or sloping ground, and gently compact the soil surface. These patches should be well drained and in open, sunny places. Ground-nesting bees seldom nest in rich soils, so poor-quality sandy or loamy soils may provide fine sites.
Do not put thick layers of mulch or landscape fabric over potential nests sites. These barriers can prevent bees from burrowing into the soil.
Create sand pits and piles. In a sunny, well-drained spot, fill a pit about 2 ft deep with a mixture of fine-grained sand and loam. Where soils do not drain well, make a pile of the sand-loam mixture instead—or fill a planter with the mixture.
Other solitary bees nest in solitary wood (or tunnel). For them, leave snags or dead tree limbs. An arborist can advise on whether a snag is really a hazard; if you can leave it, it may provide excellent nest habitat. Cut the ends off pithy-centered stems on plants such as elderberry, box elder, raspberry, or dogwood. Bees will nest in holes they dig in the soft pith. Supplement your garden with artificial nest boxes (such as a nest condo).
HOW TO MAKE A NEST CONDO
About a third of our bees, such as the family Megachilidae, nest in small cavities, often the tunnels of beetles. For these bees, leaving tree snags in your yard is a great way to keep nest sites available. If you cannot keep snags, one alternative is to build nest blocks (or buy them at a local garden center). Nest blocks can easily be made from old lumber. You will want to use untreated lumber and drill nest holes for the bees to use. Different species of bees nest in different-size holes, so I recommend you use different sizes of drill bits (3/32 to 3/8 in.). Drill the holes 5 or 6 in. deep. Each hole should be at least half an inch from the next. The inside of the hole should be smooth and closed at one end. You don’t have to use lumber for this; you can use logs or stumps. When you have the bee condo made, you can mount it. It should be somewhat sheltered and face a direction that will give it morning sun. Fence posts, trees, and buildings can all be used to mount your condo.
There is a variety of designs for bumble bee nest boxes, as well as commercially available premade boxes. These boxes have very low rates of success. A better way to attract bumble bees would be to make improvements to your garden to enhance bumble bee habitat.
Studying Bees
Studying bees through either observing or collecting them will provide wonderful insights into this marvelous group. Fortunately, it is easy to study bees in your own backyard. Bees are most active in spring and summer months when floral resources are available. Bees can be found everywhere in California from high mountain meadows to wind-swept dunes. The easiest places to find bees are the places where there are flowers. However, if you watch only flowers, you may miss some of the more interesting parasitic species that are more often found patrolling bare ground, looking for the nests of their hosts. Bees prefer open habitats. If you are new to an area, then looking along roadsides, river edges, power lines, open fields, sandy areas, and wetlands will be a great way to start.
Watching Bees
You can start by simply watching. Some of the binoculars that can focus at close range are fantastic for watching bees. One of the first things to notice is what the bee is doing on the flower. Some bees will simply be taking a sip of nectar, while others will busily be collecting pollen. You can watch and see how they move their legs, how quickly they depart after landing, how they move the pollen around their bodies. As you watch different bees on different shapes of flowers, you will begin to notice where on its body a bee is placing the pollen it collects. Sometimes you will see a bee with pollen on its face. Others will have it sprinkled on the underside of the abdomen or on its hind legs. Pollen grains that are not gathered up into pollen baskets are the key to the plants’ reproductive success, for these are the grains that get transferred to the next flower.
If you are lucky enough to find a nest, you can watch, or time, how long it takes for bees to return. Are their foraging trips long or short?
You can also participate in more formal activities like the Great Sunflower Project (www.greatsunflower.org). By using citizen scientists across the continent, all surveying pollinators in backyards, parks, and gardens, the Great Sunflower Project gathers the necessary data to address questions about the effect that declines in bee populations have had on pollination of plants. In addition, by examining pollination in their own gardens, participants get the ability to place their gardens in local, regional, and even continental contexts. Imagine being able to know enough about pollinator service to say, “Wow, my garden has many more pollinators than most of my neighbors and most of San Francisco, but fewer than average for the Bay Area in general, California, and the western United States.”
The Great Sunflower Project uses sunflowers as standardized “bee-o-meters”; for each site with sunflowers, citizen scientists record how many bees visit within the time that they watch, effectively creating an index of pollinator service. The Project also accepts data from other flowers and even counts of bees taken along trails or walkways. By also taking and uploading photographs of the visiting bees, citizen scientists help collect the data that will answer whether or not native bees are filling in where we have had a loss of Western Honey Bees through colony collapse disorder or other maladies.
