Bt (kurstaki) for tomato hornworm
Bt only works if the hornworm eats it, so thorough coverage of the leaves it is feeding on is essential — every label stresses it. It works best on very young larvae, and smaller larvae die faster. A full-grown hornworm is easier to pick off by hand. Rain and irrigation wash it off and sunlight breaks it down within days, so reapply after heavy rain. Leave any hornworm carrying white cocoons alone — it is already parasitised.
Before you order
A bacterium sprayed on leaves. Caterpillars that eat it stop feeding and die within a few days. It works only on the larvae of moths and butterflies, and has little or no effect on bees, lady beetles, lacewings or parasitic wasps. How it works, how to tell it has established and the supplier list are on the full Bacillus thuringiensis kurstaki profile.
Other controls for tomato hornworm
- Braconid waspsWild parasitoid · Nothing to release — leave the caterpillar where it is
- Trichogramma spp.Egg parasitoid wasp · About 100,000 /acre (UC IPM's rate for tomato fruitworm, whose releases also hit hornworm eggs); 1–2 /ft² in gardens (Sound Horticulture)