Just like garden vegetable crops, cannabis plants have a variety of pest insects that can hinder or destroy an otherwise healthy crop. Although most pest insects can negatively affect both indoor and outdoor cannabis crops, there are some pests that seem to be more prevalent in either an indoor or an outdoor cannabis garden. For example, slugs rarely, if ever, cause issues for indoor cannabis growers. On the other hand, fungus gnats and spider mites, which wreak havoc in indoor gardens, are usually kept somewhat in check by natural predators in an outdoor garden. Whether the cannabis plants are grown indoors or outdoors, gardeners armed with the knowledge to prevent, identify, and treat garden pests will be able to defend their precious plants from the damage inflicted by uninvited pathogenic guests.
A solid initial step toward pest insect prevention is to reduce the possibility of pests entering the garden in the first place. The number one way pest insects enter an indoor garden or greenhouse environment is by the gardener’s carelessness. When acquiring a new cannabis strain or plant, it is a good idea for the gardener to not only inspect it for bugs, but also place it in an area outside of the indoor garden for a few days of “quarantine”. The quarantine process should include a treatment with a general insecticide. After 5-7 days, if it passes inspection, the grower can introduce the new specimen to the indoor garden.
Another way pest insects enter an indoor cannabis garden is by hitching a ride on an unsuspecting gardener. If possible, gardeners should avoid entering the grow room right after doing yard work or visiting a friend’s indoor garden. If not hitching a ride on the gardener, pest insects make their way into the garden via the ventilation system or other small openings. A fine screen or mesh covering the ventilation ports reduces the chance of pest insects entering the growing space.
A thorough inspection of the grow room may reveal some small cracks or holes which should be caulked or sealed. Sometimes even the most seemingly minor repairs can make major differences in pest insect prevention. Another simple measure an indoor cannabis grower can take to prevent unwanted pests is to keep a clean growing environment. Dead leaves, discarded soil, standing water, and previously used planting containers are all places for pest insects to live and breed. A clean growing environment is beneficial in many ways, but especially in terms of pest prevention.
No matter how clean a gardener keeps his or her indoor garden or how hard they try to eliminate unwanted visitors, chances are, at some point, a pest insect problem will have to be dealt with. Early detection and proper identification are crucial to eliminating the problem quickly. The reason early detection is so important is because most pest insects have relatively fast reproductive rates. The fast reproductive rates teamed with incredibly large birth numbers are the reasons untreated pest insects are capable of taking out an otherwise healthy garden in just a matter of days.
Spider mites are one of the most destructive indoor garden pests due to their resilience and extremely fast reproductive rates. The most obvious sign of a spider mite infestation is cannabis leaves that have yellow or white speckling. This speckled look is caused by the mites feeding on the underside of the leaves. Another definite sign of spider mites is their webbing, which looks like a tightly woven spider web. When a spider mite infestation occurs, the cannabis flowers and leaves may be fully encased in webbing. There are many chemical insecticides available that are designed specifically for spider mites. These products are usually referred to as miticides.
Chemical miticides can be very dangerous if not handled properly, so paying close attention to the application directions is a must. For the “conscientious cannabis gardener”, there are organic insecticides that have shown promise in treating mites. Whatever method a grower chooses, he or she must make sure to address the mite eggs. Spider mite eggs are super tough and can withstand most insecticides. Indoor cannabis growers with a mite problem should repeat the treatment program every 2-5 days for at least two weeks. This will catch the new spider mites as they hatch and prevent them from re-establishing.
Thrips are another common indoor garden pest. They can usually be identified by the “track” marks left on the top of the leaves when the larvae are feeding. Adult thrips are winged and generally have a straw-like color. Under magnification, a thrip’s body structure resembles that of a wasp or hornet. The best way to treat thrips is to address all stages of the life cycle. Combining sticky traps for the flying adults, a foliar insecticide for the larvae and adults on the plants, and a soil drench to kill any eggs or larvae in the medium is the best way to simultaneously treat all life cycle stages for thrips. Repeating the treatment every few days for a couple of weeks will only help a grower win the battle against these pest insects.
