Hornworms

Hornworm eggs are laid singly on leaves. Eggs are round to oval, 1.5 mm in diameter, and white to light green.

While both species of hornworms have a large horn on the posterior end of the body, the tobacco hornworm has seven diagonal stripes on each side of the body in contrast to the tomato hornworm, which has eight chevron-shaped stripes. Larvae feed for 3 or 4 weeks, then burrow into the soil to pupate.

The adult moth is a strong flier with a wingspan up to 5 inches (12 cm). Development takes about 2 months in summer; the winter is passed in the pupal stage. There are two generations a year in most areas; larvae are usually most common in midsummer, but there may be a small population peak in late summer. Infestations tend to be more severe in warm inland areas.

Hornworms feed on blossoms, leaves, and fruit. At high populations they can extensively defoliate plants and scar the fruit. They are rarely a problem in the warmer interior valleys unless natural enemies are disrupted, in which case, they can do serious damage. They are mostly problems in garden situations.

In commercial tomato fields, natural enemies, crop rotation, and discing after harvest play a key role in keeping hornworm populations below damaging levels. Conserve natural enemies by not treating with disruptive pesticides, especially early in the season before fruit begin to mature.

There are several important naturally occurring parasites that help control hornworms in tomatoes. Hornworm eggs are attacked by Trichogramma parasites and the larvae by Hyposoter exiguae. Trichogramma released for control of tomato fruitworm will also attack hornworm eggs.

Discing after harvest destroys pupae in the soil. Rotations with crops that are not attacked by hornworms will also help to keep population levels low in individual fields.

Biological and cultural controls as well as Bacillus thuringiensis sprays are acceptable for use on an organically certified crop.

Treat hornworms only if they are causing extensive foliage damage, or if they are feeding on fruit. Hornworm damage can be assessed as part of the sampling guidelines and thresholds listed under beet armyworm. Look for hornworm larvae on plants that have severe foliar damage as you sample to determine if damage is the result of hornworm or armyworm activity. In addition, hornworm feeding produces larger, deeper cavities than those caused by beet armyworm. Consider spot-treating sections of a field where hornworm damage is found because it is rare for an entire field to become infested.