Collecting Bees
Insect collections are incredibly valuable resources for science. Not only do they provide documentation of the presence of species, they can be used to understand the relationships among species, how organisms evolve, and contribute to many other fields. For many insects, bees included, accurate identification of the species requires using a microscope. Because of this, we do need to collect bees. However, collections should be done thoughtfully. While there are some wonderful experiences to be had building personal collections, collected insects are most valuable in collections where scientists can get access to them. If you are interested in collecting, we would suggest working with local professional or experienced amateur entomologists and, if possible, making arrangements to donate your specimens to a local museum to ensure that the bees that you kill are able to make a contribution and are available for all to see and use.
To make a bee collection, you will need some equipment. The first item is an aerial net that can be used to catch flying bees. Most bee collectors use nets with a 12–18 in. hoop and a 3-foot handle. The larger hoop has an advantage because it covers more area; however, it is harder to swing and more likely to get snagged. I actually use a net made from an old golf club shaft because the aerodynamics are great. You can also find collapsible nets commercially. Some break down into three pieces; others telescope. The net should have a net bag that is made out of canvas material around the rim and wedding veil for the body of the net. You can either make this yourself or buy a commercially available one. Since many interesting bees are tiny, it is useful to buy or make a net bag that uses a very fine mesh.
NETTING A BEE It is best to carry your net so that you are ready to swing. I hold the handle of the net in one hand and the middle of the shaft in the other. I also usually hold the tip of the net bag in the hand on the shaft . When looking for a bee to catch, it is easier to detect the movement of a bee than the bee itself. Swing at everything. There is no harm in catching insects that are not bees, so I worry less about identifying what is moving than catching whatever it is. It is important when you swing your net that you swing quickly and swing all the way through—much like you would a tennis racket or a golf club. The only time that I check my swing is when I am working around plants with thorns or seeds that might damage my net bag. As I walk, I try to pay attention to the direction the wind is blowing. Swinging into the wind will open your net bag up, making it more difficult for bees to escape. In some instances, it is going to be easier to capture a bee by slapping your net on the ground than swinging through. This is especially true when you have small bees working plants that are near the ground. In this situation, I slap the net on the ground and then lift the end of the net bag in the air. Usually, any bee in the hoop will fly up the net bag trying to escape.
When looking for bees in a patch of flowers, I am usually conscious of where my shadow is. Bees will move away from shadows. It is best to stand several feet away from the flowers but close enough that you can quickly swing at a bee that comes into sight without taking a step. I often watch to see the pattern of flight in a patch. You will see some bees quickly flying through the patch without stopping. These are often male or parasitic bees looking for females. To catch these bees, you need to be quick. You often have a second chance though, as they seem to repeat their circuit with some regularity.
GETTING A BEE OUT OF THE NET Once you have actually captured a bee, you need to find a way to get it out of the net. If you are simply going to look at the bee and not collect (a euphemism for kill) the bee, you can use a glass jar to capture the bee. I often use a test tube with a cork inserted in the end, as I find it is easier to slip a cork in than to screw on a top with one hand. If you bring a cooler of ice with you, you can place the jar in it and the bees will become inactive and easy to examine—that is, until they warm up!
If you are going to collect the bee, you will want to have something to kill the bee. Traditionally, collectors have used collecting jars. Collecting jars are usually made with cyanide or ethyl acetate. If you are using a collecting jar, it helps to have a second set of containers to transfer dead bees to every half hour or so. The chemicals in a collecting jar can change the color of a bee and sometimes make the body stiff, rendering it more difficult to pin. I use a set of small plastic containers designed for holding beads that I purchase at a craft store. A simpler and less toxic way to collect bees (though it takes a bit more processing back in the lab) is to use a jar filled with soapy water. One or two drops of dishwashing liquid per cup of water is sufficient.
Once the bees are in the bag, I swing the net rapidly to “snap” the bees to the end of the net bag. I then trap the bees there by grabbing the net below where the bees are. The netting of the bag will bunch up and protect your hand from getting stung. I then slip the collecting jar into the net and capture the bees.
WASHING AND DRYING BEES Bees that are collected in liquids should be washed before being pinned. The easiest way to do this is to get a mason jar and replace the top with window screen. Add enough warm water to cover the bees but do not fill the jar. If your bees were collected in something other than soapy water, you should add a squirt of dishwashing liquid to the container and then shake vigorously for more than a minute. Next, rinse the bees with water until the water runs clear.
Once you have done this, rinse the bees in alcohol and then shake them out onto a paper towel to get the initial water and the alcohol off. Then, move the bees to a stack of dry towels and fold the towels over the bees to rub them dry. You can rub with some vigor without damaging them. Having a stack protects the specimens a bit. Pick up the top towel by its corners, and bounce the bees around in the basket that forms for a minute or more. This step helps to fluff the hair on the bees and really makes your specimens look better. You can also use a blow-dryer or build your own bee dryer to dry your bees, and instructions are easily found on the Internet.