Fungus gnats are usually identified by swarms of little black bugs that scatter when the soil or medium is watered or disturbed. They appear to resemble fruit flies, but, under magnification, their body structure looks more like a tiny mosquito. Fungus gnats are pesky bugs that seem to keep appearing, even after treatment. Again, the combination of sticky traps, a foliar insecticide, and a soil drench is the best multifaceted approach for a fungus gnat problem. It is also advantageous, if possible, to allow the top three to four inches of soil or medium to dry out. Fungus gnats need moisture for reproduction. Letting the top layer of soil or medium dry out will hinder their ability to reproduce.
Aphids, mealybugs, whiteflies, leaf-miners, and scales are less common pest insects for the indoor cannabis horticulturist. These soft-bodied insects can be successfully treated with an array of insecticides or beneficial insects. Many cannabis growers find insecticidal soap to be a good cure for these less intrusive soft-bodied bugs.
Although some of the same pest insects that affect indoor gardens also affect outdoor cannabis gardens, preventing and treating pest insects in an outdoor environment can differ greatly from treatment programs in an indoor garden. It is nearly impossible to use a screen or filter to stop flying insects from reaching the plants in an outdoor garden. Therefore, outdoor cannabis growers must turn to other methods of pest insect prevention. For most outdoor cannabis growers, the best way to approach pest insects is to implement an integrated pest management program. Integrated pest management, or IPM, is the practice of focusing on the long-term prevention of pests and/or pest damage by managing the ecosystem. The idea behind IPM is a comprehensive approach to dealing with pests over the long-term. All in all, IPM is the holistic approach to pest control. The principle of IPM is to look at all of the contributing factors and all of the possible remedies and then make a decision of when and how to take action.
IPM programs normally implement a combination of treatments or control practices that prevent or treat a problem from many different angles. The control practices used in an IPM program can be categorized as biological, physical, cultural, or chemical.
Biological – Biological control is the use of beneficial plants, insects, or microorganisms to control pest insects and pest insect damage. Most pest insects have a natural predator. Ladybugs and beneficial nematodes are just two examples of biological controls.
Physical – Physical control refers to a control that kills the pest insects directly or physically stops them from entering the garden space. Sticky traps to catch flying pest insects and barriers to keep rodents or deer out of the garden are examples of physical controls.
Cultural – Cultural controls are practices that reduce the pest insect’s ability to establish itself. For example, a change in irrigation practices can reduce moisture at certain times throughout the day, thus reducing particular pests.
Chemical – Chemical control is the use of pesticides. For most IPM programs, chemical control is only used when needed and is used in conjunction with other techniques to maximize effectiveness and to create long-term solutions.
Even with an IPM program implemented, it is still possible for an outdoor cannabis garden to contract a pest insect problem. Positive identification of the pest will allow a grower to find a combination of treatments specifically designed for that pest. For example, an outdoor grower who discovers aphids may want to plant fennel (known to repel aphids) and release ladybugs. This would be the biological approach. The gardener could also spray the plant with water to “wash off” some of the aphids. This would be the physical approach. If these tactics fail, it may be time to use a chemical control (pesticides).
As long as humans continue to cultivate plants for food and medicine they will have to prepare for potential pest insect problems. As with many problematic issues in a cannabis garden, prevention is key. In both indoor and outdoor cannabis gardens, growers can take steps to reduce the likelihood of acquiring a pest insect problem. Growing cannabis indoors gives horticulturists heightened control over environmental factors. Unfortunately, this “closed ecosystem” also creates a perfect environment for pest insects to establish without the danger of natural predators.
Only a grower armed with knowledge can identify and treat pest insects in an indoor garden. Outdoor cannabis growers can team up with Mother Nature and her natural biological controls. By implementing an IPM program, outdoor cannabis growers can manage their ecosystems to best prevent and treat unwanted guests. Whether indoors or outdoors, cannabis growers who encounter pest insects need to act quickly to positively identify and treat the pest insect before surmountable damage occurs.
Eric Hopper is a Professional Marijuana Grower Senior Editor.