CHAPTER 4

Insects and Plants: A Never-ending Race

Although many insects are predators or parasites, most of them eat a plant diet – in the form of either salad (living plants) or compost (dead plants – discover more in Chapter 6, here).

There are many nuances to the salad diet: insects may eat nectar and pollen, seeds or the plant itself. There may also be some advantages in this for the plant, such as pollination or the dispersal of seeds. Over 120 million years, insects and plants developed tightly in tandem. They are often mutually dependent, but at the same time it is a never-ending race in which each party is out to secure more advantages for its own side. This love-hate relationship has led to some most peculiar forms of coexistence.

Drinking Crocodile Tears

The life of a herbivorous insect is no bed of roses! Plant tissue in general is pretty meagre fare, low on vital substances such as nitrogen and sodium. For instance, while the dry weight of most insects is at least 10 per cent nitrogen (sometimes much more, see here), plants overall contain only around 2–4 per cent. This has a number of consequences for herbivorous insects. Many have a lengthy larval period to ensure that they can acquire enough nourishment before metamorphosing and facing the adult world. Other larvae (with a shorter larval period) concentrate on the most nutritious parts of plants, such as the roots (where some plants have tenant bacteria that capture nitrogen for them) or the flowers and seeds. By the way, this is exactly what we humans do too – think of our staple foods like cereals and pulses.

Many aphid-like insects, which subsist by supping nitrogen-poor plant sap, have to guzzle up massive amounts – relative to their small size – to get enough nutrition. This leads to a tremendous surplus of water and sugar, which they excrete in the form of what we often call honeydew – much to the delight of other creatures (see here).

Plants also contain very little sodium, a substance that is crucial to the functioning of all creatures’ muscles and nerve systems, among others. While members of the deer family, all herbivores, can acquire sodium by licking salt stones set out for them by friendly humans, insects must find natural sources that are rich in sodium. That is why you often see colourful butterflies sitting around a puddle, slurping up mineral-rich mud as a supplement to their nectar-based diet.

And if you can’t find a puddle, how about crocodile tears? In 2013, fascinated field biologists on a trip downriver in the jungles of Costa Rica were able to film and photograph a beautiful orange butterfly and a bee drinking the tears of a caiman – a member of the crocodile family – each sipping from one of its eyes. It turns out that this method of acquiring vital salts from reptile tears is more widespread than we had thought, it is just rarely witnessed. Drinking crocodile tears undoubtedly sounds a bit more spectacular than slurping up puddles!

Willow: The Most Important Meal of the Spring

Pollination is a win-win activity that binds insects and plants together. The insect gets food in the form of sweet nectar or protein-rich pollen; the plants get their pollen moved from one flower to another, enabling fertilisation and the development of new seeds. Although some plants rely on the wind for cross-pollination or are self-pollinating, as many as eight out of 10 wild plants benefit from insect visits to their flowers.

Some plants are especially significant ‘insect restaurants’ because they supply nectar at a critical moment. The willow tree is one example. Normally, it lives a pretty anonymous existence out in forest and farmland. But in the spring, the willow enjoys its 15 minutes of fame. Because this is when the bumblebee queen comes tumbling out of her underground bedroom, where she has lain like Sleeping Beauty since the previous autumn. And she is hungry – after all, she hasn’t eaten anything all winter. But there’s no one around to prepare a delicious breakfast for her. Not yet. All the worker bumblebees called it a day when the autumn chill arrived, along with the previous year’s queen, and now it’s up to this queen to start a new community. If she is successful, both she and we humans will eventually end up with food on the table. Because bumblebees, wild bees and other insects are, as we know, crucial for the pollination of our food crops (about which more in Chapter 5, here). First, though, Her Royal Highness must find something to eat. And this is where the willow tree comes in, serving as nature’s starting motor.

The willow doesn’t hang about once the snow is melting on the hillsides. At a time when the other trees and plants have barely started to consider what to wear this year, the willow is already fully dressed. A bit lightly clad, admittedly, because the leaves won’t arrive for some time. But the flowers are what matter here, during the first trysts of spring. The male and female flowers blossom on their separate trees: the male flowers are the familiar soft, grey catkins known in Norwegian as ‘goslings’, which eventually turn bright yellow, thanks to their pollen-bearing anthers; the female flowers are more discreet but contain more nectar than their male counterparts.

