Caterpillars are one of the sneakiest pests in an outdoor grow. By the time you see obvious damage, they’ve often been at it for days. Here’s how to get ahead of them.
The main culprits
- Budworms / Corn earworms - burrow directly into buds and are nearly impossible to spot until there’s damage
- Cabbage loopers - light green, arch their back when moving, eat leaves
- Hemp borers - bore into stems and branches
Signs you have caterpillars
- Small holes in fan leaves or buds
- Dark green/brown frass (droppings) on leaves or in buds
- Yellowing patches that don’t match any nutrient deficiency
- Silky webbing on leaves (some species)
- Bud rot appearing earlier than expected (caterpillar damage creates entry points for botrytis)
Prevention (start here)
- BT (Bacillus thuringiensis) - a naturally occurring bacteria that kills caterpillars without harming other insects, pets, or plants. Spray every 7–10 days from mid-veg onwards. This is the gold standard for outdoor growers.
- Spinosad —-another organic option, slightly broader spectrum
- Inspect plants daily during the last 4–6 weeks before harvest — this is when budworms do the most damage
If you already have them
- Remove visible caterpillars by hand (check under leaves and inside buds)
- Apply BT immediately —-it needs to be ingested to work, so coat leaves and bud sites
- Check for bud rot around any damage sites
- Repeat treatment after rain