Cannabis Pest Identification: Spider Mites, Thrips, Fungus Gnats, and More

Pests are an inevitability in cannabis growing. Whether you're indoors or outdoors, organic or synthetic, something will eventually find your plants and try to eat them. The key is identification — knowing what you're fighting determines the treatment. The wrong treatment wastes time while the pest population explodes.

This guide covers the most common cannabis pests in order of frequency and threat level. For each pest: what it looks like, the damage signature, and the recommended treatment.

Tier 1: The big three (most common)

Spider mites

Threat level: Critical. Spider mites are the most destructive indoor cannabis pest. They reproduce explosively — a population can double every 3 days in warm conditions — and they're difficult to eradicate completely.

What they look like:

• Tiny (0.5 mm) dots on the underside of leaves — tan, red, or green depending on species
• Nearly invisible to the naked eye. Use a 30x loupe or phone camera macro
• Two-spotted spider mites (the most common on cannabis) have two dark spots on their back

Damage signature:

• Tiny white or yellow speckles on the top of leaves (stippling) — each speckle is where a mite punctured the leaf to feed
• Leaves look dusty, faded, or bronzed from hundreds of feeding sites
• Webbing on the underside of leaves and between branches (advanced infestation). If you see webs, the population is massive
• Leaves dry out, curl, and drop

Where to look: underside of leaves, especially along the midrib (the main vein). Start with the lowest, oldest leaves — mites colonize from the bottom up.

Treatment:

1. Early (no webs): neem oil spray (underside of ALL leaves), every 3 days for 3 applications. Or insecticidal soap
2. Moderate (some webs): alternate neem oil and insecticidal soap every 2 days. Add predatory mites (Phytoseiulus persimilis) as biological control
3. Severe (webs everywhere): pyrethrin-based spray (kills on contact), followed by predatory mites. Consider harvesting early if in flower — pyrethrin residue on buds is undesirable
4. In flower: avoid spraying anything on buds after week 3. Use predatory mites only, or harvest early

Prevention: inspect the underside of leaves weekly with a loupe. Keep humidity above 50% during veg (mites prefer dry conditions). Never bring outdoor plants or clones from unknown sources directly into your grow without quarantine.

Fungus gnats

Threat level: Moderate. Fungus gnats are annoying and weaken plants but rarely kill them unless the infestation is extreme.

What they look like:

• Small (2–3 mm) dark flies that hover around the soil surface and fly erratically when disturbed
• Look like tiny fruit flies but with longer legs and a more mosquito-like shape
• Larvae are translucent white, 5 mm long, found in the top inch of soil

Damage signature:

• The adults don't damage the plant directly — it's the larvae that feed on roots and organic matter in the soil
• Slow growth, wilting, or yellowing that doesn't respond to nutrient changes
• In severe infestations: visible root damage when you unpot the plant
• The most obvious sign is the gnats themselves — you'll see them before you see plant damage

Where to look: soil surface, around drainage holes, flying near the base of the plant. Adults are attracted to light and often visible flying into the grow light.

Treatment:

1. Let the soil dry between waterings. Gnat larvae need moist conditions in the top inch of soil. A dry surface kills larvae
2. Yellow sticky traps catch flying adults and break the breeding cycle. Place them at soil level
3. Mosquito dunks (BTI — Bacillus thuringiensis israelensis) in the water. BTI is a biological pesticide that kills larvae specifically without affecting the plant. Crumble 1/4 of a dunk into your watering can
4. Top-dress with sand or perlite — a 1/2-inch layer of coarse sand or perlite on the soil surface prevents adults from laying eggs
5. Hydrogen peroxide drench — 1 part 3% H₂O₂ to 4 parts water. Water the soil to kill larvae. Won't harm roots at this concentration

Prevention: avoid overwatering (the #1 cause), use quality soil without excessive organic matter, and keep yellow sticky traps in the tent as early detection.

Thrips

Threat level: Moderate to High. Thrips damage leaves and, in flower, can damage buds directly. They're harder to eliminate than fungus gnats but less devastating than spider mites.

What they look like:

• Tiny (1–2 mm) slender insects, usually pale yellow, tan, or dark brown
• Move in a distinctive sliding or crawling motion on the leaf surface
• May jump or fly short distances when disturbed

Damage signature:

• Silver or bronze streaks on the top of leaves — the signature sign. Thrips scrape the leaf surface and suck the cell contents, leaving shiny, metallic-looking trails
• Small black dots (frass/excrement) on leaves near the streaks
• Leaf distortion — edges curl and wrinkle
• In flower: buds develop brown or damaged calyxes where thrips feed

Where to look: top surface of leaves (unlike mites which are on the bottom), new growth, and inside buds during flower.

Treatment:

1. Spinosad (Captain Jack's Dead Bug Brew) — the most effective organic treatment. Spray every 5 days for 3 applications
2. Neem oil — less effective than spinosad but works for light infestations
3. Predatory mites (Amblyseius cucumeris) — biological control that feeds on thrips larvae
4. Blue sticky traps — thrips are more attracted to blue than yellow (unlike gnats)

Prevention: inspect new clones or plants before introducing them to your grow. Thrips often ride in on new plant material.

Tier 2: Common but manageable

Aphids

Threat level: Moderate. Aphids are easy to spot and easy to treat, but they reproduce fast and secrete honeydew that promotes sooty mold.

