Quick answer

Pest damage on cannabis shows as stippling, chewed holes, webbing, honeydew, frass, and curled new growth. Check the undersides of leaves, the top inch of soil, and the inside of buds. Use a 10x to 30x loupe. Scouting takes 5 minutes per plant, once per week.

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Leaf damage

Spider mites

Spider mites leave pale dots on the top of leaves. The dots cluster near leaf veins. Later, fine webbing appears at the joint where the leaf stem meets the main stem. Leaves turn yellow, then bronze, then dry. Mites sit on the underside. A 10x loupe shows moving specks the size of a grain of salt.

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Broad mites and russet mites

Broad mites and russet mites are 0.1 mm to 0.2 mm long. They need a 30x to 60x loupe. Damage shows on new growth. Leaf edges curl up or down. Leaves look glossy or oily. Stems turn brown or russet. Growth stops. Buds stay small.

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Thrips

Thrips scrape leaf cells. Damage appears as silver or white streaks. Black specks of frass sit near the streaks. Leaves twist and grow with a leathery texture. Thrips also scar buds. A 10x loupe shows slender insects that move fast.

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Aphids

Aphids cluster on new shoots and leaf undersides. They excrete honeydew, a sticky clear liquid. Sooty mold grows on the honeydew. The mold is black and blocks light. Leaves curl. White shed skins stick to the leaf surface. Ants walk on the plant to farm aphids.

Whiteflies and mealybugs

Whiteflies fly up when you shake the plant. Nymphs sit under leaves and make honeydew. Mealybugs look like white cotton on stems and leaf nodes. Both pests stunt growth and cause yellow leaves.

Stem and root damage

Fungus gnats

Fungus gnat adults are small black flies. They walk on the soil surface. Larvae are clear to white, 5 mm long, with black heads. Larvae feed on roots and root hairs. Plants wilt between waterings. Roots turn brown.

Root aphids

Root aphids look like white powder or brown specks on roots. Honeydew coats the inside of the pot. Roots die. Leaves yellow and drop. Plants stop drinking water. The top of the medium dries in 1 day.

Bud damage

Caterpillars and budworms

Caterpillars chew holes in buds. Brown frass pellets sit on the bud surface. Damage opens the bud to Botrytis, a gray mold. Mold turns buds brown and soft. Check buds at week 4 of flower and every 7 days after that.

Look-alike problems

Some signs come from other causes. Nutrient burn makes leaf tips brown. pH problems cause spots and twisted leaves. Light burn bleaches the top leaves. Wind burn makes leaf edges dry. Spider mite stippling differs: the dots are tiny and sit between veins. Thrips streaks are silver. Check for insects before you change nutrients.

Scouting steps

  1. Use a 10x loupe or a 30x microscope.
  2. Pick 3 leaves per plant: 1 top, 1 middle, 1 bottom.
  3. Turn each leaf over. Look at the veins and the leaf base.
  4. Tap a leaf over white paper. Mites and thrips fall off.
  5. Hang yellow sticky cards at canopy height. Count insects after 7 days.
  6. Check buds with a loupe at week 4 and week 6 of flower.
  7. Look at the top 1 inch of soil for flies and larvae.

Write down the count and the date. A count of 5 or more spider mites per leaf calls for treatment. A count of 10 or more thrips on a sticky card calls for treatment.