APPENDIX E: PEST & DISEASE

Revised by Saul Alba

Pests and diseases are a fact of life in agriculture. Between the 1950s and the 1990s the go-to solution for pest and disease management was almost exclusively eradication with pesticides. In 1942 Rodale started the magazine Organic Farming and Gardening to promote chemical-free practices. Then in 1959, Vernon Stern introduced the concept of integrated pest management (IPM) in the landmark paper “The Integration of Chemical and Biological Control of the Spotted Alfalfa Aphid: The Integrated Control Concept.” His ideas are still the basis of modern agricultural pest management, whether conventional or organic.

What is now commonly known as the organic agriculture movement arose from the desire to protect the consumer, the farm worker, and the environment. Farmers began to embrace the use of “soft” pesticides as well as other practices that reduced the reliance on pesticide applications to their crops. To a great extent this started as a search for alternatives to banned or regulated chemicals.

Using IPM for cannabis crops is especially important. As more states legalize cannabis, the idea that a product that is primarily consumed through inhalation should be clean of toxic residues guides much of the legislation. Presently, the use of all cannabis crop protection materials is tightly regulated. Many of the pesticides and fungicides that informal market cannabis growers use have been outlawed for legal cannabis production. This has resulted in an increased interest in true integrated pest management.

IPM is an approach to pest management in which the grower relies on several control strategies. As many strategies as can work together are employed, hence the term “integrated.” The different approaches can be broken down into four principles of IPM: resistance, exclusion, disruption of the pest life cycle, and eradication. These are not mutually exclusive principles, and strategies often fit into more than one category.

The Four Principles of IPM

1. Resistance

When the grower selects a cultivar that is particularly resistant to a pest or disease, there is a greater chance of protecting the crop. Some cultivars are more resistant than others to fungal attack. Some terpene profiles are more attractive and others more repellent to certain arthropod pests. Flower architecture affects the crop too. Large, tight buds are less resistant to bud rot. When the grower chooses cultivars that are less susceptible to infestation and/or infection, pest and disease management is easier and cheaper.

2. Exclusion

There are many techniques used to prevent plant pests from interfering with the plants no matter where they are being grown.

Outdoors:

• Place netting over plants to keep out many small insects, including moths that lay eggs.
• Use high netting around the garden to prevent infiltration by some wind-borne pests.
• Create an exclusionary zone around the planting space consisting of an unplanted area or planted with pest repellent plants such as sages and mints.
• Put up high fences to protect against herbivores and prowlers.

Greenhouses:

• Screen and filter all incoming air.
• Use exclusionary spaces.
• Dust control around greenhouses prevents the entry of airborne mites, other small invertebrates, and pathogens.
• Change from street clothes to greenhouse work clothes, including shoes.
• Use hair and beard nets.

Indoors:

• Create exclusionary space around building.
• Change from street clothes to work clothes.
• Use hair and beard nets.
• Screen any air being exchanged.

3. Treating and Quarantining New Plants

Quarantine all plants entering the space in a separate area. They should be kept in quarantine for about 15 days. Any infection should be apparent during that time. Plant quarantine can prevent years of suffering from pests and diseases. Thoroughly inspect all plants when they enter quarantine and throughout the period. That should be the last thing done that day.

Sanitation practices can be thought of as exclusion. Built-in air curtains blow pests or spores off anyone who enters the space. Provide clean apparel for workers to change into. Wet sanitation mats, disposable gloves, beard and hair nets, and hand sanitizers help keep disease out of the garden.

How to Quarantine:

1. Isolate the new plants in a sanitized quarantine space

   “Social distancing” is important for plants when they are suspected of being carriers of pest and disease. An infected or infested plant cannot spread disease to other plants if they are properly isolated.

2. Inspect new clones closely

   Look for the presence of pests, their eggs, or pest damage on the plants. Use a magnifying lens or jeweler’s loupe to identify the small pests.

3. Mechanically remove pests and treat with IPM

   Either spray or dip plants in a solution to treat against potential pests. Spraying with or dipping in an insecticidal soap solution and herbal pesticide is very effective in killing soft-bodied arthropods like spider mites, aphids, thrips, and so forth. A root dip with an entomopathogenic fungal spore solution such as Beauveria bassiana will inoculate the growing medium to mitigate root pests like root aphids but not mites. Entomopathogenic fungi infect insects and arachnids and parasitize them. The pests eventually die from the infection, and the fungus will release more spores from the pest’s carcass.

4. Install sticky traps in the quarantine area

   Both yellow and blue sticky traps will show pest presence. The yellow traps show the presence of winged insects, such as winged aphids, whiteflies, and fungus gnats. Blue sticky traps attract the leaf miners and thrips.

5. Sanitize the quarantine area

   Fungal spores, zoospores, bacteria, and so on should be neutralized so that disease does not spread. Hydrogen peroxide (H₂O₂), isopropyl alcohol, bleach, hypochlorous acid, and other disinfectant products like Zerotol or OxiPhos work well as antimicrobial sprays.

6. Sanitize tools, shoes, clothes, hands

   Spores are all around: in the air, on tabletops, and on the ground. It is important to also sanitize anything that goes from the quarantine area into any area where plants are already growing. Spores can spread from clothing or hands if non-quarantined plants are handled in unsanitary conditions.

7. Install air cleaners, including a negative ion generator and UVC light air cleaners
8. Isolate new plants for as long as needed

   Typically, the new cuttings will stay in quarantine for a few days; however, if a robust and active IPM program throughout the vegetative and flowering stages is followed, the quarantine time can be abbreviated.

4. Disruption of Life Cycle

This is somewhat of a catch-all principle because it borrows strategies from the others. However, it is important to understand that all pests and diseases have a life cycle and disrupting particular stages can help control them. The life cycle stages are categorized as (1) birth or germination, (2) feeding or infection, (3) reproduction or proliferation, and (4) dissemination. Growers should become intimately familiar with the different life stages of the pest and consider ways that they can be interfered with. Approaching IPM strategy from this angle often reveals options that had not been considered.

Birth or germination is when the pest or disease starts operating. In the case of invertebrates it can be hatching or live birth, and with diseases it’s germination of spores or other propagules. Numerous strategies can be used to disrupt birth or germination.

It’s important to consider how much environmental conditions affect the life cycle stages of pests and diseases. Many fungal disease spores germinate only when exposed to particular temperatures and humidity levels. In fact, the best management strategy for the most common fungal pathogens found on cannabis is climate control. Other methods are minor tools.

Feeding or infection can be considered the first real attack by the pest or disease on the plant. After a pest is hatched or a spore germinates, penetrating the plant tissue is the life stage that follows. How can the grower disrupt feeding or germination? Azadirachtin is a neem tree extract that is known to repel insect feeding and also to reduce their appetite. Numerous herbal extracts have similar properties. Some spores need wounds in plant tissue in order to infect the plant, so minimizing wounding or treating wounds can be effective. Some plant biostimulants can elicit a resistance response that cause thickening of cell walls, thereby making feeding and infection more difficult.

Eradication

Once the grower has taken into consideration all the previous principles of IPM and developed a strategy, then it is time to think about eradication of the pest or disease. Too often growers devote excessive resources to spray materials, spray equipment, and labor, forgetting that the integrated approach can help their applications perform better. This overreliance on sprays is why the concept of IPM took hold. In fact, most farmers who practice sustainable agriculture look at sprays as a last resort for controlling a problem.

ARBICO Organics Pest Traps work as a visual attractant for a wide range of crawling and flying insect pests drawn in by the bright color. Once they are trapped, the gridded surface makes it easy to identify and count how many have landed and been captured on the trap. Adult pests are caught there and the reproductive cycle is interrupted.

One reason for rethinking the spray-first approach is that biological control, while a very effective eradication strategy, is difficult to integrate with spray programs. Monocropping disrupts natural ecology, giving an advantage to plant pests and diseases. The statement “Nature abhors a vacuum,” attributed to Aristotle, is an appropriate explanation.

Crops are artificial ecosystems that don’t contain the checks and balances of a true ecosystem. It’s conducive to the growth of plants, but also to herbivorous organisms that feed on the crop. The farmer has created a vacuum where these herbivores’ natural enemies aren’t present.

Most pests can reproduce rapidly and fill the vacuum, resulting in a population explosion. Even if natural enemies arrive, the pest and natural enemy populations are so out of balance that the predator cannot keep up with its prey’s reproductive rate.

Biological control rebalances the prey-predator relationship by increasing the population of the herbivore’s natural controls. This is done by introducing commercially reared natural enemies indoors and creating habitats that attract natural enemies from the wild in outdoor gardens.

The true goal of IPM is an effort to reduce or eliminate the need for chemical applications to control pests. Sprays and drenches can be thought of as the last-ditch effort of pest management. Sprays are sometimes likely to be used in some circumstances, but an integrated approach seeks to reduce their use and impact on the farm ecosystem and the plants themselves. In some jurisdictions no sprays of any kind are permitted. Farmers in these areas must rely on other sources of protection.

Monitoring

The foundation of modern IPM is monitoring to answer the following questions:

• What is out there?
• Where is it?
• How much of it is there?
• What stage of development is it in?

