Tobacco hornworms and tomato hornworms are large, blue-green caterpillars with a black spine (or horn) protruding from their last abdominal segment.  These insects do not typically reach damaging levels in commercial fields, however, large numbers of larvae can sporadically occur in home gardens.  Hornworms feed only on solanaceous plants, most typically tomato.  The larvae will also attack eggplant, pepper and potato.  Solanaceous weeds such as horsenettle, jimsonweed and nightshade may serve as alternate hosts.

Appearance

Hornworms are easy to identify by their blue-green color and large size.  When fully grown they can reach up to 4 inches in length.  The caterpillars have 7 (tobacco) or 8 (tomato) white stripes on each side of their bodies.  A large black spine protrudes from the posterior end of the worm, giving rise to the name “hornworm.”  The adult moth, sometimes referred to as the “sphinx”, “hawk”, or “hummingbird” moth is a large heavy-bodied moth with a wingspan of up to 5 inches.  They become active in July and are often mistaken for hummingbirds due to their size and quick movements.

Symptoms and effects

Adult hornworms are nectar feeders and do not harm tomato and related plants.  Larvae primarily feed on tomato leaves but will occasionally feed on green fruit.  Damaged fruit usually decay on the vine before harvest or will retain a dry scar.  Hornworm larvae blend in with the plant canopy and go unnoticed until most of the damage is done.  Hornworms can devour up to 4 times their weight in food each day.  Although capable of defoliating a tomato plant, the hornworm larvae are usually noticed before this occurs.  Hornworms rarely cause economic damage to tomatoes in Wisconsin.

Life Cycle

Tomato hornworms overwinter as pupae in the soil and become most active in mid to late summer.  The adults emerge in late June and lay a single pale green, spherical egg on the lower leaf surface of tomato plants.  Upon hatching, the larvae immediately begin feeding and feed continually for about 1 month before dropping from the plant to pupate.  In the upper Midwest there can be up to 2 generations of hornworms per year.

Control

Hornworms rarely cause enough damage to warrant the use of insecticides.  In smaller plantings and gardens, frequent monitoring from early July through August and hand picking the larvae from infested plants is recommended.  Tilling the soil after harvest is recommended to destroy the burrowing larvae which are attempting to pupate.  Tillage has been shown to cause up to 90% mortality.

The egg stage and early instar larvae are often preyed upon by various predatory insects such as lady beetles, green lacewings and predatory wasps.  Trichogrammatid wasps offer natural control by parasitizing hornworm eggs.  Braconid wasps often lay their eggs on the bodies of the hornworm larvae.  When the eggs hatch, the larval braconids feed inside the caterpillar.  If left unharmed, these parasitized caterpillars will produce wasps that can repeat the process on other hornworms, thereby providing a continual source of biological control.

Commercial tomato growers with large acreages of tomatoes should monitor their fields and treat them if an average of more than two hornworms per plant is detected.  Infestations are often localized and spot treatments will usually take care of the problem.  With all products, treatments should be applied when larvae are still in the early instars, as late instars are difficult to kill.  There are several insecticides available to control the tomato hornworm.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

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