Braconid wasps for tomato hornworm
A hornworm covered in white cocoons has already lost. The wasp larvae fed inside it, then chewed out to pupate, and NC State says a parasitised hornworm eats only about a fifth as much as a healthy one. It stops feeding and dies while the wasps pupate on its back (UF). Killing it destroys the next generation of wasps, which is why UMN and NC State say to leave it on the plant. A clean hornworm with no cocoons is the one to pick off.
Before you act
Small parasitic wasps (the common Cotesia species are only 2–3 mm long) whose larvae develop inside a caterpillar or an aphid and kill it. The white, rice-grain cocoons standing up on the back of a hornworm are theirs, and a hornworm carrying them should be left on the plant. How to recognise it, what it is confused with and how to keep it around are on the full Braconid wasps profile.
Other controls for tomato hornworm
- Bacillus thuringiensis kurstakiMicrobial insecticide · Label rate — Thuricide 1–3 tsp per gallon for tomato hornworm; Monterey B.t. 4 tsp per gallon
- 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)