Decision-making about control options is handicapped until these questions are answered. The larger the pest infestation, the harder it is to control. It is easier to deal with a small population or infection than a large one. Monitoring helps the farmer find sparks or smoldering embers and extinguishes them before they become raging wildfires. It also helps the grower rein in pest management costs by identifying pest or infection “hot spots” so that more resources can be allocated to those areas.

Pests and infections can be identified by their signs and symptoms.

Signs are the pests themselves or something they leave on the plant that can be used to identify them. Eggs, mycelia, excrement, shed exoskeletons, and webbing are all evidence pests leave behind.

Symptoms are the effects observed on a plant that are caused by an infection or pest attack. Common symptoms can be feeding damage, yellowing, leaf deformation and wilt, weird growth, rot, stem damage, unhealthy color, and lack of vitality. It is of utmost importance that the grower is intimately familiar with both signs and symptoms of the infections and pests that can injure the crop.

A very useful technique for monitoring pests is “beat sheet” scouting, which involves using a white sheet and beating on the foliage to knock pests off the plant and onto the sheet, where they can be seen. Pheromone lures are available for some pests. Indicator plants that preferentially attract pests can be used as an early detection tool.

Prevention

Pests are among the most annoying and difficult problems facing gardeners. The best way to deal with them is to prevent them from entering. No matter the growing method, pests will infect the garden if they are given a chance. Here are some ways of preventing them from getting in.

Pets: Pests enter using pets as transportation. Don’t let pets in the garden. Their fur and dander can also accumulate on sticky buds.

Clothing: Clothing worn outdoors can carry in pests. Even a single pest can be a vector for a fast-growing colony. After going outside, wash and dress in freshly laundered clothes.

Plant Quarantine: Before letting a plant enter a pest-free garden, place it in quarantine and use a pesticide/fungicide spray or dip just to be sure.

Planting Mix: Make sure the planting mix is composed of inert or pasteurized ingredients. Planting mix that is not inert or pasteurized may contain pests and diseases.

Pest Highways & Airways: Make sure all air coming into the garden space is filtered. Repair any cracks, holes, or other open spaces.

The Common Pests of Cannabis

The following section provides information to help the grower recognize and control pests that commonly affect cannabis. A description of each pest is provided to identify both signs and symptoms, and some effective control options are reviewed.

Invertebrate Pests

Ants herding aphids. Photo: Alexander Wild

Aphids

Aphids are small oblong or pear-shaped soft-bodied insects about one to three millimeters long. There are thousands of species that vary in color from green to yellow, black, or brown. Depending on the species and the stage of life the aphid is in, it may have wings and a wax or “wool” made from their secretions.

A common trait of aphids that distinguishes them from all other insects is the pair of tailpipe-like cornicles that extend from the tail end of their abdomens.

Aphids colonize the stems and undersides of plant leaves. Some species, such as the black bean aphid, are quite noticeable because their color stands out from the plant. Others, such as the green peach aphid or the cannabis aphid, are often colored spring green and blend in with young leaves. The most problematic aphid pest of cannabis by far is the cannabis aphid, which evolved on cannabis and is highly specialized to feed and reproduce exclusively on this plant. Another problematic aphid is the subterranean rice root aphid, which evolved feeding on grass roots but has been able to host on roots of several broadleaf plants, cannabis included.

Aphids are true bugs. They puncture stems, branches, and leaves and suck sap from them using a straw-like mouth, called a stylet. To obtain enough nutrients, aphids must suck a lot of dilute sap, refine it, and excrete the excess concentrated sugar solution, referred to as “honeydew.” It attracts food for ants that herd the suckers, protecting them from predators. Honeydew is a growth medium for sooty mold fungus, which causes necrosis of plant tissue and interferes with photosynthesis.

Heavy aphid infestations cause leaf curl, wilt, and stunted and delayed growth. Aphids are vectors for hundreds of diseases and can quickly cause an epidemic. They transfer viruses, bacteria, and fungi from plant to plant.

During warm weather, each generation takes only a few days to complete a life cycle, and each aphid can produce numerous offspring, depending on the species. This high reproduction rate makes aphids seem to appear overnight. Indoors, with no predators to keep them in check, they can overrun a garden very quickly.

The Cannabis Aphid

It’s believed that the cannabis aphid evolved along with the cannabis plant. No other plant host is known. This makes it a formidable pest of cannabis, as the plant meets all its predator’s nutritional needs, allowing it to reproduce incredibly fast.

The cannabis aphid has a similar appearance to other green aphids, so the grower must pay attention to its distinguishing behaviors and features. It colonizes the underside of leaves but also infests branches. A branch infestation resembles a freeway traffic jam. Other aphids that feed on cannabis tend not to feed on branch tissues.

It is usually almost white during early nymph stages and light green at older stages and adulthood, but it has a broad color range, including brown. Some populations have three green stripes running lengthwise along the dorsal surface of the abdomen. These stripes are more prominent in the darker-colored forms. The winged form is generally a greenish yellow with a dark spot on its back. The main distinguishing features can only be seen under high magnification. They are two horn-like projections between the antennae. The hairs associated with these projections and the antennae have a bulbous tip.

The Rice Root Aphid

The rice root aphid is a pest of cereal crops and some broadleaf crops such as celery and cannabis. Like most aphids, it has a broad range of colorations. Most are green with an orange tail end. Many are completely orange, but their tail ends tend to be a darker orange. These aphids usually feed on cannabis roots but do occasionally infest and feed on leaves.

Monitoring

• Signs: check for colonies, which tend to be most prevalent along the upwind edge of the garden, close to outside sources of aphids; check leaf undersides, where many species hang out. Root aphid colonies are easily identifiable feeding in clusters on roots.
• Signs: look for winged forms.
• Signs: look for shed exoskeletons. The shed exoskeletons are white and for foliar aphids can be seen on leaves under the infestation. Root aphids are found near colonies.
• Signs: look for honeydew. Often exoskeletons stick to the honeydew.
• Signs: look for sooty mold.
• Signs: look for ant presence.
• Symptom: leaf curling.

Resistant Varieties

The industry is still too young for aphid-resistant varieties to be recognized. There are anecdotal observations that some varieties are more susceptible to infestations.

Exclusionary Practices

• Use filtration and screening. Generations of some species are airborne for part of their life cycle, so a fine filter in the air intake prevents aphid entry.
• Hang air curtains at entries to the garden.
• Weed along the perimeter of the garden.
• Use quarantine procedures for new plants.
• Use of repellents such as herbal insecticides, garlic oils, or other herbal extracts.

Disruption of Life Cycle

• Use molting disruptors such as azadirachtin.

Eradication

• Biological controls: aphid parasitoid wasps (A. colemani, A. ervi) are extremely effective. They lay eggs in the developing aphid and emerge “Alien” style. Other good options: the aphid predatory midge (A. aphidimyza), generalist insect predators (lacewing larva, Orius insidiosus), lady bird beetles, sprays (foliar aphids) and drenches (root aphids) of entomopathogenic fungi (B. bassiana, I. fumosorosea).
• Sprays: herbal/botanical oils (rosemary, thyme, garlic, peppermint, neem, capsaicin, cinnamon oil, cloves, coriander oil, garlic), insecticidal soaps, pyrethrum.

Spider Mites

Spider mites are very common and are among the most serious pests in the cannabis garden.

Spider mites are barely visible to the naked eye, since they are only 0.02 inch (0.4 mm) long. They are arachnids (relatives of spiders), and like other arachnids they have four pairs of legs and no antennae. Their color range includes red, brown, black, yellow, and green, depending on their diet, species, and the time of year. Spider mites are so tiny that most of these details are visible only with magnification.

Spider mites leave behind telltale webbing on the colas. Photo: Ed Rosenthal

Spider mites live on the plants, mostly on the underside of the leaves, but are also found around the buds. They can be found moving from leaf to leaf and plant to plant using their silvery webbing as highways.

Spider mites are sap-feeders, like many other garden pests. They are a major threat due to their high rate of reproduction. Spider mites pierce the surface of leaves and then suck plant juices from them. These punctures appear on the leaves as tiny yellow spots. As the population grows, the mites produce webbing that they use as a pedestrian bridge between branches or plants.

Spider mites multiply quickly. They are most active in warmer climates than cold ones.

Reproduction Rate & Life Cycle

Newly hatched mites have a 3:1 ratio of females to males, and each female lays up to 200 eggs. This life cycle can repeat as often as every eight days in warm, dry conditions such as an indoor garden. This means that a spider mite population can explode with shocking speed, and this rapid reproduction is what makes them so troublesome.

Monitoring

• Signs: colonies
• Signs: eggs
• Signs: shed exoskeletons
• Signs: webbing
• Symptoms: stippling (puncture) damage

Exclusionary Practices

• Control dust along the perimeter of the garden.
• Use fine filtration of air inlets.
• Use air curtains.
• Weed along the perimeter of the garden.
• Quarantine new plants. Almost all spider mite infestations enter the garden on an infested plant.

Disruption of Life Cycle

• Control relative humidity. Spider mites thrive in dry climates. High humidity slows spider mite development and reproduction. This can be used in vegetative and early flowering stages to slightly slow population increase.

