Tomato Hornworm

• Large sections of leaves and stems stripped — often overnight
• Green caterpillars up to 4 inches long, nearly invisible against stems
• Dark green or black pellet-shaped frass on leaves and soil below the damage
• Defoliated branch tips with only bare stems remaining
• Chewed unripe tomatoes with clean, scoop-shaped bite marks

The adult is a large sphinx moth that flies at dusk. It overwinters as a brown pupa in the soil, emerging in early summer. Eggs are laid singly on leaf undersides. Caterpillars feed for three to four weeks before burrowing into the soil to pupate. There are one to two generations per year depending on latitude. Parasitic wasps (Cotesia congregata) often parasitize caterpillars — white rice-grain-shaped cocoons on the caterpillar's back indicate parasitism.

1. 01Scout regularly starting in July — look for frass on leaves before looking for the caterpillar
2. 02Hand-pick caterpillars and drop into soapy water. Early morning is easiest because they don't move until warm
3. 03Leave parasitized caterpillars (those with white wasp cocoons) in place — they will not keep feeding and the wasps will kill them
4. 04Apply Bt (Bacillus thuringiensis) when caterpillars are small — it is most effective on young larvae
5. 05Cultivate soil shallowly in fall to expose overwintering pupae to birds and cold

Hornworm pressure can vary a lot year to year. If you're seeing very heavy populations three or more seasons in a row, an extension specialist can advise on whether tobacco budworm is also present and whether treatment thresholds in your situation warrant a different approach.

• Tomato Hornworm— University of New Hampshire Extension
• Hornworms on Tomato— UC ANR Integrated Pest Management