And this is the queen bumblebee’s great stroke of luck: a fortifying breakfast that combines protein-rich pollen with a strengthening source of sugar nectar, all served up in the willow trees. This provides the energy that is urgently needed when you’re about to set up a whole new community of pollinators single-handedly.

Once the bumblebee queen has eaten her fill, she will find a suitable nest site, either underground or above ground depending on the species. Here, she will gather a ball of pollen in which to lay her first batch of eggs, before covering the ball with wax. Later, the newly hatched larvae will eat their way through this pollen-stuffed nursery. In the meantime, the queen is not being idle – she also builds a wax honey pot and fills it with regurgitated nectar. In this way she makes sure she can feed herself while incubating her eggs. Bumblebee eggs need to be kept at around 30°C to develop properly and the queen broods her eggs just as birds do. The queen actually has a bare spot on her abdomen to help disseminate the heat from her body to the eggs. In this first period she must sometimes leave the nest for short foraging trips, but as the colony grows, the workers take over the gathering of pollen and nectar and the queen concentrates on laying eggs.

Later in summer, the bumblebee queen stops producing female workers. Instead, she lays unfertilised eggs that develop into males, and the larvae from her fertilised eggs are now fed in a way that makes them develop into new queens. With autumn drawing close, the new queens and the males mate. For the old queen, the males and the rest of the summer’s colony, it’s game over. Only the new, now-mated queen survives and crawls into a cosy space underground, ready for a long sleep, until spring awakens and the cycle begins again.

Globeflower: Nature’s Bed and Breakfast

Coupledom can be complicated, and that applies to the pollinating relationship between insects and plants too. The pollination of the globeflower is a case in point. With its bright yellow, almost closed head, the globeflower is easy to spot in meadow and waterside locations in the United Kingdom, but not so easy to access.

Only three or four insect species, all belonging to a family known as globeflower flies, are able to find their way into this tightly packed miniature sun of a flower. But they are amply rewarded: it turns out that the globeflower is a bit like a bed and breakfast, offering its visitors a hearty meal!

The globeflower provides the very best it has to offer: its own seeds. I’m not sure whether they contain as much protein as bacon and eggs, but they must certainly taste pretty good to an exhausted fly. Strictly speaking, the adult flies aren’t the ones helping themselves to the grub, either – they just lay eggs in the ovules inside the flower, and that’s where the larvae grow up. In fact, the only place they are able to develop is inside a globeflower seed.

So, how in the world do globeflowers organise things so that a steady stream of flies can move from flower to flower with pollen? It’s a question of collaboration and an ingenious balance between flower and fly. Because these particular flies are the only ones that can pollinate the globeflower, without their visits there would be no globeflower babies – in other words, no seeds. No wonder the flowers bend over backwards to offer the very best they have at their disposal. Yet it is an incredibly fine balancing act. If the flies eat up all the seeds, there will be no more globeflowers, and over the long term that means there will no longer be any hosts offering board and lodging – and this in turn means no more new flies either, so it is crucial for the flies to lay eggs in a suitable proportion of the seeds. Quite how the flies work that out remains an open question, but the fact is, it works.

An Innocent Pizza Herb? Far From It!

Oregano is another example of complex interaction between plants and insects because this green herb, much used in Italian cuisine, is party to a cunning intrigue involving powerful alliances, disguises and forgery.

Picture an arid, sun-drenched hillside in northern Italy emanating a heady scent of oregano, thyme and marjoram. One of the oregano plants feels a tickling sensation around its nether regions: a bunch of Myrmica ants have decided to set up their nest beside the plant’s roots. Now and then they gobble up a few small roots as they go about their work. This is hardly beneficial for the plant, which responds to the ants’ munching by upping its production of carvacrol, a substance that defends it against insects. Most ants have zero tolerance for the insecticide, but this particular species has learnt to cope with it and stands its ground down there among the roots. We humans prize this defensive substance: carvacrol is what gives oregano its intense herbal scent. But the aromatic substance has several functions. In Italian flower meadows, it also serves as an SOS, a kind of shout in the language of scent directed at a totally different species. The recipient is a beautiful butterfly known as the Large Blue. It lays its eggs on the plant and the larva then spends a couple of weeks there, developing and simultaneously preparing a disguise that would be the envy of any undercover agent. We’re not talking false moustaches and hair dye here, because the visual aspect isn’t especially important for ants. Scent is, however, which is why the butterfly larva veils itself in an alluring cloak of ant scent, perfectly adapted to the aroma of the ants living beneath the flower.