Eradication

• Biological controls: predatory mites (P. persimilis, N. californicus, A. andersonii, N. fallacis, G. occidentalis), the spider mite predatory midge Feltiella acarisuga, the spider mite ladybird beetle predator Stethorus punctillum, generalist insect predators (lacewing larva, Orius insidiosus).
• Sprays: herbal/botanical oils (cinnamon, clove, lemon grass, rosemary, thyme, garlic, peppermint), insecticidal soaps, miticidal sulfur sprays.

ARBICO Organics Leafhopper Assassin Bugs (Zelus renardii) are excellent general predators in gardens and greenhouses because they remain predatory in all life stages. Their rapid feeding habits and tendency to kill for the sake of killing make them ideal for continued control of various pest insect populations including leafhoppers, aphids, mealybugs, thrips, small caterpillars, and more. Assassin Bugs also have the ability to feed on larger prey than many of their beneficial insect counterparts. Their wide range of food sources and high reproduction rate allow Assassin Bugs to maintain and even grow their population without high pest numbers. They are active in a wide range of temperatures and have a two-month lifespan, providing continuous control.

Release fast-feeding Phytoseiulus persimilis at first sign of spider mite infestation. P. persimilis are red relative to the pest mites they feed on and are best used to combat active mite populations. P. persimilis will leave the area in search of alternative food sources if pest levels drop too low. P. persimilis consume egg, nymph, and adult stages of pest mites.

Hemp Russet Mites (HRM)

Hemp russet mites are very small in comparison with other cannabis pests, and an individual is nearly impossible to see without magnification. As a size comparison, the length of their bodies is roughly the diameter of a spider mite egg. The body shape of HRM is very different from most other mites in that it is elongated and almost worm-like, with their legs near the head at the thick end of the body. They resemble tiny carrots.

Their only known host is cannabis; hence, HRM can be a devastating pest to the crop that can completely destroy flowers. It is an insidious pest in that often the symptoms of an infestation are only visible after the population is enormous. These sap-feeding mites cause a leaf deformation typically called “canoeing” because the leaves curl downward and resemble a canoe. This symptom is not always indicative of HRM feeding. Another symptom of HRM infestation is a “rust” discoloration of branches and young flowers. Finding this symptom usually takes a trained eye.

The primary strategy for control is exclusion of infested plants. Most russet mite infestations were inherited by less-than-dedicated growers who did not quarantine and scout plants that were brought in from other gardens. The mites are also suspected of using wind currents to get from one plant to another.

Once in a garden they are nearly impossible to eradicate. The go-to strategy is the use of elemental sulfur sprays, but these are risky to consumers and also when used shortly after applying oil sprays because it dramatically increases the risk of phytotoxicity.

Herbal sprays such as Ed Rosenthal’s Zero Tolerance control the mites when used regularly.

All mite sprays should be applied using a fogger so that they coat the undersides of the leaves, where the mites hang out.

There are anecdotal reports that commercially available natural enemies can control an HRM infestation. One promising exception would be the Western Predatory Mite, Galendromus occidentalis, which controls russet mites of other crops. One thing that the grower needs to consider before attempting to use predatory mites for HRM control is that they will likely feed on their preferred prey if it is present and ignore HRM.

Ed Rosenthal’s Zero Tolerance Pesticide has been formulated from food-grade cinnamon, clove, rosemary, and thyme oils to create a powerful tool to kill and repel plant pests and pathogens. It has been designed for repeated use.

Monitoring

• Signs: colonies.
• Symptoms: leaf deformation (“canoeing”).
• Symptoms: stem discoloration (“rust-like”).
• Symptoms: flower discoloration (“rust-like”).

Resistant Varieties

• The industry is still too young for HRM-resistant varieties to be recognized; however, there have been observations of preferences for plants with certain terpene profiles.

Exclusionary Practices

• Use air curtains.
• Weed out “volunteer” hemp plants along the perimeter of the grow.
• Employ quarantine procedures for new plants.
• Enforce worker hygiene protocols.

Eradication

• Potential for biological control of early infestations by Galendromus, Amblyseius, and Neoseiulus predatory mites (G. occidentalis N. californicus, A. andersonii, N. fallacis).
• Sprays: elemental sulfur sprays and clone dips, insecticidal soaps, and paraffinic oils.

Broad Mites

Broad mites are a microscopic mite pest of cannabis. Unlike hemp russet mites, broad mites are pests of many other crops too. The body shape of broad mites is similar to that of other mites, but there is a sizable gap between its two front legs, which tend to point forward, and its two hind legs, which tend to be oriented to the back. Unlike hemp russet mites, they are very mobile.

One other sign of broad mite infestation is the presence of their eggs on leaf surfaces. These eggs are tiny, oblong, and translucent, and have what appear to be a dimpled surface. They are very distinct, and scouts should know what to look for.

Symptoms of broad mite infestation can be similar to those of hemp russet mites. Leaf deformation from feeding tends to be more “blister-like,” and there is often what appears to be more of a sheen to the leaves. Leaves often lose their sheen and tend to curl up, so “canoes” or downward-curling leaves are more likely to indicate russet mites.

As with hemp russet mites, the primary strategy for control is exclusion of infested plants and worker hygiene. Since they are very mobile, they can be introduced on clothing and have even been reported to hitchhike on whiteflies.

Herbal oil and sulfur sprays can also be used for broad mite control. Two commercially available predatory mites, Galendromus, Amblyseius and Neoseiulus mites successfully control them in a number of crops.

Monitoring

• Signs: mobile adults.
• Signs: distinct eggs.
• Symptoms: leaf deformation (“blistering”).
• Symptoms: leaf deformation (upward curl of the leaf edge).

Exclusionary Practices

• Use air curtains.
• Employ quarantine procedures for new plants.
• Enforce worker hygiene protocols.

Eradication

• For biological control of early infestations use Galendromus, Amblyseius, and Neoseiulus predatory mites (G. occidentalis N. californicus, A. andersonii, N. fallacis).
• Sprays: herbal oils, elemental sulfur sprays, and insecticidal soaps.

Caterpillars

In spring and summer, caterpillars are common outdoors, but rare indoors.

Caterpillars are the larval stage of butterflies and moths. They have soft, segmented bodies with a head, thorax, and abdomen. The thorax contains three pairs of jointed legs that have hooks and the abdomen up to five pairs of stumpy, “false” legs. Caterpillars are often the same color as leaves, so they are hard to spot. There are a range of caterpillars that commonly infest cannabis.

Photo: Ed Rosenthal

Corn Earworm

Adults are gray to brown with a wingspan of about 35 mm (1.3 inches). Coloration of the larvae varies from green to brown, and they have a dark stripe that runs the length of their backs as well as along both sides. There is typically a lighter coloration of the underbelly. Eggs can be laid singly or in clusters.

Cutworms

The adults are gray to dark brown moths with wingspans of 1.25-1.75 inches (3-4.5 cm). The caterpillars grow to 1-1.5 inches (2.5 to 3.75 cm) long. Colors include brown, green, gray, and black. Eggs vary widely by species but are usually laid on the stems or the upper sides of leaves in clusters. The caterpillars hide in the soil and leaf litter and feed on the aboveground parts of the plant at night.

Cabbage Worms

The most common cabbage worm is the Imported Cabbage Worm. The adult moths are off-white with one or two black spots on each wing. They have a wingspan of about 1.5-2 inches (3.75-5 cm). The caterpillars are green and may grow up to 1.5 inches (3.75 cm) long. Eggs are ridged and dome-shaped and usually laid singly on the undersides of leaves.

Corn Borers

The adults are yellow or tan-colored nocturnal moths with wingspans of about an inch (2.5 cm). The caterpillars are about one inch long, light brown in color with a brown head, and spots on each segment. Eggs are white to pale yellow in color, laid in clusters of 20 to 30 on the undersides of leaves.

Caterpillars eat both leaves and the soft stems. Others bore into the stem and eat the pith, the stem’s soft inner tissue. Cutworms feed at night and spend the day in shallow burrows near the plants. Corn borers attack mature plants: they need a stem large enough to hold their bodies. After the eggs are laid on the leaves, they hatch and the young larvae eat the leaves around the eggs for two weeks to a month, leaving close clusters of tiny holes. To catch borers early, look for these small holes. Later in the season look for small holes in the plant stalks, possibly covered with thin silky webbing. After borers have been at work for a while, they sometimes cause the stalk to develop “fusiform galls,” or bulges in the plants’ stalks that widen in the middle and taper at both ends. The borers may leave visible trails on the stalks leading to the galls.

The branches and leaves above the caterpillar’s feeding damage wilt, since they receive no water or nutrients. If it is a main stem, the whole plant dies. If it is a side stem, only that branch succumbs. In addition to the direct damage they cause, caterpillars leave behind damaged tissues that are vulnerable to infection as well as feces that can serve as inoculation points for numerous microbes.

Caterpillars are voracious eating machines and can savage plants very quickly. They chew continuously to support their high growth rate. They can destroy a tray of seedlings overnight.

Leaf-eaters leave large holes as calling cards in the leaves they dine on. Corn earworms, cabbage worms, and other caterpillars also infest buds. A bud that turns brown and wilts “for no reason” may house a caterpillar consuming it from within.

