How to Identify Cannabis Pest Damage: Leaves, Stems, Roots, Buds
Pest damage on cannabis shows as stippling, holes, webbing, honeydew, and frass. Check leaf undersides and buds with a 10x loupe.
Bud worms on cannabis are the larvae of moths, most often the corn earworm (Helicoverpa zea). You stop them with three moves: scout flowers at dusk, hand-remove every caterpillar you find, and spray Bacillus thuringiensis var. kurstaki (Bt) every 5 to 7 days while moths are active. Spraying alone fails once a worm is boring inside a cola, so inspection comes first and spraying second.
How to Identify Cannabis Pest Damage: Leaves, Stems, Roots, Buds
Why Bud Worms Are a Serious Problem
A single caterpillar can chew through several grams of flower in a few days. The damage is only half the issue. Caterpillars leave frass and open wounds, and those wounds are the entry point for Botrytis, the gray mold that turns a cola to mush and spreads to neighboring plants. A worm you catch early costs you a small section of bud. A worm you miss in week six can cost you the plant.
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Prerequisites
- A bright flashlight or headlamp
- A white paper plate or sheet of paper
- Tweezers and disposable gloves
- A jar of soapy water
- Clean pruning shears wiped with isopropyl alcohol
- An OMRI-listed Bt kurstaki product
- A pump sprayer reserved for biologicals
- Pheromone traps rated for corn earworm
- A hand lens or loupe
Step-by-Step: How to Stop the Worms
- Start scouting as soon as the first flowers form, and repeat twice per week.
- Walk the garden at dusk or with a headlamp after dark, when larvae leave the bud to feed on the surface.
- Hold a white plate under a cola and tap the stem to knock loose frass, eggs, and small worms.
- Inspect the plate and the bud surface with a hand lens for dark green pellets, silk, and chewed calyxes.
- Pick any caterpillar off with tweezers and drop it into the jar of soapy water.
- Cut away webbed, browned, or frass-covered flower with clean shears, removing about an inch below the damage.
- Open the cut section to check for gray fuzz or brown mushy tissue, which signals Botrytis.
- Bag and discard any moldy material off-site. Do not compost it.
- Mix Bt kurstaki at the label rate and fill the sprayer with fresh solution.
- Spray in the evening so ultraviolet light does not break the Bt down before larvae eat it.
- Cover the entire plant, including leaf undersides and the outside of every cola, until the solution drips.
- Repeat the Bt spray every 5 to 7 days while pheromone traps keep catching adult moths.
- Switch to spinosad if you find larvae longer than half an inch, since Bt works best on small worms.
- Check the pre-harvest interval on the label and stop spraying in time to meet it.
- Wash harvested buds in a hydrogen peroxide and water bath to remove frass, eggs, and mold spores.
Prevention for the Next Run
- Drape 40-mesh exclusion netting over outdoor plants before moths start flying in midsummer.
- Hang corn earworm pheromone traps around the perimeter and check them weekly for moth counts.
- Release Trichogramma wasps, which parasitize moth eggs before they hatch.
- Cut nitrogen rates in late flower, since soft, dark growth attracts egg-laying females.
- Turn off outdoor lights near the garden at night to reduce moth traffic.
- Clear crop debris and till the top few inches of soil after harvest to destroy pupae.
- Rotate cannabis away from corn, tomato, and pepper plots, which share the same pest.
What Not to Do
Do not spray during the day, because sunlight cuts the effective life of Bt. Do not use broad-spectrum insecticides in flower, because they kill the predatory insects and spiders that eat worm eggs. Do not leave a chewed cola on the plant hoping it will finish, since the mold will not stay in one place. Do not reuse spray solution that sat in the tank overnight.