Next comes a critical moment: the larva lets go of the plant and tumbles to the ground. A Myrmica ant comes toddling by on its way home from the eternal round of food gathering. It finds the butterfly larva, is tricked by the scent into thinking it’s a larva from the ants’ nest and carefully carries the butterfly larva into the darkness of the nest, where it is adopted on the spot. Even though it differs from the ant offspring in both size and colour, it is nursed, cared for and fed on regurgitated food by adult worker ants, who tend to it as diligently as they care for the nest’s own young. But the butterfly larva, which needs to multiply its weight by several hundred per cent before it’s done, is not content with recycled sugar water. As soon as its adoptive mothers turn their backs, the greedy butterfly larva tucks into the nest’s stock of ant larvae. It supplements its aromatic disguise by imitating the sounds made by the ant queen – a kind of clicking song. This convinces the workers that the butterfly larva is a high-ranking ant and so none of them intervene while it runs amok in the nursery.

By the end, the butterfly larva has more or less polished off the entire colony. Peace returns to the area around the oregano plant’s roots and the larva can pupate. If it isn’t reared in the right kind of ants’ nest, the butterfly has no chance of producing future generations. Who’d have thought so much drama lay behind that scattering of green herbs on your pizza?

Seeds Play a Crappy Trick on Dung Beetles

In the case of oregano, both the plant and the butterfly benefited from the collaboration, but sometimes one of the parties has the upper hand and ‘tricks’ the other. Like when the red-tailed robber bumblebee Bombus wurflenii can’t be bothered to crawl past the stamen buried in Northern Wolfsbane flowers to get to the nectar. Instead, it takes a shortcut, simply biting through the flower head and helping itself to the good stuff, having done nothing whatsoever to earn it. Because that way, there won’t be any pollination.

Other times, the plant draws the long straw, as in the case of the reed-like Ceratocaryum argenteum, which only grows in South Africa. Cleverly enough, it produces seeds that look like dung: big, round dark-brown lumps identical in appearance to the leavings of the local antelopes.

Just as some clothing chains pre-perfume the garments they sell, this plant ensures that its ‘sales products’ – the seeds – have an attractive scent too: the scent of dung. Because it is targeting a very particular group of customers.

Normally, it’s daft for seeds to have a strong smell since that makes it easier for hungry little seed-eating creatures to find and eat them. The explanation for this mystery came as a surprise. It was discovered by a group of scientists from Cape Town University who were actually supposed to be researching whether small rodents ate these strange heavy seeds. They set out nearly 200 Ceratocaryum seeds in a nature reserve in South Africa, sort of like free samples. And, as in the human world, the whole thing had to be documented photographically, of course, so motion-sensitive cameras were set up beside all the seeds.

It turned out that the seeds were removed not by rodents in search of food but by gullible dung beetles who fell hook, line and sinker for the seeds’ advertising offensive. These beetles believed the smelly balls were antelope dung of the type they bury and then lay their eggs in.

Incidentally, dung beetles perform an extremely important service to the ecosystem by burying genuine animal muck, as this prevents pastureland from drowning in dung, and ensures that the nutrients return to the soil (see here and here). In this case, though, the beetles were tricked: they trustingly trundled away the spherical dung-like seeds and buried them a couple of centimetres beneath the earth. At least a quarter of the seeds were sown in a new location – job done!

And what did the dung beetles get for their labour? Nothing. The scientists hid in the bushes and dug up the seeds as soon as the mother beetle had shuffled off. They found no sign of eggs and no trace of any attempt to eat the seed either. Apparently, the beetles ultimately discovered that they had been tricked and gave the whole thing up as a bad job. If beetles were capable of blushing, perhaps we would have seen the beetle mother’s cheeks burn red when her naivety was exposed live on camera. Imagine being taken in by a reed – a pretty mean trick!