Cutworms are perhaps the most obvious of all caterpillars: plants damaged by cutworms are literally chewed through at the soil line, causing the plant to topple. Seedlings and young plants are completely consumed.

Moths can produce many generations per year in warm climates. Each female lays several hundred eggs. The adults mate in spring to early summer, and the caterpillars emerge in the early summer to fall. The caterpillars feed until they are ready to enter the pupal stage. They then spin cocoons or dig burrows and continue their development until they emerge as adult moths. The generation that emerges in late summer and fall often overwinter as caterpillars, emerging in early spring to begin feeding again. This is especially common with cutworm species.

Caterpillars reproduce slowly compared with many pests, but they have large appetites, and each one can cause a lot of damage.

Monitoring

• Signs: adults fly around lights at night or are caught on pheromone traps.
• Signs: eggs.
• Signs: larva (caterpillars).
• Signs: leaves that are rolled with silk (some species).
• Signs: pupae.
• Signs: silky webbing.
• Signs: feces.
• Symptoms: chewing damage.
• Symptoms: “Fusiform galls” from corn borer feeding.
• Symptoms: dried-up buds.

Exclusionary Practices

• Use light traps outside the garden.
• Screen air inlets.
• Use pheromone mating disruption of some species.
• Use row covers to prevent egg laying.
• Weed alternate hosts.
• Protect young plants by growing vegetatively indoors.
• Use insect trapping glue applied as cutworm barriers on the stems of seedlings and young plants.

Eradication

• Biological control: BT Trichogramma wasps, spined soldier bugs, generalist insect predators (lacewing larva, Orius insidiosus), parasitic nematodes (S. feltiae) for subterranean caterpillars, Bacillus thuringiensis kurstaki (Btk) sprays, praying mantis.
• Sprays: herbal/botanical oils (rosemary, thyme, garlic, peppermint, neem), herbal teas (cinnamon), insecticidal soaps, insect neurotoxins (pyrethrins, spinosads), commercially available polyhedral occlusion bodies of numerous caterpillar viruses.
• Hand removal of the larva.

Thrips

Thrips are not commonly considered major pests of cannabis. However, in some indoor and greenhouse conditions they can be serious pests.

Thrips are tiny, no more than 0.06 inch (1.5 mm) long, but can still be seen by the naked eye. Adults have wings but do not fly well; they jump when startled. The head and body range from yellow to dark brown. The larvae are about half the size of adults, lighter in color, and wingless.

Thrips attack the leaves and are seen on both surfaces.

Thrips use a saw-like structure to pierce and scrape the leaf surfaces until sap begins to flow. They then suck up the juices and leave a surface of patchy white or silvery scrapings often called “rasping” damage. The leaf surface looks scarred or scabby. If the infestation is heavy and goes unchecked, the leaves lose much of their chlorophyll and turn white. Thrips leave behind greenish black specks of feces on and under leaves. The scar tissue shows up in silver patches. Thrips damage can resemble that of spider mites or leaf miners at first.

Damaged leaves can’t be healed, and their ability to absorb light is compromised. If the thrips are not controlled, the plants die. Thrips also carry pathogens that they transfer through feeding, although it is yet to be confirmed that any of these pathogens can infect the cannabis plant.

Outdoors, thrips hibernate over the winter in soil and plant debris. Thrips become active when the temperature climbs above 60°F (16°C). The warm, stable temperatures of indoor gardens allow them to be active year-round. Thrips are a more serious problem indoors because of this and also because a natural soil-dwelling fungus that infect thrips pupae may not be present in indoor grows.

Reproduction Rate & Life Cycle

Females lay eggs (anywhere from 40 to 300 depending on species) in plant crevices or insert them into the leaves and stems. The larvae of Western Flower Thrips feed until they enter what is known as a pre-pupal stage, when they fall to the ground, burrow, spend two developmental stages in the soil or leaf litter, and then emerge as reproductive adults.

Depending on the species and temperature (optimum is 77-82°F/26-28°C), the larval thrips hatch, pupate, and mature into egg-laying adults in 7 to 30 days.

Monitoring

• Signs: identify adults and larvae either on the plant by using the “beat scouting” technique or on blue or yellow sticky cards.
• Signs: green to black feces near feeding damage.
• Symptoms: “rasping” feeding damage.

Exclusionary Practices

• Barrier placed between soil and plant canopy.
• Use air curtains.
• Thrips screening.
• Weed along the perimeter of the grow.
• Employ quarantine procedures for new plants.
• Western Flower Thrips pupae live in the soil after they drop from the plant. By placing a barrier around the top of the container, the pupae can’t get to the soil and die. As with fungus gnat larvae, a layer of diatomaceous earth on top of the soil also helps destroy the thrips pre-pupae.

Disruption of Life Cycle

• Molting disruptors such as azadirachtin.

Eradication

• Biological control of early infestations by Amblyseius predatory mites (A. swirskii, A. cucumeris), generalist insect predators (lacewing larva, Orius insidiosus), entomopathogenic fungi (B. bassiana, I. fumosorosea), parasitic nematodes (S. feltiae), soil predatory rove beetle (D. coriaria formerly Atheta), soil predatory mite (S. scimitus formerly Hypoaspis).
• Sprays: herbal/botanical oils (rosemary, thyme, garlic peppermint, neem), herbal teas (capsaicin, cinnamon oil, cloves, coriander oil, garlic), insecticidal soaps, insect neurotoxins (pyrethrins, spinosads).
• Trapping with yellow or blue sticky tape, trap crops such as marigolds.
• Crop vacuuming.

ARBICO Organics offers six species of beneficial nematodes to choose the best options for the indoor garden, greenhouse, row crop, or container. Beneficial nematodes mainly parasitize insect pests that have soil-dwelling larvae or pupae; however, they have been known to parasitize above ground stages of certain pests as well. Their wide range of prey makes them exceptional for general pest control in chemical-free growing environments.

Fungus Gnats

Fungus gnats are common indoors. They are found outdoors occasionally in moist warm areas.

Fungus gnats are a tenth to a twelfth of an inch (3-4 mm), dark grayish black, and have a slender build with delicate long legs and long wings. The larvae are clear to creamy-white with a shiny black head and can be up to a quarter inch (6 mm) long.

Adults fly close to the soil level and through the lower region of the plant canopy. Fungus gnat larvae live at the root level, usually from one to three inches (2.5 to 7.5 cm) below the soil line. In shallow containers the larvae may be found wriggling in the drain tray after watering. Outdoors, adults and larvae live in moist, shady areas.

Fungus gnat larvae weaken the plant by eating roots, root hairs, and organic matter growing in planting mix, rockwool, soil, and other planting mediums. They can also be vectors for disease. Adult gnats do not eat. They live only to reproduce. However, when they develop into large-enough swarms, they can stick to trichomes on flowers.

Adult females lay eggs at the surface of moist soil, near the plant stem. The larvae hatch out in 4 to 10 days, depending on temperature, and feed off algae, fungus, and plant matter (including plant roots), and then pupate in the soil and emerge as adults. The total time from egg to reproductive adult is about four weeks, and females lay several hundred eggs in small batches over their lifetime. Indoors, they breed continuously throughout the year and reproduce very rapidly.

Monitoring

• Signs: adults flying or crawling under the canopy or trapped in yellow sticky cards.
• Signs: larva in the soil or feeding on potato halves. Potato halves can be placed on the soil surface cut-side down and checked periodically. The larva will be found feeding on the potato.

Exclusionary Practices

• Use air curtains.
• Use screening.
• Employ quarantine procedures for new plants.

Disruption of Life Cycle

• Barrier placed between soil and plant canopy. Paper, cloth, or another barrier placed over the soil prevents larvae from completing their life cycle, part of which is spent in the soil.
• Control irrigation so that there is no overwatering. Algae attracts adults that grow in areas of standing water and pot surfaces.
• A layer of diatomaceous earth can prevent the fungus gnat adult from laying eggs in the soil. If algae develops on the surface of the diatomaceous earth, then its effectiveness diminishes.

Eradication

• Biological control with parasitic nematodes (S. feltiae), soil predatory rove beetle (D. coriaria formerly Atheta), soil predatory mite (S. scimitus formerly Hypoaspis), Bacillus thuringiensis israelensis (Bti) drenches.
• A drench with hydrogen peroxide / peracetic acid solutions can kill fungus gnat larvae.
• Sprays: herbal/botanical oils (cinnamon, rosemary, thyme, garlic, peppermint, neem), insecticidal soaps, insect neurotoxins (pyrethrins).
• Trapping: putting yellow sticky tape under the canopy lowers population levels.

Whiteflies

Whiteflies are common pests indoors and outdoors. Whiteflies resemble tiny moths but are neither moths nor true flies. They are relatives of aphids and scales. They are 0.04 inch (1 mm) long, and their soft bodies are covered in a powdery wax, which gives them protection.

Photo: Nature’s Control

Whiteflies infest the undersides of leaves. If the plant is disturbed, they take flight and are seen fluttering around the plant.

