Hornworms

7/14/22
This resource is offered as part of MOFGA’s Pest Reports Fact Sheet Series

Pest: Hornworms (Manduca sexta, M. quinquemaculata)

Pest/disease identification and lifecycle, most common damage symptoms and crops affected:

Hornworms are probably the most destructive insect that attacks tomatoes. They are giant caterpillars that grow rapidly and can do a vast amount of eating in a very short amount of time. Sometimes it seems that healthy-looking tomato or pepper plants are stripped of their leaves overnight, leaving bare stems. The hornworms will also attack fruit, eating gouges out so large that they look more like bites of a furry animal pest than an insect.

Hornworm stages

Look for feeding damage and frass (insect poop) around tomato and pepper plants. The frass is often the first noticeable sign of their presence. The frass of young hornworms is small, but the hornworms grow larger quickly and so does the frass. Many growers don’t bother looking for the caterpillars because they blend in so well, and instead just look for the frass.

Tomato hornworm frass

The adults are large, fast-flying hawk moths, which in flight may look like a hummingbird. At dusk they hover over flowers sucking nectar. Eggs are laid on tomato leaves and hatch in five days. Adults are most often found dead or dying among or nearby a tomato crop after having laid their eggs.

Hornworm moth. Christa Bahner photo

Management options:

Hornworm caterpillar on tomato plant

Cultural:

Hand picking hornworms is a bit frightening but does work and you can bring them to chickens who enjoy fighting with the challenging pest. The problem with hand picking is that hornworms blend in very well and it is easy to overlook one or two caterpillars, which can do significant damage in a day or two. Many growers report that using a black light at night helps to find them, as the caterpillars fluoresce brightly under ultraviolet light.

Organic pesticides (as a last resort):

Bt works very well on these caterpillars, as long as it is still feeding. Spinosad is also very effective against these caterpillars, but Bt is preferred as it has lower potential to negatively impact non-target species.

What are those white things on tomato hornworms? Tomato hornworm larvae are parasitized by a number of insects. One of the most common is a small braconid wasp, Cotesia congregatus. Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate. The cocoons appear as many small white projections protruding from the hornworm’s body. Parasitized hornworms should be left in the field to conserve the beneficial parasitoids. The wasps will kill the hornworms when they emerge from the cocoons, and will seek out other hornworms to parasitize. Photo by Vern Grubinger
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