QUICK ANSWER
Tomato hornworm is the big green caterpillar that eats whole tomato leaves from the top of the plant down, and it is easy to miss because its color matches the foliage. Look for dark, blocky droppings on the leaves or the soil, then search the stems and the undersides of leaves. Handpick any caterpillar you find and drop it in soapy water, but leave one alone if white, rice-like cocoons cover its back. Those are braconid wasps developing, and they finish the job for you.
Tomato Hornworm: What It Looks Like and Why You Cannot See It
A full-grown tomato hornworm is a bright green caterpillar that reaches 3 to 4 inches long, which makes it one of the largest caterpillars in a home garden. Even so, it is hard to find. Its green body blends in with tomato leaves, and it feeds in the upper part of the plant where you are not looking.
Here is what to look for:
• Size and color. Large caterpillars are green and up to about 4 inches long. Newly hatched ones are pale yellow to white with no markings, so they are easy to overlook until they grow.
• V-shaped markings. Big tomato hornworms carry eight white V-shaped marks along their sides and a black horn on the tail end.
• A red horn means a different species. The tobacco hornworm looks almost identical but has a red horn and seven diagonal stripes instead of V shapes. Both feed on tomato plants and are handled the same way.
Because the caterpillars hide so well, you will usually spot the damage and the droppings long before you spot the animal.
Signs a Tomato Hornworm Is Feeding on Your Plant
The clearest sign is that whole leaves disappear. Hornworms eat whole leaves rather than leaving small holes, and they can strip a plant fast. Feeding starts at the top, so a plant can look fine from the side while the upper branches are already bare stems.
• Droppings are the tell. Frass is the best early warning. Look for dark green or black pellets scattered on the leaves or on the soil beneath the plant. The bigger the pellets, the bigger the caterpillar above, and late-stage ones leave droppings about the size of a pea.
• Bare stems and missing leaves. Damage is often concentrated on interior leaves or scattered across the plant. Deer damage, by comparison, usually strips one side of the plant.
• Gouges in ripe fruit. Older caterpillars also eat tomatoes, leaving large scooped-out gouges, especially in dry weather.
• Most of the damage comes late. The final growth stage eats nearly as much as all the earlier stages combined, which is why a plant can seem untouched one week and shredded the next.
If your plants are in the danger window, check them at least twice a week. Spraying the plant with water can also dislodge caterpillars so they are easier to see.
How to Get Rid of Tomato Hornworms
Handpicking is the most effective method for a home garden, and it works because the caterpillars are so large. Follow the droppings up the plant, check the stems and the undersides of leaves, and toss each caterpillar into a bucket of soapy water.
• Use Bt on young caterpillars. Bacillus thuringiensis (Bt) only affects caterpillars and does not harm bees. The caterpillars have to eat it for it to work, and it works best on very young ones, so spray before they get big. Always follow the label.
• Skip the broad-spectrum sprays. Insecticides are rarely needed for hornworms, and harsh residual sprays can kill the beneficial insects that keep them in check.
When to Leave a Tomato Hornworm Alone: White Cocoons
If a hornworm has white cocoons that look like grains of rice sticking out of its back, do not kill it. Those cocoons belong to braconid wasps, which lay their eggs on the caterpillar. The wasp larvae feed inside it and then spin cocoons on the outside.
A parasitized hornworm stops eating and dies, so it will not do more damage. Leaving it in the garden lets the wasps finish their life cycle and go on to parasitize other hornworms. That is free pest control, and it is the reason handpicking comes with one rule: leave the ones wearing cocoons.
Other predators help too. Lady beetles, minute pirate bugs, green lacewing larvae, and spined soldier bugs eat hornworm eggs and young caterpillars, which is one more reason to avoid sprays that kill everything.
Where Tomato Hornworms Come From and How to Stop Them Next Year
Understanding the life cycle tells you what to do in fall. Hornworms spend the winter as pupae in the soil. Full-grown caterpillars dig 4 to 6 inches into the soil without spinning a cocoon and turn into reddish-brown pupae about 2 inches long that wait out the winter underground. Large gray-brown moths emerge in spring and summer and lay single eggs, usually on the underside of leaves. There are one to two generations a year, and the second one feeds into late summer or early fall.
That gives you two jobs once the season is over:
• Turn the soil after harvest. Tilling or turning the bed in fall or spring can expose pupae to weather and predators, and it can destroy burrowing caterpillars.
• Pull weeds that host moths. Weeds in the nightshade family, such as horsenettle, jimsonweed, and nightshade, give moths more places to lay eggs.
Is It a Tomato Hornworm or Something Else?
Chewing damage has several causes. Whole leaves gone from the top down, plus dark pellets, points to hornworms. Small holes chewed in leaves, rather than whole leaves missing, point to other insects, and damage on one side of the plant suggests deer or other animals. If you cannot tell, run your symptoms through the Plant Problem Finder. Flowers dropping instead of leaves disappearing is a different issue, and blossom drop from heat covers that one.
Hornworms also feed on pepper plants and other crops in the same family, so check those if tomatoes are being hit.
Related Questions
What is the difference between a tomato hornworm and a tobacco hornworm?
Tobacco hornworms have a red horn and seven diagonal stripes on each side, while tomato hornworms have a black, less curved horn and V-shaped markings. Both can reach 3 to 4 inches long, both feed on tomato plants, and the control steps are the same.
How fast can hornworms strip a tomato plant?
Fast once they are large. Caterpillars pass through four to five growth stages over about a month, and the final stage eats nearly as much as all the earlier ones combined. That is why a plant can look fine and then lose its leaves quickly.
Is Bt safe to use for tomato hornworms?
Bt only affects caterpillars and does not harm bees, but it only works when the caterpillars eat it and it is most effective on small ones. Spray early and follow the product label.
What does a tomato hornworm turn into?
A large gray-brown hawkmoth with a wingspan of 4 to 5 inches and orange-yellow spots along the abdomen. The moths lay the eggs for the next generation, which is why turning the soil and removing weeds helps.
Sourced from University of Maryland Extension (tobacco and tomato hornworm on vegetables and braconid and ichneumonoid wasps), University of Minnesota Extension (tomato hornworms), and Penn State Extension (keep an eye out for hornworms).