They are true bugs, using a modified mouth shaped like a straw to puncture plants and suck sap. They are vectors for viruses. Like aphids, whiteflies exude sticky honeydew, and this can contribute to mold problems on the plants. Leaves appear spotty, droop, and lose vigor.

Whiteflies quickly build up a large population and suck the life out of the plants, but they are not difficult to get rid of. Plants are easily checked for whiteflies by shaking them. If they are present, they’ll fly off and then settle right back onto the leaves.

Reproduction Rate & Life Cycle

Females each lay about 100 tiny eggs on the underside of leaves. Eggs hatch in about 7 to 10 days, and the larvae drain sap from leaves. Larvae mature in two to four weeks, and the adults live for four to six weeks after that. Like all arthropods, their reproductive rate is temperature dependent: most whitefly species do best in a temperature range of 80 to 90°F (27-33°C).

Monitoring

• Signs: adults flying or feeding under leaves or trapped in yellow sticky cards. The glue from yellow sticky cards removes their waxy covering so they appear clear.
• Signs: waxy exudates left on the leaves where there was a colony.
• Signs: eggs laid in a circular pattern.
• Signs: nymphs that look like miniature trilobites, usually with red eyes.
• Signs: pupae that look like hockey pucks. Some species have long hair protruding from them.
• Signs: translucent, empty pupal cases.
• Signs: honeydew.
• Signs: sooty mold.
• Signs: ant presence.

Exclusionary Practices

• Use air curtains.
• Screen.
• Weed areas around the garden.

Disruption of Life Cycle

• Molting disruptors such as azadirachtin.

Eradication

• Biological control with parasitoid wasps (E. formosa, E. eretmocerus), Amblyseius swirskii (the predatory mite), the Mullein Bug Dicyphus hesperus, generalist insect predators (lacewing larva, Orius insidiosus), entomopathogenic fungi (B. bassiana, I. fumosorosea).
• Sprays: herbal/botanical oils (cinnamon, rosemary, thyme, garlic, peppermint, neem), limonene-oil-based repellents, insecticidal soaps, insect neurotoxins (pyrethrins).
• Trapping with yellow sticky tape under the canopy.
• Crop vacuuming.

Slugs & Snails

Snails and slugs occasionally attack greenhouse and outdoor gardens, especially young plants with leaves close to the ground. They are rare indoors.

Slugs range in color from pale gray to tan and grow to as long as two inches (5 cm) long. Their bodies are soft and fleshy, and glisten with a clear slime that the slugs secrete to retain moisture and help their movement. Two small “horns” atop the slug’s head are actually the slug’s “eyes,” which sense light (slugs have no sense of sight).

Snails are slugs with shells. They are built almost identically to slugs, except for a coiled shell of calcium carbonate that protects most of a snail’s body. Snails can withdraw completely into their shells when threatened. Shells of common garden snails can reach up to an inch and a half (3.75 cm) in diameter, and are gray, brown, and black.

Snails and slugs are found on the leaves and edges of leaves and flowers when it is moist, usually after dusk. When it is dry or light, they hide in dark moist areas such as piles of fallen leaves, crevices, and moist shady areas.

Snails and slugs eat leaves. Holes in leaves and/or clipped edges of leaves and flowers, accompanied by a silvery, slimy trail, indicate snail or slug damage. A single snail can savage many small plants in one night.

These pests thrive in moist, dark environments. They hide in mulch, short and stubby plants, under boards, and in soil. They avoid sunlight, so they are seldom seen during the day, but come out to feast at night.

There is one particular kind of snail that should be left alone. Decollate snails sometimes attack plants, but their main food is other snails and slugs. The fastest way to tell a “good” snail from a plant-eating pest is the shape of the shell: common garden snails usually have round shells that coil in a simple spiral. Most species of decollate snails have cone-shaped shells. If these are the only snails found in the garden, then go ahead and get rid of them, because they eat plants if there is no other food in their habitat. But if there are other snails present as well, then the decollate snail is a friend.

Reproduction Rate & Life Cycle

Slugs and snails are hermaphroditic, so every individual lays eggs in clutches of 30 to 120 eggs, one to two inches (2.5 to 5 cm) in moist soil. When conditions are suitable (not too dry or too cold), slugs and snails can lay eggs as often as once a month, so their numbers can increase rapidly during damp spring and fall weather.

Monitoring

• Signs: adults and eggs that are laid in the soil.
• Signs: dried slime trails.
• Signs: feces.
• Symptoms: feeding damage.

Exclusionary Practices

• Screen.
• Eliminate areas of excessively humid soil.
• Use copper-based barriers (copper salts, copper tapes).

Eradication

• Biological control with decollate snails, arthropod predators (rove beetles, carabid beetles), all manner of vertebrates (amphibians, reptiles, birds, and mammals).
• Iron granular slug baits.
• Trapping with beer traps.
• Hand picking.

Ants

Ants are abundant both indoors and outdoors. Most of the species that affect cannabis do so because they protect honeydew-producing pests such as aphids and other sap-sucking bugs.

Ants can be found in the soil or planting medium, where they nest. They climb the stalk and graze their herds of aphids and other sap feeders on the leaves. They nest in underground colonies, occasionally causing damage to the roots.

Ants are attracted to plants that already have colonies of aphids or any other sap-feeding bugs. They tend to these pests and protect them from predators, often moving them to new grazing areas. It is important to control ants not so much for the damage they might cause to the plant directly but because they make control of other pests more difficult.

Ants are social creatures, living in colonies centered around queens and supported by workers. Some species have only one queen per colony, while others may have several. The ant life cycle begins with an egg laid by a queen, progresses through a larval and pupal stage, and then adulthood. In many species only the oldest adults work outside the colony. Ninety percent of the ants work in the nest. Colonies reproduce when a newly hatched queen selects several males and either walks or flies to a new location.

Ants regulate their reproductive rates depending on conditions in the colony and the outside environment. They do this partly by regulating the length of the pupal stage and partly by varying how many larvae or pupae are transformed into queens. With suitable weather, ample food, and water, the reproductive rate increases. To protect the plant, the colony must be eliminated.

Monitoring

• Signs: adults crawl on floors and walls, on the plants, on the soil and pots and near colonies of sap-sucking insects.

Exclusionary Practices

• Place ant poison baits along the perimeter of the garden.
• Place ant traps along the perimeter of the garden.
• Use perimeter insecticidal sprays or powders such as ground cinnamon or cloves.
• Destroy ant colonies adjacent to the garden.
• Use water moats. Ants don’t swim, so moats prevent them from crawling from floor or table to the container. A simple moat can be made using a wide tray and a support such as a thick piece of Styrofoam or a block of wood. Place the plant container on top of the support. Fill the tray with water or place the table legs in a moat.
• Spread diatomaceous earth along the garden perimeter.
• Use sticky substances painted on or wrapped around plant stalks and stems, pots, and tables, which can interfere with their harvesting behavior.

Eradication

• Sprays: herbal/botanical oils (rosemary, thyme, garlic, peppermint, neem), insecticidal soaps, insect neurotoxins (pyrethrins, spinosads).
• Bait stations placed within the garden. Poisonous bait stations are a fast and efficient way to control ant infestations. Boric acid is an organic material that when mixed with a sugary bait will be taken back to the colony and fed to others. If ingested regularly it will kill the colony.

Vertebrate Pests

Vertebrate pests tend to be problematic in the outdoor grow environment, although they can also wreak havoc inside the greenhouse. The best way to manage them is through exclusionary practices.

Burrowing Mammals: Gophers & Moles

Gophers and moles are subterranean mammals. Gophers are more of a problem for farmers because they feed on plant roots and kill them, while moles are predators of invertebrates and don’t feed on plant material. Both can cause damage to roots if they dig near plants. These tunnels can be extensive, and injuries to workers who step into them are another potential problem. Both can also damage irrigation lines.

Gophers are medium-size rodents ranging from about 5 to nearly 14 inches (13-36 cm) long (not including tail). Their fur is very fine and ranges in color from nearly black to pale brown. The forepaws have strong claws. The head is small and flattened, with small ears and eyes and very prominent incisor teeth.

Gophers feed on plants in three ways; (1) they feed on roots that they encounter when digging their tunnels, (2) they may venture short distances (only a body length or so) from their tunnels to eat vegetation on the surface, and (3) they pull vegetation into their tunnels from below. Gophers may also attract predators that feed on them, and these may cause considerable damage when digging for their food.

Moles are also burrowing mammals about five to seven inches (13-17 cm) long, weighing three to four ounces (85-115 g). They have soft dark fur, very small eyes, pointed snouts, and strong digging claws on their front feet. Moles seldom appear on the surface, but the gardener usually notices their burrows instead.

Moles build tunnel complexes in rich soil. They eat insects and earthworms, and therefore favor moist soils with a lot of soil-dwelling insect life.

When gophers are suspected, the first task is to make sure that they aren’t moles instead.

First, check their diggings: a molehill tends to be a rough cone with a hole or an earthen “plug” near the center. A gopher mound is more fan-shaped, with the hole or plug near one edge. Next, look for damage. Moles generally cause very little damage. Gophers may chew the plants’ roots, causing them to wilt and making it possible to pull them up with just a slight tug. If plants are chewed off completely at the soil line, or completely gone roots and all, then chances are good that there is a gopher problem.