What they look like:

• Small (2–3 mm) soft-bodied insects, usually green, but can be black, white, or pink
• Cluster in groups on stems, new growth, and the underside of leaves
• Often found in dense colonies — where there's one, there are dozens

Damage signature:

• Curled, distorted new growth
• Sticky residue (honeydew) on leaves below the colony
• Black sooty mold growing on the honeydew
• Yellowing and wilting of infested shoots
• White shed skins on leaves (aphids molt repeatedly)

Treatment:

1. Blast with water — a strong spray from a hose knocks aphids off (outdoor) or a spray bottle (indoor). They can't fly back easily
2. Insecticidal soap every 3 days for 2 weeks
3. Ladybugs — the classic biological control. Release 100–200 in the tent; they'll eat hundreds of aphids each
4. Neem oil as a preventive spray

Broad mites / Russet mites

Threat level: High. These microscopic mites (invisible without a 30x+ loupe) are often misdiagnosed as a nutrient problem because the damage doesn't look like typical pest damage.

What they look like:

• Invisible to the naked eye — you need a 60x+ microscope
• Broad mites are 0.2 mm, oval, translucent
• Russet mites are even smaller, elongated, tan-colored

Damage signature:

• New growth is distorted — twisted, glossy, and "wet looking" even when dry
• Leaves curl and wrinkle at the edges without typical pest marks
• Growth slows dramatically
• Often mistaken for pH problems, herbicide damage, or genetics
• By the time you diagnose correctly, the population is large

Treatment:

1. Abamectin (miticide) — the most effective treatment
2. Sulfur spray — works but requires multiple applications
3. Predatory mites (Amblyseius andersoni or Neoseiulus californicus)
4. Heat treatment — some growers report success raising canopy temperature to 115°F for 15 minutes with lights off. Risky — monitor carefully

Prevention: quarantine all new clones for 2 weeks before introducing to the main grow. Broad/russet mites most commonly enter on infested clones.

Caterpillars (budworms)

Threat level: High in flower (outdoor). The corn earworm (Helicoverpa zea) and cabbage looper burrow into cannabis buds and eat them from the inside, creating ideal conditions for bud rot.

What they look like:

• Green, brown, or striped caterpillars, 10–30 mm long
• Often hidden inside the bud — you find the damage before you find the caterpillar

Damage signature:

• Small dark pellets (frass/droppings) on and around buds
• Brown, dead sections in otherwise healthy buds (the caterpillar ate the interior)
• Bud rot developing from caterpillar damage — the wounds provide Botrytis entry points
• Holes in leaves

Treatment:

1. BTK spray (Bacillus thuringiensis kurstaki) — a biological pesticide that specifically kills caterpillars without affecting other organisms. Safe on buds up to 2 weeks before harvest
2. Hand-pick caterpillars — inspect buds daily and remove by hand
3. Spinosad — effective but avoid in late flower if possible

Prevention: outdoor growers should apply BTK preventively every 7–10 days during flower, especially after moth activity (moths lay the eggs that become caterpillars). See our bud rot guide — caterpillar damage and Botrytis are often found together.

Whiteflies

Threat level: Low to Moderate. Whiteflies are common in greenhouses and can spread viruses, but they're easy to manage.

What they look like:

• Tiny (2 mm) white-winged insects that fly up in a cloud when you shake the plant
• Rest on the underside of leaves
• Nymphs are translucent and scale-like, stuck to the leaf underside

Damage signature:

• Yellow spotting on leaves (similar to but coarser than spider mite stippling)
• Sticky honeydew on lower leaves
• Sooty mold on honeydew
• Plant weakness and reduced vigor

Treatment:

1. Yellow sticky traps — very effective for catching flying adults
2. Insecticidal soap on the underside of leaves (where nymphs live)
3. Neem oil as a preventive
4. Encarsia formosa — parasitic wasps that attack whitefly nymphs (biological control)

Quick identification table

When not to spray

In flower, after week 3–4: anything you spray ends up on the buds that will be smoked. Most sprays are safe on leaves during veg, but residue on buds is unacceptable for consumption. After early flower, rely on:

• Biological controls (predatory insects)
• Manual removal
• Environmental management (humidity, temperature)
• Harvesting early if the infestation threatens the crop

Exception: BTK (for caterpillars) is considered safe for food/flower crops up to day of harvest and is approved for organic agriculture.

Common mistakes

1. Spraying the top of leaves only — most pests live on the UNDERSIDE. If you don't spray the underside, you're missing the target
2. Spraying once and calling it done — pests lay eggs that survive treatments. You need 2–3 applications spaced 3–5 days apart to break the reproductive cycle
3. Not quarantining new clones — the #1 way pests enter a clean grow is on infested plant material. Quarantine new clones for 2 weeks in a separate space before adding them to the main grow
4. Confusing pest damage with nutrient issues — broad mites look like pH problems. Spider mite stippling looks like calcium deficiency. Always inspect with a loupe before treating with nutrients
5. Ignoring the problem because "it's just a few" — pest populations grow exponentially. 10 spider mites become 80 in a week. Act immediately at the first sign

For integrated pest management strategy, see our IPM guide. For bud rot from pest damage, see our bud rot guide. For plant recovery after pest damage, see our emergency recovery guide.