Seeds That Serve Up a Packed Lunch for Ants

There are plenty of other plants that get insects, mostly ants, to disperse their seeds for them in return for a reward. We know of this in more than 11,000 different plants, or almost 5 per cent of all plant species. It is common for the plant to ensure that there is some sort of payment in the form of a valuable supplement: a packed lunch for the ant. The ant carries the whole thing home to the anthill, but while the packed lunch is being served up to the hungry ant babies, the seed is thrown away, generally beneath the earth in or near the anthill. Some of the seeds are also lost in transit.

In the United Kingdom, too, ants help many plants, including common cow wheat, violets and wood anemone. One cunning adaptation may be for plants to flower and produce seeds early, before the ants have much else worth eating, as this increases the chance of getting help with transport. The next time you see a common hepatica in spring, take a closer look when the flowers fall off and you’ll see the little white ant packed lunches sitting on each seed.

Other plants have taken the collaboration with ants a step further, not just serving up food but also building the ants a house. Acacias are the classic example: some develop enlarged thorns where the ants can live, and provide them with nutritious food in the form of small packets of oils and proteins. In return, the ants keep peckish herbivores at bay and graze on competing vegetation around the acacias.

Wood Wide Web – Underground Internet for Plants

Collaboration may be the smartest option when insects are on the warpath. And here, plants are helped by an entirely different species: fungi. There’s a lot more to the chanterelle or porcini mushroom than the cap that catches your eye while you’re out mushrooming in the autumn. A large part of these mushrooms is hidden beneath the forest soil, forming the wood’s concealed communication system – a network of fungal threads that links trees and plants, allowing them to communicate. Yes, communicate. We are constantly learning more about this close collaboration between fungi and roots, known as mycorrhiza (literally ‘fungus roots’), which we actually find in 90 per cent of all the plants on Earth.

One thing this relationship does is help plants grow, as the fungus transfers water and plant nutrition from the soil. We have known about that for a long time. But the fungal network can also be used to send messages – about an insect attack, for example. Just like a school sending emails to all parents when the school nurse finds head lice in Class 6b, or the public health institute issuing an internet warning about the onset of this year’s influenza bug, a plant attacked by insects can send chemical signals via the subterranean internet to say: ‘Watch out, here come the aphids again!’

In an ingeniously constructed study, British scientists planted beans and allowed some of the plants to develop fungus roots but stopped others from doing so. Next, they eliminated the possibility of sending signals through the air by wrapping the plants in special bags that prevented signalling molecules from getting through. The next step was to let aphids loose on certain plants. According to the scientists’ findings, plants that didn’t get chewed themselves but did have contact with the plants under attack via the fungal internet developed defensive substances to protect them against the aphid attack; the isolated plants did not.

In forests, trees also use this underground internet – call it the Wood Wide Web, if you will – to send each other carbon. Some scientists think that the oldest and largest trees in the forest, ‘the mother trees’, help the young saplings in the early phase of their lives by sending a kind of food parcel through the network. Even trees of different species may send each other nutrition in this way. Perhaps we need to reconsider the way we think about forests: individual trees may be more closely linked than we had realised.

Working Your Land

Agriculture and animal husbandry are the basis of our modern civilisation. They have permitted high population density, with all the opportunities that entails, but we humans were pitifully late starters compared to insects. Our agricultural revolution only happened 10,000 years ago. By then, ants and termites had already been practising agriculture for 50 million years and ants had been keeping livestock for twice as long. Is it any wonder that ants beat us hands down when it comes to the number of individuals on the planet, and that the combined weight of these minute but multitudinous six-legged creatures matches the weight of all the human beings on Earth?

Insects don’t grow plants – they grow fungi. Specially adapted fungi that only grow in the ants’ fields, in the same way as our crop plants have adapted to a life ‘in captivity’. In South and Central America, leaf-cutter ants are common. Long columns of workers trudge off to cut off suitably sized sections of leaves and bring them back to the nest beneath the earth. The machinery that then takes over is so well oiled that it would exceed the wildest dreams of any industrial magnate. A long column of ants, all of slightly differing sizes, do exactly what is needed – without demanding longer lunch breaks, better shifts, or an end to zero-hour contracts.