Reproduction Rate & Life Cycle

Gophers mate once a year, in the spring, and produce a litter of up to five young in late spring to early summer. They live for up to 12 years.

Moles generally have one litter of two to five pups per year, in mid to late spring. Except for the spring breeding period, they tend to be solitary and highly territorial. They fight other moles even to the death if one invades another’s tunnel system.

Monitoring

• Signs: tunnels and piles of earth from their digging.
• Signs: gophers occasionally exit their tunnels
• Symptoms: wilted plants, roots show chewing damage or have been completely eaten.

Exclusionary Practices

• Install gopher screens under the soil before planting.
• Use traps. Gopher and mole traps can be very different, since they are designed to take advantage of their different burrowing and feeding habits.
• Use perimeter repellents. Gophers and moles are repelled by several different substances. Castor oil is a commonly used organic repellent, but synthetic repellents are also available. Odor repellents such as predator urine can also work.

Eradication

• Biological control: owl boxes are often erected near grows to attract females to nest. Owls hunt at night when gophers may find it safer to emerge from their burrows.
• Fumigants: toxic substances such as poison gases, carbon monoxide (engine exhaust fumes), and carbon dioxide, either as a gas or as dry ice, can be pumped into their tunnels to kill entire colonies. Commercial fumigants are generally paper or cardboard cartridges filled with charcoal and potassium nitrate. They’re ignited and dropped into the tunnel openings, and the gasses they produce as they burn kill the gophers. Watch for wisps of smoke rising from the ground, as these may mark other exits from the gopher’s tunnel. Sealing the exits with packed earth or heavy rocks is critical to keeping the toxic gases inside the burrow and also to prevent escapes while the fumigation is performed.
• Poison baits: there are commercially available organic and conventional toxic baits for burrowing pests of agriculture. Some of these are very fast acting; others require multiple feedings. What must always be considered are any nontarget effects on predators of gophers and moles. If predators feed on a poisoned pest, the toxins that killed it are ingested by them (dogs, cats, owls, raptors) and may in turn poison them.

Deer

Deer populations vary widely, both geographically and by habitat. Deer favor light forest and grasslands near forested areas, as they dislike getting more than a few hundred yards from cover. Gardens in suburban areas built near suitable forestland may have problems with grazing deer. Garden plots set up in wild or rural areas are very likely to be visited by deer if the habitat supports them.

Deer are grazers with graceful bodies, thin legs, and long necks. They vary greatly in size depending on species, age, and sex, but usually range between four and six feet (1.2 and 1.8 m) long and weigh 80 to 220 pounds (35 to 100 kg). Usually the heavier species are found in the north and the smaller lighter ones in the south. The males carry antlers beginning in late summer and usually shed them in very late winter or spring.

Deer emerge from forest cover at night to browse on plants, but flee quickly when approached. They have excellent senses, so most of the time the gardener knows them only by their tracks and the damage they leave. Some deer have become accustomed to humans and don’t flee on sight.

Deer prefer fresh leaves, fruit, and other rich plant matter. In the cannabis garden they tear up and eat entire small plants, strip plants of leaves, and trample them. Cannabis evolved cannabinoids in part as protection against herbivores. Most mammals find the leaves and flowers unpleasant. Deer are among the few exceptions. This means that in areas that have large deer populations, they may be attracted to the garden simply because it’s a food supply that most other herbivores leave alone. Even so, they prefer young, tender plants. As cannabis matures and cannabinoid levels increase, they become less palatable to deer.

Deer lack upper incisor teeth. They do not bite plants the way a rabbit or similar animal might. Instead the deer takes hold of leaves with its lips and lower teeth and then tears them off. This makes for ragged browse damage, very different from the neatly clipped leaves left by rodents. Look also for deer tracks and droppings near the garden.

Reproduction Rate & Life Cycle

Deer follow a normal mammalian life cycle. Adults mate in the late autumn through midwinter, and the female gives birth to one or occasionally two fawns in late spring to early summer. Deer usually mature in 1 to 2 years and live for 10-20 years if not killed by predators or disease.

Monitoring

• Signs: adults and young.
• Signs: tracks.
• Signs: feces.
• Symptoms: leaf feeding damage that appears ragged. Younger plants eaten entirely.

Exclusionary Practices

• Use fencing. Fencing must be tall enough so that the deer can’t jump over it. A deer can jump any fence under eight feet (2.5 m) high if it can get close enough. Fencing can be electrified to make it more effective. A variant called a “Minnesota fence” actually uses an attractive bait such as peanut butter to get the deer to lick the fence or a foil tag attached to the fence. The deer get a mild shock and avoid the fence completely after that.
• Use perimeter repellents. Deer find the odors of garlic, capsaicin, and rotten eggs offensive, and several brands of commercial repellents containing these ingredients are available. Anything that carries a human’s scent such as barbershop clippings, worn clothing, or human urine also works. Urine or scat from dogs or other predators is another option. Predator urine is available commercially. Scat is sometimes available from a local zoo or pet store. Scented bar soap can be used suspended in a net bag near the plants. If it is unknown whether a repellent is safe for use on food plants, surround the cannabis plants with other plants that will not be ingested. Replace the repellent according to the manufacturer’s instructions or every few days for the other scent repellents listed above.
• Use sounds and lights. Anything that startles or frightens deer is effective. Buy several home motion detectors at a hardware store and set them up in a perimeter around the grow site. Depending on the resources at the site, motion detectors can be rigged to trigger high-pressure water sprinklers (these are sold as “scarecrow sprinklers”), bright lights, battery-powered radios, or ultrasonic noise when a deer approaches. Remember to set up lights to point away from the plants, and if discretion is important, then shield the lights or lower the radio volume so the deer can detect them but nearby watchers cannot.

Eradication

• Use traps.
• Hunt them (follow local regulations).

Rodents: Rats & Mice

Rats and mice are not common pests in cannabis gardens, but sometimes they kill plants by gnawing or digging. They are an environment-specific problem, as they view cannabis as a target of opportunity.

Rats and mice are common wherever humans live, although they are not always visible. Some rats live in the wild, feasting on insects, nuts, fruits, and nature’s detritus. They lair in burrows, walls, piles of trash, dense brush, attics, or wherever they can build a secure nest.

Cannabis is not a primary food source for rats, but they like to chew the woody stalks of plants. This cuts the plants down. These rodents’ teeth grow constantly, and gnawing behavior is instinctive.

Rodents can also be indirect pests of cannabis. Many growers use slow-release sachets of predatory mites as a biological control strategy for several different pests such as spider mites, thrips, and whiteflies. These sachets are essentially tiny breeding colonies of predatory mites that are hung in the canopy where their prey live. As the members of the breeding colony reach maturity, they emerge from the sachets and begin hunting in the crop for their pest prey. The sachets contain a wheat bran carrier that is attractive to rodents. If the rodent population in and around the garden is sizable, they can destroy all the sachets within a few nights. If the grower wishes to use slow-release sachets as part of a biological control program, it is essential that rodents be controlled.

Rodents are a problem for the garden when the garden is close to something that they like to eat. Gardens near cornfields, orchards, food warehouses, areas with nuts or berries growing wild, and other similar places are at risk. Food at campsites draws rats and mice close to the garden, so secure all food and destroy or remove all food scraps.

Rodents are prolific breeders. They breed year round if they have sufficient food and a warm place to keep their young. They produce numerous litters a year, but they adjust their population automatically to the local food supply.

Monitoring

• Signs: rodent runways. When rodents leave their nests, they prefer to move close to walls following scent paths left during previous excursions. These pathways are known as runways and can often be identified visually by the gray or brown marks on the walls adjacent to the runway that oil from their fur has left behind.
• Signs: feces. These are prominent along runways and entrances into walls. Products that dye rodent feces with a fluorescent compound after they are digested can be used to find feces.
• Signs: holes in slow-release sachets.
• Symptoms: chewing damage on stalks.

Exclusionary Practices

• Repair cracks in the walls of grow rooms or greenhouses. The repair material should be made of something that rodents can’t chew through such as steel plate or steel wool.
• Use traps. There are numerous different kinds of traps. Lethal traps are usually set and kill the individual that triggered it. Live-catch traps can capture an individual rodent or they can be multi-catch, which do not have to be reset. Runways are the most strategic location to place traps.
• Use perimeter repellents. Castor oil is a commonly used organic repellent, but synthetic repellents are also available. Odor repellents such as predator urine can also work.

Eradication

• Biological control: owl boxes, cat sentries.
• Poison baits: there are commercially available organic and conventional toxic baits for rodent pests of agriculture. An organic poison is cholecalciferol, a Vitamin D3 that requires multiple feedings. As with other poisons, the grower needs to be careful to not poison their non-target predators. Predators feeding on rodents killed by cholecalciferol are much less likely to be poisoned.

The Common Diseases of Cannabis

Disease can strike cannabis plants at any stage. The diseases that affect cannabis fall into three broad categories: fungal, viral, and bacterial. The spores that cause plant diseases are ubiquitous. Viral and viroid particles live either in the plant or inside an insect that vectors them. A garden’s susceptibility to disease is often traceable to environmental imbalances in temperature, moisture, light conditions, airflow, and pH, among others.