PINNING BEES Collected bees are usually stored on pins. Bees are pinned using entomological pins (sizes 1–3). Size 2 pins are useful for virtually all bees that we have in California. Bees are usually pinned directly from the collecting jar or drying towel into a box. Bees are pinned in the thorax right between the wings. Usually, the pin is inserted through the right side of the bee, leaving the left side intact. The bee is positioned toward the top of the pin, leaving enough room at the top for someone to safely grab the pinned specimen without contacting the bee. You can use a pinning block to determine the correct height.
STORING BEES There is a variety of commercially available boxes, drawers, and storage cabinets that can house your collection. For temporary collections, I use cardboard specimen boxes. These have the advantages of being inexpensive and easy to store, and you can write on them. For more-permanent collections, I use a glass-topped hinged museum drawer.
When pinning into a storage box prior to making permanent labels, I start by creating a label with the date, site, and collection ID. This is often the tag that I slipped into the vial when I was collecting the insects in the field. I then pin all the insects from that site to the right of the tag. I signal the end of that group by placing the next tag next to the final specimen. In general, I try to keep bees from a single collecting trip or project in the same box. On the outside of the box, I write the appropriate identifying information so I can quickly relocate the specimens. You will want to take some steps to keep pests, particularly dermestid beetles (family Dermestidae), out of your collection. You can do this by regularly freezing (for 3 days) your boxes, by keeping a mothball or other chemical deterrent in the box, or by simply storing the box in a plastic ziplock bag. We find that a 2 gal ziplock is the right size for most standard insect boxes. If you live in a very humid area, you will also want to be careful about mold and mildew.
Identifying Bees
It is almost impossible to identify bees while they are flying, so to identify a bee you need to be able to take a good look at it. If you are not planning to collect the bee, you can often get a good look by simply cooling the bee down. Put the bee in the refrigerator or in a cooler with ice, and wait 5 minutes or so. This will immobilize bees, although you need to be aware that they will become active again as they warm up. Then, you can use a hand lens or dissecting microscope to look at the bee. It helps to place your bee on a white piece of paper because this will allow you to look at some of the characteristics of the wings more closely. You can use the matrix in Appendix 2 to differentiate among the genera. The matrix starts with the size of the bee. Is the bee larger or smaller than your average Western Honey Bee worker? Then you will want to examine the color of the bee. Bee colors can be subtle; to distinguish among the dark blue, black, and green colors, it can be easier to look at the sheen of the bee. Next, look for markings on the bee. These are most common on the face or the abdomen of a bee.
HONEY BEES
Arguably the most familiar bee to humans is the honey bee. Honey bees are not native to California. We think honey bees originated in eastern Africa and migrated from there into Europe and Asia. The species we see in California is sometimes called the European Honey Bee, but a better name might be the Western or European Honey Bee (see the genus account for Apis).
Some of the earliest records of the relationship between bees and humans come from late Paleolithic cave paintings like those of the Cuevas de la Araña found near Valencia, Spain. Ancient beehives have also been discovered in Israel that date back 3,000 years, and Egyptian beekeeping operations are depicted on ancient tomb paintings. In North America, the first honey bees were brought to Jamestown, Virginia, by European settlers in 1622. Interestingly, these were not the first members of the genus Apis (the honey bees) in North America. A honey bee fossil of a different species (Apis nearctica), complete with hairy eyes, was found in Nevada. This species may have crossed to North America from Asia in the Beringia migration about 30,000 years ago and gone extinct due to changes in climate at the end of the Miocene era. Apis mellifera, our Western Honey Bee, has spread now to most continents and is the most important partner for agriculture.
Western Honey Bees are considered social bees. This means that there is a social differentiation among the bees based on their functions. The division of labor in a Honey Bee hive is complex and fascinating and worthy of a book unto itself. Every Honey Bee in a hive belongs to one of three castes: queens, female bees called workers, or male bees called drones.
In each hive there is a single queen whose main purpose is to lay eggs. She is able to lay over 1,500 eggs per day. The queen lives for 2 to 8 years. She is larger than the workers and drones. Her ovipositor is smooth and curved, so she can use it over and over again.
