Cannabis Pest Control: An IPM Guide to Mites, Aphids, Thrips, and Gnats
Cannabis pest control uses IPM: prevent with sanitation and quarantine, monitor with traps and scouting, then treat with beneficials or minimum-risk sprays.
The short answer
Spider mites, aphids, thrips, fungus gnats, and whiteflies cause most of the trouble indoors. Outdoors you can add caterpillars that bore into buds, leafhoppers, and root aphids. Two-spotted spider mites and hemp russet mites are the ones that wreck a crop fastest, because populations explode before the damage is obvious to the eye.
The sap suckers
These insects pierce plant tissue and drink. The leaf ends up stippled, curled, or coated in sticky honeydew that later grows sooty mold.
How to Identify Cannabis Pest Damage
- Spider mites. Look for fine webbing on lower leaves and pale speckling that turns into bronze or yellow patches. Hot, dry rooms make them worse. They hide on the undersides, so check there first.
- Aphids. Clusters on new growth and along stems. Winged adults show up when the colony gets crowded. Ants tending the plants are a hint.
- Thrips. Silvery streaks and black specks of frass on leaves. They also scar flowers.
- Whiteflies. A cloud of small white insects when you bump the plant. Common in greenhouses and on clones brought in from outside.
- Hemp russet mites and broad mites. Too small to see without a scope. New growth comes in shiny, twisted, and bronzed, and buds stop filling out.
Root zone pests
Fungus gnats are the usual suspect. Adults fly around the medium while larvae live in wet topsoil and chew on roots and root hairs. Root aphids look like a white, waxy crust on the root ball and cause plants to stall, yellow from the bottom up, and wilt in a way that looks like under or overwatering. Damping-off pathogens often follow both.
how to prevent pest infection in indoor grow
Chewers and borers
Outdoor plants get caterpillars, mostly corn earworm and other budworms. They chew into flower clusters, leave frass, and open the door to botrytis, also called bud rot. Leafhoppers feed on leaves and can carry pathogens. Grasshoppers and slugs take bites out of young plants. Cutworms clip seedlings at the soil line, which kills a plant overnight.
How to tell them apart
Scope the undersides of leaves at 60x to 100x. If you find mites, note whether they are spotted (spider mites) or translucent and worm-like (russet or broad mites). Yellow sticky cards catch winged adults and give you an early warning. Tap a branch over white paper and watch what falls. Root problems show up as wilting in moist soil, so pull a plant and inspect the root ball before you treat the leaves.
What works
Start with physical control. Rinse foliage, remove infested leaves, and keep the room clean. Quarantine new clones and seedlings for a week or two before they go near the rest.
For biological control, predatory mites (Phytoseiulus persimilis, Amblyseius andersoni), lacewing larvae, and lady beetles handle mites and aphids. Steinernema feltiae nematodes and the rove beetle Dalotia coriaria go after fungus gnat larvae in the medium. Bacillus thuringiensis var. kurstaki is the standard for caterpillars, and it works best while larvae are small.
For sprays, insecticidal soap, horticultural oil, and neem-based products knock down soft-bodied insects. Spinosad covers thrips and caterpillars. Rotate modes of action so you do not select for resistance, and never spray anything on flowers close to harvest. Check your state's list of pesticides allowed on cannabis, because the label is not the last word. Plenty of products registered for food crops are still off limits on cannabis in some states.
Prevention that pays
- Filter incoming air and keep the room under positive pressure.
- Change clothes and shoes before entering a grow.
- Do not overwater. Let the top layer of medium dry between feeds to slow fungus gnats.
- Keep humidity and temperature in range. Mites like it hot and dry, so 60 to 70 percent RH and moderate temps help.
- Inspect weekly with a scope. A five-minute check beats a two-week recovery.
- Clean tools, pots, and trays between runs.
If you catch an infestation early, most of these are manageable without losing the crop. If you find it at week seven of flower, your options shrink fast, and the honest move may be to harvest what you have, sanitize, and start clean.