Fungus grows when it finds the right levels of moisture, temperature (the range varies by species), acidic conditions, and a reliable source of nutrition.

Viruses or viroids live in plant tissues, and in cannabis are transmitted mechanically.

Bacteria are much more likely to invade when the environment has been compromised, in conditions such as oxygen deprivation, which make their attack more successful.

Once disease hits, it is important to act quickly. However, prevention is the best solution.

Gray Mold & Brown Mold (Botrytis)

Gray mold, Botrytis cinerea, is found almost everywhere and can cause disease on most plants, including cannabis. It causes damping off, stem canker, and bud rot. It is one of the most common fungal diseases that attack cannabis.

Botrytis infected plants Photo: Ed Rosenthal

The fungus can germinate only on wet plant tissue when the temperature is between 55 and 70°F (13-21°C). This often happens in dry weather as dew accumulates on the leaves. Once it starts growing, it can tolerate a wide range of humidity and temperatures, but high humidity and cool temperatures help it thrive. Lowering the humidity often stops it from continuing to grow.

Gray mold, like most other fungi, enters and easily infects any part of a plant that is either wounded, damaged from pests and pruning, or beginning to die. Thus, it is very important to sanitize pruning equipment between cuts.

Cuts and lesions are a normal part of plant life, so all plants are subject to attack when conditions are favorable to the mold. Unhealthy or shaded areas of plants or crevices in buds are ideal conditions for mold. Spores travel mostly via wind and rain and even in tap water, but they can be brought into gardens on clothing and pets.

Shaded areas of the plant that do not get a lot of light are usually infected first. Then the disease spreads quickly through growth and spores.

Gray mold does the greatest amount of damage during flowering. It attacks the flowering tops, leaves, and stalks. Seedlings and seeds can also be infected and killed.

The mold starts out whitish like powdery mildew, but then darkens to a smoky gray or brown. It has a fuzzy appearance, and light to dark brown rot forms in the damaged tissue.

Leaves and buds turn yellow. In higher humidity, the gray mold leaves a brown slimy substance on the leaves and turns the bud to rot, especially when the tissue is dense late in flowering.

Stems with unhealed breaks can be infected with B. cinerea, causing stem cankers, which then affect the rest of the plant by depriving it of nutrients and water.

Monitoring

• Signs: spore-trap sampling of the air and surface swabs. There are numerous methods to identify and quantify the spore load in an air space or on surfaces, which have been developed for other industries. Some of these use spore identification by microscopy; others require the captured spores to be cultured on a nutrient media for identification; and others use PCR technology.
• Signs: transmission-light microscopes can be used to identify botrytis. The structure to look for is a branched reproductive mycelia with oval spores in clusters on the tips of the branches. They are reminiscent of grape bunches.
• Symptoms: gray or brown fuzz inside buds that eventually advances to the outside, turning green buds gray or brown.
• Infected area turns brown and dries up. It may be soft if wet or brittle if it has dried.
• Stem canker or stem rot.

Resistant Varieties

• Although this pathogen can be devastating to crop yield, several resistant cultivars are available. Varieties that produce smaller flowers are likely to be more resistant than those that produce large, compact flowers that create a micro-environment conducive to fungal germination, proliferation, and sporulation. Cultivars that have a much more open branch architecture may allow for better airflow and humidity control. It’s quite possible that cultivars which prefer tropical environments will be less susceptible to this pathogen.

Exclusionary Practices

• Use HEPA filtration of air inlets.
• Decontamination of the grow environment air and incoming air can be accomplished using peroxides, ultraviolet light, chlorine dioxide, ozone gas, ionizers, and antimicrobial fogging. Hospitals, food processors, and laboratory clean rooms use different technologies and materials to reduce pathogen loads in the air.
• Create work procedures that forbid going from infected to uninfected rooms, and general worker hygiene to prevent movement of spores from infected areas to clean areas.
• Sanitize hands and tools.
• Fungicidal clone dips.
• Remove plant debris post-harvest and around the vicinity of the garden to exclude aerial spores from disseminating to clean plants. Botrytis can form a protective structure called a sclerotium, which allows it to survive in debris and soil.
• Rogue infected tissue: carefully remove infected flowers to prevent spore dissemination.

Disruption of Life Cycle

• Climate control is especially important as harvest approaches. The best strategy to control botrytis is to keep the temperature and humidity outside the range that allows for spore germination, mycelial proliferation, and sporulation. This includes the macroclimate of the garden and the microclimate in the canopy and inside the flower. The temperature should be raised to a minimum of 75°F (24°C) day and night if possible. Outdoors, heaters can be used to keep small gardens warm.
• Control air movement: air currents can disseminate spores to plants in areas of the garden that haven’t been infected or to younger plants.
• Use bio stimulants: numerous substances are known to elicit defensive responses that cause plants to thicken their cell walls. This thickening of the cell walls can make it much more difficult for a spore that has germinated to penetrate the flower tissue and infect it.

Eradication

• Preventative biological control with numerous competitive microorganisms (B. subtilis, B. amyloliquefaciens, S. lydicus, U. oudemansii).
• Use fungicidal sprays: some materials that can help control the spread of spores are herbal sprays containing cinnamon or clove oil. These kill the organisms and dry the infected areas in place.
• Use a 10% milk solution (32 ounces per gallon) to prevent mold. It can be used after rain or during humid weather.
• Use a 1% potassium bicarbonate (KHCO₃) solution (1.25 ounces per gallon). Sodium bicarbonate (baking soda, NaHCO₃) can also be used, although it delivers sodium, which presents problems if it builds up.

Chlorine Dioxide for Pest & Disease Control

Chlorine dioxide (ClO₂) is a yellowish-green gas that has antimicrobial properties and shows great potential as a disinfectant in cannabis cultivation, drying, curing, and trim rooms. It is less corrosive than other gaseous disinfectants such as ozone; however, it has been shown to neutralize fungal spores for crops other than cannabis (Lee et al. 2020). This paper shows that ClO₂ is very effective in killing Aspergillus spores for storing fresh coffee beans. Plant pathogens such as powdery mildew (PM) are also susceptible to ClO₂ (Sharma et al. 2017).

Many cultivars are especially susceptible to PM infection, and one of the most effective preventative measures against infecting a crop (applying sulfur to the canopy) should not be used once the flowers appear on the stems. Sulfur application will impart an unpleasant aroma and flavor on cannabis flowers. The application of sulfur does not kill PM spores; however, it creates an environment that makes it difficult for the spores to germinate.

Using a slow-release ClO₂ product, such as ProKure D, in combination with sulfur can be the one-two punch that prevents PM from rearing its ugly head. A slow-release ClO₂ product sanitizes the air and the plant tissue, so that the PM spores are neutralized. Once the flowers develop in the canopy, stopping the application of sulfur and switching to hanging ClO₂ packets above the canopy at a regular interval will prevent PM without leaving a residual aroma or flavor on the cannabis flower. ClO₂ is gaseous and breaks down into chloride ions and oxygen. Chloride is naturally found in plants and does not impart any flavor.

This product is OMRI listed and registered with the EPA as a fungicide, disinfectant, algaecide, virucide, and deodorizer, so it goes beyond just eliminating PM from the canopy.

Powdery Mildew

Powdery mildew is a fungal disease that affects a wide range of plant species. Each species of powdery mildew has a very limited host range, but all are characterized by an easily recognizable white or gray powdery growth on the upper surfaces of leaves. The strains that attack hops also attack cannabis.

Powdery mildew. Photo: Ed Rosenthal

Mildew spores can be found everywhere. Powdery mildew is a common problem for both indoor and outdoor growers whenever the temperature and humidity fall into its favored range.

Mildew spores are ubiquitous and endemic. In areas where grapes, strawberries, cannabis, or hops are being grown, wind and air ventilation are the main vectors. Another major factor is contaminated cuttings. Clothing, pets, and wild animals can also deliver spores to the garden.

Spores can remain dormant until triggered by environmental factors, which include a suitable host, adequate humidity, moderate temperatures, low light intensity, and acidity.

Powdery mildew is most likely to attack young leaves, up to two or three weeks old. The infection spreads over the plant and spreads to other plants in the garden. It affects buds, stems, stalks, and leaves.

The first signs of an infection are raised bumps on the upper leaf surfaces. Plant leaves look like they’ve been dusted with flour or confectioners’ sugar. At first, it might appear on just a small portion of the leaf in an irregular circle pattern. It quickly spreads, and soon the entire leaf is covered as if it had been powdered.

Infected plants prematurely yellow, brown, and eventually die. If untreated, black specks can arise in the white powdery mildew. Buds have a stale, moist smell and are coated with white powdery-looking mycelia. Powdery mildew hinders photosynthesis, crippling yields. Infected buds and leaves are not acceptable for smoking.

As with other aboveground pathogenic fungi that affect cannabis, climate control is the key. Many of the strategies for identification and control of botrytis apply to powdery mildew as well. One major difference is that powdery mildew is much easier to control with sprays than botrytis since it primarily infects leaves. However, it can move into flowers if left unchecked.