The workers are the daughters of the queen. Different workers do different tasks in a hive. When they are young, they work within the hive constructing comb, managing the brood, cleaning, regulating hive temperature, and defending the hive. As the workers age, they become foragers. These foragers gather nectar, pollen, and water from outside the hive. Interestingly, as workers age, certain regions of their brain increase in size. It is thought this may be associated with their spatial learning from foraging. During the main period of hive activity, a worker only lives about 6 weeks. Workers are also morphologically different from the queen. Aside from being smaller, they have a corbicula (pollen basket) on each hind leg, an extra stomach for carrying nectar, and special glands on their underside for secreting bees-wax. Their ovipositor also differs, as it is straight and barbed. This is why you always hear that a bee dies after stinging. A worker Honey Bee’s stinger is ripped out of her abdomen when she stings, which kills her.
Drones are male bees. They, therefore, have no ovipositor. The only reason for drones to exist is to mate with new queens. Drones have bigger eyes than workers and queens and have longer abdomens. They lack the specialized structures for collecting pollen and nectar.
The first Honey Bees brought to North America were kept in hives and managed for their honey production. However, Honey Bees have escaped from management and successfully colonized the United States. In 1862, L. L. Langstroth, a Philadelphia minister and beekeeper, used the knowledge that Honey Bees will fill in any gaps in their hives that are larger than 1 cm, a concept called bee space, and invented the modern beehive with movable frames. This invention really led to the development of modern apiculture, as it allowed beekeepers to harvest honey, move their hives, and manipulate their colonies. Here in California, we have both escaped wild Honey Bee colonies and managed colonies.
Wild hives can be found in the walls of houses, in cavities in trees, or in any large cavities. Wild hives build honeycomb and produce and store honey just like managed hives. When you see a swarm of Honey Bees, these are bees that have left their natal hive (either a wild hive or a managed hive) and are following their queen as she searches for a new home. While swarming, the queen is emitting a pheromone that is all but irresistible to the accompanying worker bees, and they will follow her for miles and days.
The swarm will eventually settle into a new home. Sometimes feral swarms are captured by beekeepers and moved into artificial hives; other times, the swarm finds its own cavity. Once the queen settles in, the workers begin to construct honeycomb from wax and to provision that comb with pollen and nectar—for the new eggs that the queen will lay and to provide food resources for the bees of the hive. Honey represents the food storage of the hive. Individual worker bees bring nectar back to the hive. They regurgitate that nectar into a cell in the honeycomb. Once in the honeycomb, the water from the regurgitated nectar evaporates, making it thicker syrup. Bees can accelerate this evaporation by fanning the comb with their wings. Once the honey in a cell reaches the right consistency, the bees plug that cell with wax, and the honey is stored until needed.
In California, wax is only produced by bumble bees and Honey Bees. It is produced from eight internal wax glands found in the abdomen of the bee’s body. As bees age and begin to take on the role of forager, the wax glands decrease in size. The wax produced by the bee is clear; after the bee chews it, it becomes white, and then as pollen oils and propolis are added, the wax becomes yellower and browner.
HONEY BEE PROBLEMS
Over the past 60 years, Honey Bee colonies have declined by 59 percent because of the effects of a parasite, the Varroa Mite, among other things. The USDA and Apiary Inspectors of America have reported that in both 2006 and 2007, commercial beekeepers in the United States lost a little over 30 percent of their Honey Bee colonies. Much of this loss has been attributed to colony collapse disorder, a condition that results in the rapid loss of a colony. As far as we know, colony collapse disorder does not affect other species of bees, only Western Honey Bees.
COLONY COLLAPSE DISORDER Honey Bee hives have been known to collapse at several different points in the history of apiculture. However, the term colony collapse disorder was first used for the big decline in Honey Bee colonies in late 2006. Initial reports attributed colony collapses to factors that ranged from cell phone usage to the development of new pesticides, particularly neonicotenoids such as imidacloprid that affect the central nervous system of insects. Surveys looking at differences between colonies affected by colony collapse disorder and ones that are not have shown that the affected colonies have higher levels of viruses, pesticides, and parasites, suggesting that environmental stress may lead to a series of events within the colony that render it more susceptible to parasites and pathogens. More recent genetic work has found that affected colonies all are infected by both a virus (invertebrate iridescent virus type 6) and a spore-forming (microsporidian) fungus (Nosema ceranae).
HONEY BEE ENEMIES In addition to the players implicated in the studies of colony collapse disorder, Honey Bees have a variety of predators, parasites, and diseases. Individual bees are eaten by birds, bats, flies (family Asilidae), dragonflies, wasps, and other organisms. Social bee nests, which have lots of food resources as well as tasty larvae, are raided by everything from ants to raccoons to bears (which my student Amber discovered after placing bumble bee nests in high Sierra meadows).