Tomato Hornworms: Where They Come From and How to Get Rid of Them

Where Do Tomato Worms Come From?

The big green tomato worm is the caterpillar of a hawkmoth. A large gray-brown moth lays single eggs on tomato leaves, the eggs hatch in about 5 days, and the caterpillars feed for about a month before they burrow into the soil. To get rid of hornworms in a home garden, pick them off by hand and drop them into soapy water. They seem to appear from nowhere because the moth flies in the evening and the caterpillar is the same green as the leaves.

The full cycle has four stages, and three of them are easy to miss. Hornworms spend the winter as pupae in the soil. University of Maryland Extension reports that the adult moths emerge over a long period, as early as May and as late as early August. Utah State University Extension puts the life of the adult moth at 2 to 3 weeks, which is long enough to mate and lay eggs on tomatoes, peppers, eggplant and potatoes.

The caterpillar starts eating as soon as it hatches. It passes through about five growth stages in roughly one month and ends up 3 to 4 inches long. Then it drops to the ground, burrows 4 to 6 inches down and turns into a dark brown pupa 2 to 3 inches long. Most regions see one or two generations a year, so a second wave of caterpillars can show up in late summer after the first has gone to ground.

The Tomato Worm Moth: What the Adult Looks Like

The adult tomato hornworm is the five-spotted hawkmoth. It is a heavy-bodied gray and brown moth with a wingspan of up to 5 inches, large front wings, small zigzag-marked hind wings, and a row of five yellow spots along each side of the abdomen. The adult of the tobacco hornworm is the Carolina sphinx, which is slightly smaller at up to 4 inches across and carries six yellow spots on each side.

Both moths fly in the evening and hover in front of flowers while they drink nectar through a long, coiled tongue. People often mistake them for hummingbirds. The moths do no damage themselves, and seeing one near the garden at dusk is a useful warning to start checking leaves for eggs.

Hornworm Eggs on a Tomato Plant

Hornworm eggs are smooth and round, about 1/16 inch in diameter, and range from pale green to white. Clemson’s fact sheet notes that they start light green and turn white before hatching. The moth lays them one at a time instead of in clusters, usually on the underside of a leaf, though they also turn up on the upper surface.

Because each egg sits alone, they are slow to find on a big plant. Hornworms begin on the upper leaves, so check the top of the plant first. Rub off any egg you find with a thumb, or pinch off the leaflet. Lady beetles and green lacewings eat hornworm eggs, and tiny Trichogramma wasps parasitize them, so some eggs never produce a caterpillar.

Tomato Hornworm vs. Tobacco Hornworm

Two species feed on garden tomatoes, and many gardens have both. Each is a large green caterpillar with a horn on the rear end, and each reaches 3 to 4 inches. Control is identical, so the difference matters mostly for curiosity.

Feature	Tomato hornworm	Tobacco hornworm
Side markings	Eight white V-shaped marks	Seven diagonal white stripes
Horn	Black and straighter	Red and curved
Adult moth	Five-spotted hawkmoth	Carolina sphinx
Yellow spots on each side of the moth	Five	Six
Moth wingspan	Up to 5 inches	Up to 4 inches

The horn looks alarming but it is harmless. Tomato fruitworms feed on the same plants and are significantly smaller. If the holes on your plants don’t match a hornworm’s work, the leaf damage identification chart covers the other common culprits.

Hornworm Frass and Other Signs of Damage

Frass is the caterpillar’s droppings, and it is usually the first thing a gardener notices. Hornworm frass is dark green to black and blocky, and Rutgers notes that the pellets can look like small stars or flowers. It collects on leaf surfaces and on the ground under the plant. Pellet size tracks caterpillar size, and a late-stage hornworm leaves pellets as big as a pea.

The feeding itself starts at the top. Hornworms eat the upper leaves first and work downward, leaving only stubs. When foliage runs short they chew large pieces out of green fruit. University of Illinois Extension notes that one or two caterpillars can defoliate a five-foot tomato plant in less than three days, which is why a plant can look fine on Monday and stripped by Thursday. Damage is most common in midsummer, with a smaller peak possible in late summer.

How to Find a Large Tomato Caterpillar

Start from the frass and look straight up, since the pellets drop from wherever the caterpillar is feeding. Then follow the nearest stripped stem back toward the main stalk and turn over the leaves around it.

A few habits make the search faster:

1. Begin checking plants in early July, before the midsummer peak.
2. Scout in the early morning or the evening. The caterpillars tend to hide during the day.
3. Shake part of the plant over a sheet of cardboard to knock loose any you missed.
4. Go out after dark with a handheld UV flashlight. Under black light the tomato foliage looks deep red or orange and the hornworms show up bright green.

Check nearby peppers, eggplant and potatoes as well, because both species feed on the whole nightshade family.

How to Get Rid of Hornworms on Tomatoes

Handpicking gives enough control in most home gardens, and extension services recommend it ahead of any spray. Pull the caterpillar off the stem and drop it into a bucket of soapy water. Garden shears work too if you would sooner not touch them.

Leave one kind of hornworm alone. A caterpillar carrying white cocoons that look like grains of rice on its back has been parasitized by a braconid wasp. The wasp larvae fed inside it and have come out to pupate. That hornworm has stopped feeding, and the wasps that hatch from the cocoons will go on to attack others.

For a large planting, two insecticides are widely listed for hornworms: Bacillus thuringiensis, sold as Bt, and spinosad. Both work best on small caterpillars. A young larva needs a smaller amount of Bt than a 4-inch one, so spray when you first see eggs hatching or small chewing damage and pick the big ones by hand. Follow the label for rates and for the waiting period before harvest. The same approach works for other caterpillars in the vegetable garden, including cabbage worms.

Stop Next Year’s Hornworms in the Soil

The pupa is the weak point. It sits 4 to 6 inches down for the whole winter and cannot move away from a tiller or a fork. Wisconsin, Illinois and Rutgers extension publications all report that tilling after harvest can destroy up to 90 percent of them. Tilling in spring before planting also turns pupae up to the surface.

In a no-till bed, you can still thin them out. Destroy any pupae you uncover while planting or digging. They are easy to recognize by the curved handle on one end, which holds the developing moth’s mouthparts. Gardeners who grow in containers should watch for pupae when they empty or refresh old mix, and the steps for reusing old potting soil cover that job.

Protecting the insects that hunt hornworms pays off the following summer. Paper wasps take small caterpillars, and the braconid wasps handle many of the large ones. Ohio State University notes that broad-spectrum insecticides such as carbamates and pyrethroids disrupt the wasp that parasitizes hornworms, which is one more reason to rely on handpicking. Our guide to companion herbs for pest control covers plantings that go with this approach.