Monitoring

• Signs: spore-trap sampling of the air and surface swabs. Numerous methods to identify and quantify the spore load in an air space or on surfaces have been developed for other industries. Some of these use spore identification by microscopy, others require the captured spores to be cultured on a nutrient media for identification, and others use polymerase chain reaction (PCR) technology.
• Signs: transmission-light microscopes can be used to identify powdery mildew. The structure to look for is a reproductive mycelium terminating in a chain of oval spores. They are reminiscent of a pearl necklace.
• Symptoms: white, powdery mycelia that emerge from the leaf upper surfaces.

Resistant Varieties

• Powdery mildew is a common problem in cannabis production, and there may be varieties that are less susceptible to it. As with botrytis, strains that have a much more open branch architecture might allow for better airflow and humidity control.

Exclusionary Practices

• Use HEPA filtration of air inlets.
• Decontaminate the grow environment air and incoming air by using UVC light, chlorine dioxide, ozone gas, ionizers, antimicrobial fogging, clean uniform changes, and restricting worker movement.
• Sanitize hands and tools.
• Fungicidal clone dips.
• Removal of plant debris.

Disruption of Life Cycle

• Climate control: this is especially important as harvest approaches. The best strategy to control powdery mildew is to keep the temperature and humidity outside the range that allows for spore germination, mycelial proliferation, and sporulation. This includes the macroclimate of the garden and the microclimate in the canopy. Do not let the temperature dip below 75°F (24°C) day or night.
• Control air movement: air currents can disseminate spores to plants in areas of the garden that haven’t been infected or to younger plants. Use UVC lights to clean air-streams of living organisms
• Biostimulants: numerous substances are known to elicit defensive responses that cause plants to thicken their cell walls. This thickening of the cell walls can make it much more difficult for a spore that has germinated to penetrate the flower tissue and infect it.

Eradication

• Use preventative biological control with numerous competitive microorganisms (B. subtilis, B. amyloliquefaciens, B. pumilus, S. lydicus).
• Use fungicidal sprays: elemental sulfur, potassium bicarbonate, organic acids such as sorbic, citric, peracetic, and acetic, hydrogen peroxide, fungicidal soap, herbal/botanical oils, copper soaps and salts, and skim milk.

Is Powdery Mildew Systemic, and Does It Matter?

It is easy to see the means of powdery mildew transmission. The white powdery-looking growth on top of leaves is the reproductive organs of the fungus. The spores’ transmission can be by airborne invasion, by touch from plant to plant or plant to animal to plant and splashing water. It dies in streams and standing water.

This doesn’t answer the question of whether powdery mildew is systemic. Asked another way, does it invade the plant’s vascular system? A systemic disease may be asymptomatic at times. However, given the right conditions (such as temperature, humidity, and food source), it quickly invades and digests cells in the leaf, then produces reproductive organs.

Plants that have expressed signs of powdery mildew must be monitored for symptoms. When cloning, it seems as though plants that have expressed powdery mildew but are asymptomatic can still transmit the disease. These might be indications of systemic disease. However, the general consensus in the scientific community is that it doesn’t travel through the plant’s vascular system but along the leaf epidermis, eventually covering the plant completely, though it might remain asymptomatic until the right conditions occur, and then the pathogen has an explosive eruption that can turn the plant white.

The fungus can infect the entire plant, so it doesn’t matter whether it happens through internal or external action. Once the plant is infected, the fungus may be controlled but not eliminated.

The Threat of Viral Cannabis Diseases

Cannabis cryptic virus (CanCV) was recently identified in several European industrial hemp cultivars, and infected plants exhibit symptoms that may previously have been confused with tobacco viruses. Alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV), and Arabis mosaic virus (ArMV) all originate in other plants and have also been identified in European industrial hemp cultivars.

Recently, lettuce chlorosis virus (LCV) was found in Israeli glasshouse medicinal cannabis crops, and beet curly top virus (BCTV) was reported from plants grown outdoors in Colorado. All could pose economically significant problems in the future.

Out of the 900 plant viruses that have been described, only a limited number have been studied for cannabis, and so far just two cannabis-specific viruses have been identified: the hemp streak virus (HSV) and the hemp mosaic virus (HMV), whose spread is facilitated by viral vectors including aphids, thrips, and whiteflies.

Hop latent viroid infected plant. Photo: Dark Heart Nursery

Hop Latent Viroid (HpLVd)

The Hop Latent Viroid (HpLVd) is a recently identified pathogen of cannabis that has the potential to become a pest of serious economic importance. Hops is a relative of cannabis, as both plants belong to the Cannabaceae family. It is unknown how it was transmitted from hops to cannabis.

It is important to understand that this is not a plant virus. Viruses are composed of nucleic acids (like DNA and RNA) and have a protein coat. Viroids do not have a protein coat and are composed entirely of nucleic acids. These nucleic acid chains are also very short, making viroids the smallest known pathogens to infect plants. Similarly to viruses, viroid particles enter a cell and hijack the cell’s gene replication machinery in order to replicate themselves. It is still not known how this process causes the symptoms associated with infection.

Ironically, HpLVd is of major concern in cannabis, although it is mostly asymptomatic in hops. As of publication, the California Department of Food and Agriculture (CFDA) has given HpLVd a temporary disease rating of “Z” in hops, meaning that it “may be expected to score low as a pest of agriculture or the environment and/or that are of common occurrence and generally distributed in California.”

The temporary Z rating means that not enough research is available to conclude that it is a pathogen of significance in California hops. It is expected that this will be changed to a rating of “C,” meaning that it is “used for pests that score medium to low as a pest of agriculture or the environment and that are of general distribution.”

In a recent CDFA publication, an HpLVd disease rating of C was also given to the viroid in hemp production.

Hops production in California is very small in comparison with cannabis. Research and time will determine what economic impact this pathogen will have on cannabis production.

Transmission of viroids is thought to be primarily mechanical, meaning that it is transmitted when the particles from an infected plant are moved through a wound or an opening in an uninfected plant. This vectoring can occur through pest feeding or through contaminated tools. Aphids are known to readily transmit viroids in other plant species. Scissors used for pruning or taking clones can also move it through a garden. Contaminated hands can also move the viroid.

Tracking and tracing of the viroid is critical. What makes this pathogen so insidious is that it is frequently asymptomatic in plants, and even those that will show symptoms sometimes may not until a few weeks into flowering. It is believed that an asymptomatic plant may develop symptoms as a result of some stressor. Once a plant has tested positive, it is essential that all progeny either be tested and found clean or destroyed. This is a very difficult task and can become very expensive. The viroid was only recently identified, and many states have had a legal cannabis industry for years, so one can assume that it is everywhere now. As new states legalize cannabis, it may be necessary to introduce legislation on mandatory testing and certification.

Monitoring

• Signs: genetic testing reveals the presence of the viral RNA. Genetic testing is absolutely essential to monitoring for this viroid because it is a latent viroid, meaning that many plants are asymptomatic. These asymptomatic plants can still serve as a source for infection.
• Symptoms: reduced trichome production or diminished size of the gland heads.
• Symptoms: reduced flower yield.
• Symptoms: stunting.
• Symptoms: fragile stalks and stems.

Exclusionary Practices

• Test all incoming plant material for presence of the virus and keep the new plants quarantined until negative results are confirmed.
• Maintain populations of clean tested plants as the sole source for propagation material.
• Hand and tool sanitation is the most important control strategy available to the grower. Proper protocols and training of all workers who come in contact with plants is essential.
• Worker hygiene.

Disruption of Life Cycle

• There are no known strategies for interfering with the viroid’s life cycle.

Eradication

• Destroy plants that have tested positive or show symptoms.
• Micro-propagation of apical meristem tissue is one way to remove the viroid.

Hop Latent Viroid Screening Protocol

It is ineffective to screen for infection by looking for short, stunted plants early in vegetative growth because clones can be cut at different sizes and from different parts of the mother. Lateral stem growth is the best screening method outside of getting plant tissues tested at a lab.

Starting at the second week of vegetative growth, as the lateral branches start to develop, each plant is examined to determine abnormal lateral growth. Each cultivar will have different lateral stem angles, so the abnormal lateral growth will differ from cultivar to cultivar if there is a symptomatic infection. Qualitative assessment is usually sufficient to screen most of the plants. If a lateral branch looks abnormally more horizontal than average, lightly push down on it. If it easily separates from the main branch, it’s likely infected with HpLVd. Repeat screening as plants are transitioning from vegetative growth to flowering; however, at this stage the infected plants should also show signs of stunting.

Process:

• At week 2 of vegetative growth, screen each plant for abnormally horizontal lateral branching.
• Lightly press down on the lateral branch. If it easily separates from the main branch, it is likely infected with HpLVd.
• Separate/quarantine plants that are assumed to be infected.
• Do NOT cut on these plants without sanitizing the cutting tool.
• Isopropyl alcohol does not sanitize the tools quickly enough.
• Bleach (at full strength or 10% dilution) or Virkon are the only cleaning agents that work quickly enough.
• Follow the instructions from a qualified testing lab and send plant tissue to be tested. If results come back as positive, the quarantined plants must be removed and destroyed.