# Caterpillars and budworms on cannabis

> Caterpillars, mainly corn earworm (Helicoverpa zea) outdoors, bore into buds, leave frass and start bud rot; scout from early flower, use pheromone traps for moth flights, exclude moths with netting, and use Bt (kurstaki or aizawai) while larvae are small, since it must be eaten and breaks down in sunlight within days.

- Canonical: https://genetixbay.com/knowledge/pests/caterpillars.html
- Category: Pests · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Flower, Ripening · Systems: Soil, Living soil, Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What is eating my buds?** Buds with brown dead patches, small holes and dark crumbs of frass inside are usually being eaten by caterpillars. Outdoors in North America the main culprit is the corn earworm (Helicoverpa zea), with the Eurasian [hemp](https://genetixbay.com/knowledge/glossary/hemp.md) borer tunnelling stems and buds in some areas. Indoors caterpillars are uncommon and usually arrive through open intakes or doors. Open a damaged bud to check for tunnels.

**Why is there brown mush inside my [cola](https://genetixbay.com/knowledge/glossary/cola.md)?** Brown mush inside a cannabis cola is [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md), and if dark frass pellets and tunnels are present a caterpillar started it. Larvae bore into the bud, and their wounds and frass let Botrytis in, so the loss is often more rot than chewing. Rot without frass is plain bud rot. Cut out the damaged section with a margin and bag it.

**How do I stop budworms on outdoor cannabis?** Stop budworms on outdoor cannabis by scouting upper colas for eggs and young larvae from early flower, hanging pheromone traps at the border by late July or early August to time moth flights, and covering plants with fine insect netting where practical. Hand-pick larvae at dusk or dawn. If you treat, use Bt on larvae under about 12 mm (0.5 in).

**Is Bt safe to use in flower?** Bt (Bacillus thuringiensis subsp. kurstaki or aizawai) is one of the few biological options listed for hemp in some US states, but whether it is permitted on cannabis in flower depends on where you live, so check registration and follow the label. It only works when larvae eat it and breaks down in sunlight in under a week. Avoid leaving buds wet overnight.

**When do caterpillars attack cannabis?** Caterpillars attack cannabis mainly in flower, when moths lay eggs singly on developing buds and nearby leaves. Corn earworm eggs hatch in about a week, and larvae reach about 25 mm (1 in) in 2 to 3 weeks while feeding on and inside flowers. Start scouting in early flower and use pheromone traps from late July or early August to know when moths are flying.

**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 veg.
- 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](https://genetixbay.com/knowledge/glossary/sugar-leaf.md) 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](https://genetixbay.com/knowledge/glossary/bract.md) and [pistils](https://genetixbay.com/knowledge/glossary/pistil.md).

## 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](https://genetixbay.com/knowledge/glossary/action-threshold.md) | none established for hemp | Scout buds for eggs and larvae [1] |

## What to check first

1. **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](https://genetixbay.com/knowledge/diseases/botrytis-bud-rot.md).
2. **Scout** from early flower: inspect upper colas and bracts for eggs and young larvae [1].
3. **Trap moths** to know when flights start [1].
4. **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].
- 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).

### 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](https://genetixbay.com/knowledge/glossary/beauveria-bassiana.md):** combined with NPV in Oregon trials [1].
- **Trichogramma egg parasitoids** are used against Helicoverpa in other crops.
- **Natural predators** (birds, wasps, [lacewings](https://genetixbay.com/knowledge/glossary/lacewing.md)) 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 jurisdictions. 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.

## Evidence

- Corn earworm lifecycle and bud damage: *Established*, mostly from other crops, with hemp extension confirmation [1][3].
- Bt and NPV [efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) on hemp: *Supported* by field trials [1][2].
- Exclusion netting: *Grower practice*; little cannabis-specific research.

## Sources

1. Oregon State University Extension. Corn earworm management in hemp (EM 9626). https://extension.oregonstate.edu/catalog/em-9626-corn-earworm-management-hemp
2. Virginia Tech. Corn earworm pest problems on hemp and results of recent insecticide tests. https://ag-pest-advisory.vaes.vt.edu/corn-earworm-pest-problems-on-hemp-and-results-of-recent-insecticide-tests/
3. Utah State University Extension. Corn Earworm (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-corn-earworm
4. Cranshaw, W. Hemp pests (CSU, 2020). https://webdoc.agsci.colostate.edu/bspm/InsectInformation/Talks2020/HempPests.pdf
5. Utah State University Extension. Hemp bud rot. https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot