One of the more challenging aspects of growing cannabis is preventing, detecting, and controlling infestations of harmful biological pests. Smart growers do everything they can to keep pests from gaining a foothold in their gardens because it’s easier than dealing with them once they’re on the plants. If you find yourself plagued with these three common cannabis pests, here are some guidelines for combating the nasty little buggers.
First, let’s go over some basic guidelines for keeping your plants healthy and pest-free.
Next, we’ll examine three of the most common cannabis pests and some of the more effective ways of eradicating them. Use a magnifying glass and compare pests with images found online to positively identify them before attempting to treat your plants.
Spider mites are the most common pest associated with cannabis, and also one of the most potentially devastating. These are tiny (.4 mm) arachnids that generally live on the underside of leaves, where they puncture the leaf surface to suck out the plant’s juices. They are very hard to spot with the naked eye, but infested leaves show stippling on their top surface, little white dots that some people initially confuse as mold or mildew. Spider mites also spin little webs, so if you see anything that looks like miniature cobwebs on your plants, you can be sure you have a significant infestation.
Spider mites thrive and reproduce quickly in warm, dry environments. To slow an infestation, lower temperatures and raise the humidity in your room. While there are many commercial miticides available, most of them are quite toxic and should never be used on cannabis.
Pyrethrum, however, is a naturally derived insecticide that is very effective at killing mites. Three applications at 5-10 day intervals should be enough to completely eliminate spider mites. Either spray a solution on your plants or use room foggers. Keep in mind, however, that pyrethrum is not recommended for flowering plants.
Things like insecticidal soap, horticultural oil, and neem oil are safe for use on flowering plants and are generally quite effective. Concentrated neem products like Azamax and Azatrol are very popular with cannabis growers and are safe for use throughout the grow cycle, but are expensive.
Biological controls such as predatory mites and ladybugs are also viable options but cannot be used in conjunction with insecticides for obvious reasons.
Fungus gnats are grey or black long-legged flies that are usually 2-4 mm long and resemble tiny mosquitoes. Their larvae are white or transparent, 4-6 mm long, and live in the growing medium where they feed on roots, organic matter, and fungus. A severe infestation will lead to pale foliage and a general loss of vigor that leaves plants susceptible to disease, especially root-rot. And while adult gnats do not directly harm the plant, they are vectors for disease, and also easily become trapped in resinous flowers.
Keeping the humidity low and making sure not to overwater are key in preventing fungus gnat infestations. The most effective way to prevent or end an infestation is to place a physical barrier over your grow medium. A two-inch layer of sand, perlite, or other similar product (Gnat Nix works wonders) on top of your grow medium prevents adult gnats from laying their eggs in the medium, effectively ending their life cycle.
The naturally occurring soil bacteria Bacillus thuringiensis, subspecies Israelensis, kills fungus gnat larvae and is available in many inexpensive forms like Summit Mosquito Bits, Gnatrol, and Microbe-Lift BMC. Insecticides like pyrethrum and neem oil can be effective when used as a soil drench, but must be applied several times and can have negative effects on plant roots.
Finally, yellow sticky traps can be used to catch adults, hopefully before they lay hundreds of eggs in your grow medium.
Perhaps the most difficult cannabis pest to eliminate are grape phylloxera, or root aphids — tiny aphid-like insects that feed on plant roots. About 1 mm long and ranging in color from yellow to green to brownish-orange, root aphids are often confused with fungus gnats as some adults will grow wings once a population has reached critical mass. Plant roots turn yellow, swell, and then harden as the root aphids feed on them, leading to secondary fungal infections and dead spots. Eventually, plants will become stunted and yields will be greatly diminished.
Once infestation occurs, it can be nearly impossible to eliminate root aphids from your plants. The number one treatment is BotaniGard, an insecticide composed of the living fungus Beauveria bassiana, which infects and kills the aphid and then releases spores to infect the next victim. It can be used in conjunction with pyrethrum, neem, and citrus oils, but care should be taken with these to avoid harming the plant. Apply as a soil drench every other day for a minimum of six treatments. Predatory nematodes can also be quite effective in controlling root aphids.