Short answer. If a cola has a brown, dead patch with dark crumbs (frass) inside, a caterpillar bored into it and bud rot followed. Outdoors the usual culprit is corn earworm. Start scouting in early flower, trap moths to know when they are flying, cover plants with fine netting where practical, and if you treat, use Bacillus thuringiensis (Bt) on small larvae: it only works when eaten and lasts only a few days in sunlight.
What's happening
Moths lay eggs on or near developing buds; larvae feed on and inside flowers. The wound and frass let Botrytis in, so the loss is often more rot than chewing [1][5].
Main species: - Corn earworm / budworm (Helicoverpa zea): the key caterpillar of outdoor hemp and cannabis in North America [1][3][4]. - Eurasian hemp borer (Grapholita delineana): small larvae that tunnel stems and buds (listed as an emerging hemp pest by CSU [4]). - Others: beet armyworm, cabbage looper and other leaf-feeders mainly in vegbeing reviewed. - Indoors, caterpillars are uncommon and usually arrive via open intakes or doors.
Lifecycle (corn earworm). Females lay up to ~25 eggs a day and about 1,500 over a 2–3 week life, singly on flower buds and nearby leaves. Eggs hatch in about a week; larvae go through five instars and reach ~25 mm (1 in) in 2–3 weeks; mature larvae drop to the soil to pupate, and summer adults emerge about two weeks later. In autumn pupae overwinter in soil [1]. Hemp-specific data are limited; much comes from corn, cotton and tomato [1].
How to recognise it
- Brown, wilted or dead sections of cola, sometimes a single dead sugar leaf poking out of a bud.
- Dark frass pellets in buds or on leaves below.
- Small holes in leaves (early instars), webbing in buds (hemp borer).
- Larvae: mostly green on hemp for corn earworm [1], striped along the body; up to 25 mm.
- Moths at dusk; pale, single eggs on bracts and pistils.
Numbers that matter
| Parameter | Value | Notes |
|---|---|---|
| Eggs per female | up to ~25/day; ~1,500 lifetime | [1] |
| Egg hatch | ~1 week | [1] |
| Larval development | 2–3 weeks to ~25 mm | [1] |
| Bt use window | larvae under ~12 mm (0.5 in) | [3] |
| Bt persistence on foliage | often < 7 days in sunlight | [1] |
| Re-treatment interval used in trials | every 2–3 days during heavy moth pressure | Trial practice, VA Tech [2]; follow your label |
| Trap timing | pheromone traps at the border by late July/early August, check twice weekly, change lures every 2 weeks | Oregon guidance [1] |
| Action threshold | none established for hemp | Scout buds for eggs and larvae [1] |
What to check first
- Is it caterpillar damage or plain bud rot? Open the bud: frass and tunnels mean caterpillar. Rot without frass: see kb-diseases-botrytis-bud-rot.
- Scout from early flower: inspect upper colas and bracts for eggs and young larvae [1].
- Trap moths to know when flights start [1].
- Indoors: check intake screens and door habits.
How to fix it
Registrations differ by jurisdiction. Bt products are among the few options listed for hemp in some US states, but check what is permitted on cannabis where you live and follow the label exactly [1][2]. No product names or rates are given here.
Cultural
- Remove crop debris after harvest and work the soil where allowed; pupae overwinter in soil [1]being reviewed.
- Keep weeds and alternate hosts (e.g. corn, tomato nearby) in mind; moths move in from them.
- Plant date can shift flowering away from peak moth flights (Grower practice)being reviewed.
Physical
- Exclusion: fine insect netting or a greenhouse with screened vents keeps moths off (Grower practice; strong in principle).
- Hand-picking: walk plants at dusk or dawn; remove larvae and cut out damaged bud sections, bagging them.
- Pheromone traps for monitoring only; lures attract males [1].
Biological
- Bacillus thuringiensis subsp. kurstaki and subsp. aizawai: bacterial insecticides; larvae must eat it, so coverage of buds matters; apply early morning or evening; short residual in sunlight [1][2][3].
- Nucleopolyhedrovirus (NPV) of Helicoverpa: a virus-based biological, effective in Oregon trials [1].
- Beauveria bassiana: combined with NPV in Oregon trials [1].
- Trichogramma egg parasitoids are used against Helicoverpa in other cropsbeing reviewed.
- Natural predators (birds, wasps, lacewings) help outdoors; avoid broad sprays that kill them.
Least-toxic options
- The biologicals above are the least-toxic options. Spinosyn-class (spinosad) materials are labelled on hemp in some jurisdictionsbeing reviewed. Check residue rules for cannabis testing.
What not to do in flower
- Do not use synthetic broad-spectrum insecticides on buds; most are not registered on cannabis and residues fail testing.
- Do not spray once and assume protection; Bt breaks down within days [1].
- Do not apply sprays late in the day in a way that leaves buds wet overnight in cool weather; that feeds bud rot.
- Do not leave damaged bud sections on the plant; cut a margin around them.
How to confirm it worked
- No new frass or fresh dead bud sections on twice-weekly checks.
- Trap counts fall as the season ends.
- At harvest, open buds: no larvae, no rot pockets.
Common mistakes
- Waiting until buds rot to start scouting.
- Spraying Bt on large larvae inside buds where it never reaches them.
- Ignoring the rot that follows and letting it spread at drying.
