One of the more challenging aspects of growing cannabis is preventing, detecting, and controlling infestations of harmful bugs. Prevention is best, to keep pests from gaining a foothold in your garden—it’s easier than dealing with them once they’re on plants.
Three common little bugs that can contaminate cannabis plants are:
- Spider mites
- Fungus gnats
- Root aphids
If you find yourself plagued with these three common cannabis pests, here are some guidelines for how to get rid of them.
How to prevent pests in your cannabis grow
Here are some basic guidelines for keeping cannabis plants healthy and pest-free:
- Indoors, make sure your grow room is completely sealed. Use caulk or spray foam to fill in gaps or cracks, seal doors and windows with weather stripping, and place filters over air inlets.
- Don’t let clothes cross-contaminate your plants. Change clothes before entering a grow space so you don’t bring in any “hitchhikers.”
- Keep pets out of the grow area; they can bring in all types of bugs.
- Keep equipment sterile. Always use new soil and grow media. Sterilize all tools and containers, wash hands before handling plants, and remove all plant debris immediately.
Spider mites on cannabis
Spider mites are the most common pest associated with cannabis, and can be one of the most devastating. These tiny critters (.4 mm) live on the underside of leaves, where they puncture leaves to suck out the plant’s juices.
They’re hard to spot with the naked eye, but infested cannabis plants show little white dots on the back of leaves. Spider mites also spin little webs, so if you see anything that looks like miniature cobwebs on top of marijuana leaves, it’s likely spider mites.
How to get rid of spider mites on marijuana plants
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 are quite toxic and should never be used on cannabis.
However, pyrethrum—derived from chrysanthemum flowers—is a natural insecticide that is very effective at killing mites. Three applications at 5-10 day intervals should significantly knock down and even eliminate spider mites.
Either spray a solution on 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.
Beneficial insects such as predatory mites are also viable options but can’t be used in conjunction with insecticides.
Fungus gnats on cannabis
Fungus gnats are grey or black long-legged flies 2-4 mm long, and they resemble tiny mosquitoes. Their larvae are white or transparent, 4-6 mm long, and live in soil where they feed on roots, organic matter, and fungus.
A severe infestation will lead to pale foliage and a general loss of vigor that will leave plants susceptible to disease, especially root rot. Adult gnats do not directly harm the plant, but they are vectors for disease, and also easily get trapped in resinous flowers—nobody wants to smoke bugs in their buds.
How to get rid of fungus gnats on weed plants
Keep humidity low and make sure not to overwater plants to prevent fungus gnat infestations. The most effective way to prevent or end an infestation is to place a physical barrier over your grow medium, such as a two-inch layer of sand, perlite, or other similar product—Gnat Nix works wonders. This prevents adult gnats from laying eggs in the soil, effectively ending their life cycle.
The naturally occurring soil bacteria Bacillus thuringiensis, 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 before they lay eggs in soil.
Root aphids in cannabis
Root aphids are tiny 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.
Plant roots turn yellow, swell, and harden as root aphids feed on them, leading to secondary fungal infections and dead spots. Eventually, plants will become stunted and yields will be greatly diminished.
How to get rid of root aphids in cannabis soil
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 more root aphids.
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 and other beneficial insects can also be effective in controlling root aphids.
FAQs on pests and little bugs on weed plants
What does spider mite damage look like?
The telltale sign of spider mites is many white or yellow dots on the top of fan leaves. If the infestation is really bad, there will be webbing on the buds. Actual spider mites appear on the underside of fan leaves as tiny white dots, hard to see with the naked eye.
What causes spider mites?
Spider mites are usually unknowingly brought into a weed grow from outside or from another weed grow. They can also travel on pets.
How do you prevent spider mites?
A lot of growers will change clothes when they enter their grow space; having a separate set of work clothes will ensure any bugs you may have picked up will stay outside the grow space. Some growers will also have a floor mat with bleach or other disinfectants to kill bugs on shoes.
How do you get rid of spider mites?
You can spray weed plants with safe, approved insecticides to get rid of spider mites. Be sure to check your state’s list of approved insecticides.
Beneficial insects, such as persimilis, or other predatory mites, can also be applied to weed plants to combat spider mites.
Trevor Hennings contributed to this article.
Read more of Leafly’s guide to growing marijuana
- How to grow weed: Basics of growing marijuana
- 4 stages of marijuana plant growth
- Marijuana plant anatomy
- How to grow weed indoors
- How to grow marijuana outdoors
- Cannabis seeds 101: How to grow marijuana from seed
- How to clone cannabis plants
- Marijuana seedling and plant care
- How to harvest marijuana plants
- Troubleshooting common cannabis plant problems
- Buyer’s guides for cannabis seeds and growing equipment
- How to grow marijuana using hydroponics, aeroponics, or aquaponics