The leaves are chewed up and distributed across the ‘kitchen garden’. Other, smaller ants lick the fresh mass of leaves, thereby transferring fungi from the established parts of the garden. Even smaller ants shuffle carefully around in the garden, removing ‘weeds’, which in this case means bacteria or the wrong kinds of fungus. Once the fungus has grown and spread across the new section of the garden, certain ants work to harvest the nutrient-rich parts of the fungus and send the candy-floss-like food around to all the rest, including the growing generation of ant larvae.

© Carim Nahaboo 2019

As in a well-run factory, this sort of streamlined production requires good access to raw materials. In the course of a year, an average leaf-cutter ant colony clears and maintains 2.7 kilometres of ant paths, radiating out from the colony like the spokes of a bicycle wheel.

The agriculture practised by termites resembles that of the leaf-cutter ants, but in this case the nest is built of earth and wood pulp mixed with spit, and lies partly beneath and partly above the ground. A sophisticated air-conditioning system ensures that the temperature is maintained at optimal levels in the subterranean fungus gardens (see here). And termites don’t bring in green leaves: they carry home sticks, grass and straw. With the aid of their fungus partner, the plant matter is broken down and converted into more digestible termite food. The two parties, termite and fungus, are dependent on one another.

Certain bark beetles that live in wood also rely on fungus. This enables them to convert cellulose into edible material. These ambrosia beetles, as they’re called, pretty much take a dinner-in-a-box kit with them when they move into a newly dead tree: they have special cavities in their body (mycangia), where they store a certain type of fungus. Once installed in their new accommodation, a dying or newly dead tree, they’re not content just to lay eggs in a crack; no, they excavate splendid chambers and corridors beneath the bark and there they plant the fungus that will be grown as in a kind of allotment to provide healthy, nourishing food for the beetle babies. And this is probably necessary, since the beetles’ family life is not quite like our own. After the beetle mum lays her eggs, she does a runner, leaving the kids to butter their own bread, so it’s a good thing she has at least taken the trouble to fill the pantry before leaving!

We don’t know how ants and termites manage to maintain high, stable production, even in such an extreme monoculture where they cultivate only a single species. It would be good news for our own future food production if we could wheedle this secret out of the insects.

Aphids as Dairy Cattle

Ants’ animal husbandry is no less impressive. As described earlier (see here), aphids produce large amounts of sweet surplus liquid and some ant species provide bodyguard services in return for this substance. For the ants, easy access to carbohydrates is so attractive that they happily, and aggressively, defend their herd of ‘sugar cows’ against anything that might even think of eating them. An ant colony can easily harvest 10–15 kilograms of sugar from aphids over a summer; some estimates go as high as 100 kilograms of sugar per anthill per year.

Ants have also been found to ‘herd’ their cattle by restricting the aphids’ capacity to disperse to other plants. Just as we humans clip the wings of geese and other winged livestock, ants may bite the wings off aphids. They can also use signalling chemicals to prevent the development of winged individuals or limit how far the aphids stray on foot.

The ants’ nurturing of these sap-sucking insects can be a disadvantage for the host plant – hardly surprising, perhaps, since aphids and their relatives hoover up huge amounts of plant sap. Some American scientists found proof of this when they were actually supposed to be studying the co-dependent relationship between ants and tiny cicada-like insects called treehoppers on the yellow rabbitbrush shrub out in the field in Colorado. To their irritation, black bears kept turning up and destroying the ant nests in some of the areas they planned to study (taking a chunk out of the field equipment along the way).

In the end, the scientists decided to shift their focus and look at how the bear influenced the system instead. They then discovered that the plants grew better where the bear was present, owing to a sophisticated domino effect. When the bear ate the ants, there were fewer ants to frighten off the ladybirds. That meant there were more ladybirds. Since these were now left to eat in peace, they helped themselves to the treehoppers and other herbivores. As a result, there were fewer bothersome insects on the plant, which therefore grew better. That is how the presence of bears can improve plant growth – by keeping the ants in check!

Small Creatures, Great Significance

Connections don’t always work the way we think they do. One example of this stems from the wheat fields in the arid parts of Australia. In this case scientists wanted to study the positive contribution of insects, especially ants and termites, so they compared the wheat harvest in fields where these insects had been eliminated by insecticide and fields where ants and termites were allowed to remain.

It turned out that the wheat harvest rose 36 per cent where crops had not been sprayed. Why? In areas as arid as this, there are no earthworms, so ants and termites do the earthworm’s job instead, creating corridors that allow more water to trickle down into the soil. The water content was twice as high in the soil where these insects were allowed to live as in the soil where they had been eliminated. In addition, the nitrogen content was much higher. This may be because termites’ guts contain bacteria that capture nitrogen from the air. And as if it wasn’t already enough that the insects improved the soil’s supply of water and nutrients – seed-eating ants also ensured that there were only half as many weeds in the unsprayed fields as in the sprayed fields.

But there’s no need to travel halfway across the planet to see the significance of ants. We can find other examples in Europe, right outside our own back door. A Swedish study of coniferous forests shows how tiny ants might affect massive issues, like the climate, by influencing carbon storage in forests.

Take a walk in any woodland around your way and find yourself an anthill. This is the home of wood ants – hill-building ants of the Formica genus. In an experiment conducted in northern Sweden, scientists excluded these ants from small areas of the forest floor with major consequences.

First, the entire community changed. The four most common herbs became even more common. This increased the supply of nutrients to the soil in the forest because woodland herbs like cow wheat and Linnaea borealis, or twin flower decompose more easily than woody berry shrubs. The increase in nutrients was like putting a rocket under the tiny caretakers of the woodland soil. Most notably, the activity of various bacteria increased. This also led to the decomposition of old and seasoned remnants of dead plants.

So, what was the net result of keeping wood ants at bay? Well, because the changes in the decomposition community meant that old, stored carbon was now suddenly being broken down, the scientists observed an overall 15 per cent decline in the forest soil’s storage of carbon and nitrogen. If this result holds true when scaled up, it means that in the absence of ants, a substantial proportion of the large carbon stock in boreal forest soil would be lost. Bearing in mind that northern forests cover 11 per cent of the Earth’s surface and store more carbon than any other type of woodland, it is clear that, despite their modest size, the wood ants have a major influence on fundamental processes such as the circulation of nutrients and carbon storage.

A Troublesome Cactus

We humans have long exploited the close relationship between insects and plants, and between predatory and herbivorous insects. Ancient Chinese documents, apparently from around the year 300 BCE, tell farmers how to move the papery nests of a certain ant into the lemon groves to reduce the pests on the lemons. It was also common to set up bamboo ‘suspension bridges’ between the trees to make it easier for the ants to move from tree to tree and keep away the pests. This appears to be one of the first examples of what we call biological control – the use of living organisms in the battle against pests, as an alternative to using chemicals.

We have moved species from one end of the planet to another, often quite intentionally, and with extremely variable results. Sometimes, things have gone terribly wrong. Like in Australia in the 1800s, when somebody had the bright idea of setting up production of cochineal dye (see here) and optimistically imported a few shiploads of prickly pears from Mexico. The cochineal production went down the drain but the prickly pears spread like wildfire. By 1900, the cactuses covered an area the size of Denmark. Just 20 years later, the area was six times larger. A region the size of Great Britain was totally unusable for grazing or crop growing because it was overgrown by spiky cactuses. It was a crisis. The authorities offered a generous reward to anybody who could come up with a way of combating the prickly pears – the reward was never claimed.

In the end, one World War and much desperation later, the solution arrived – in the form of an insect from South America. An anonymous moth-like Lepidopteran of the Cactoblastis genus, whose larvae chew passageways through prickly pears, was brought in, tested and bred in massive numbers. A hundred men in seven lorries drove around the whole of Queensland and New South Wales, handing out paper quills filled with Cactoblastis eggs to the landowners. In the five years from 1926 to 1931, more than two billion eggs were distributed.

It was a spectacular success. By as early as 1932, the moth larvae had killed off the cactuses in large parts of the fallow land. This is still one of the prime examples of successful biological control.

But there’s always another side to the coin. After the success in Australia, the moth was used to biologically control cactuses in several other places, including the Caribbean islands. From there, Cactoblastis moths have spread to Florida, where they now threaten to wipe out unique local cactuses.