Tomato hornworm

Why the damage looks so sudden

A tomato plant that was fine on Monday can be stripped by Wednesday. That is not an exaggeration and it is not a slow-moving disease: Wisconsin Extension records that hornworm larvae devour up to four times their own weight in leaves and fruit every day, and Illinois Extension puts the arithmetic plainly at one or two caterpillars defoliating a five-foot tomato plant in under three days.

The reason it feels sudden is that you never saw the small ones. A young hornworm is the exact green of a tomato leaf and sits along the midrib where it reads as part of the plant. By the time the caterpillar is big enough to notice, it is four inches long and has already eaten most of what it was going to eat.

Finding it when it is still small

Do not look for the caterpillar. Look for what it drops.

Dark green or black pellets on the leaves below, on the soil, or on the path beside the bed are the reliable signal, and they appear well before the damage does. Follow them straight up: the caterpillar is almost always on the underside of a leaf directly above.

The other early sign is a stem with the leaves gone and only the bare midribs left, usually near the top of the plant. Hornworms work from the top down, which is why the first damage is often at eye level rather than where you are looking.

Evening and early morning are the productive times to search. In full midday sun they move to the shaded interior of the plant and are genuinely hard to see.

Which hornworm you have, and why it barely matters

Two species turn up on the same plants and are routinely confused. Wisconsin Extension separates them by markings: the tomato hornworm carries eight V-shaped marks along each side with a straighter, blue-black horn, while the tobacco hornworm has seven or eight diagonal white stripes and a red horn.

For the gardener the distinction changes nothing. They eat the same plants, do the same damage and are handled the same way. It is worth knowing only so that a caterpillar identified online as the “wrong” species does not send you looking for a different answer.

Both feed on solanaceous plants: tomato above all, and less often eggplant, pepper and potato. They also live on nightshade, jimsonweed and horsenettle, which is where the ones in your garden generally came from.

The one caterpillar you must not remove

If a hornworm is covered in what look like small white grains of rice standing up from its back, leave it exactly where it is.

Those are the cocoons of a braconid wasp, Cotesia congregata, which has laid its eggs inside the caterpillar. The larvae have fed internally and emerged to pupate. Clemson Extension notes that a parasitized hornworm has already stopped feeding, so it is doing no further damage to your plant.

Killing it destroys the next generation of the insect that controls hornworms for you. Leaving it produces dozens of wasps that go looking for the caterpillars you have not found yet. This is the single highest-value thing to know about this pest, and it is the opposite of what the sight of it suggests.

What to do about the rest

1. Pick them off by hand

   Clemson Extension is direct that handpicking is an effective control in a home garden. Drop them in soapy water, or move them to a nightshade weed away from the beds if you would rather not kill them.

2. Check twice a week from early July

   Wisconsin and Illinois both put the monitoring window at early July through August. Twice a week is enough to catch them while they are small.

3. Look under the leaves, not on top

   Eggs are laid singly on leaf undersides and that is where the small larvae stay. The top of the plant is where the damage shows; the underside is where the animal is.

4. Till the bed after harvest

   They overwinter as pupae in the soil. Wisconsin reports that post-harvest tilling can destroy up to 90 percent of them, which is the cheapest control available and the one almost nobody does.

5. Pull the nightshade weeds nearby

   Horsenettle, jimsonweed and nightshade are alternative hosts. A tidy fence line is a smaller hornworm population next summer.

When a spray is justified, which is rarely

Illinois Extension’s position is that hornworms rarely cause enough damage to warrant insecticides in home gardens, and that monitoring plus handpicking is the appropriate response at that scale.

That is worth taking seriously rather than treating as caution. A broad-spectrum insecticide kills the braconid wasps along with the caterpillars, and the wasps are doing more work than the spray will. A garden that is sprayed hard in July often has a worse hornworm problem in August than one that was not.

Where a treatment is genuinely wanted, a Bacillus thuringiensis product aimed at caterpillars is the narrow option, and it works only on small larvae. On a four-inch hornworm it is a waste of money; that one comes off with your fingers. Whatever you use, the label on the container you own is the authority on rates, timing and pre-harvest interval.

The life cycle, and where it hides

Almost everything surprising about this pest comes from the fact that you only ever see one stage of it. The rest is underground or in the air at night.

Two practical consequences. The eggs are laid one at a time on leaves, so there is no egg mass to find and destroy the way there is with some pests; the search has to be for young larvae instead, which is what the section above is about. And because the pupae spend the winter in the soil under the plants, cultivating the bed after the crop comes out is one of the few things that reduces next year’s numbers rather than this year’s.

The moth it becomes, and whether to kill it

This question comes up more than almost any other about this insect, and it deserves a straight answer rather than a slogan in either direction.

The tomato hornworm is the larva of the five-spotted hawk moth, Manduca quinquemaculata. The adult is a large night-flying moth that hovers motionless in front of a flower and feeds through a long proboscis, which is why people who see one mistake it for a hummingbird. It navigates by scent and by the brightness of the flower, and UMN Extension names the moonflower vine among the blooms it works.

Its value is not sentimental. A hawk moth visits more flowers in a single night than a bee does in a day, including flowers that bees will not work at all, because the deep tubular night-scented flowers are shaped for a long proboscis and nothing else can reach into them. Removing hornworms removes night pollinators.

So the honest position is a matter of scale rather than principle.

• A few caterpillars on established plants are worth tolerating. A mature tomato carries a surprising amount of defoliation without losing its crop, and what you get in exchange is the moth.
• A serious infestation is a different matter. Because of their size and appetite, a small number of hornworms defoliates a plant quickly, and a stripped tomato does not recover its season.
• If you are removing them, move them rather than killing them where you can. They will keep feeding on any nightshade, and a volunteer or a sacrificial plant at the far end of the garden does the job.
• Never remove a parasitized one. That is covered above, and it is the single most consequential thing on this page.

What does not survive scrutiny is the framing that this is a pest with no other identity. It is a pollinator with a larval stage that eats your tomatoes, and both halves are true at once.

What happens next in the season

Adults are large hawkmoths with a wingspan up to five inches, active at dusk and regularly mistaken for hummingbirds. They emerge from late June and lay pale green eggs singly on leaf undersides.

The upper Midwest sees up to two generations a year, so clearing the July caterpillars is not the end of it. Keep checking into August, when the second flush arrives on plants that are carrying fruit and can least afford to lose their leaves.

Frequently asked questions

Will the plant recover if it has been stripped?

Usually, if it happens early and the stems are intact, though at a cost: fruit already on the plant loses the leaf cover that shades it and often scalds in the sun. A plant stripped in August with a full crop on it is in more trouble than one stripped in early July.

What are the white things on the caterpillar’s back?

Braconid wasp cocoons. That caterpillar has stopped feeding and is producing the wasps that parasitize the next ones. Leave it alone; it is the best pest control in the garden and it is free.

Where did they come from? I have never had them before.

They overwintered as pupae in your soil, or the moths flew in and laid eggs. Nightshade, jimsonweed and horsenettle growing near the garden keep a local population going even in years you grow no tomatoes.

Do they eat anything besides tomatoes?

Yes, but less often: eggplant, pepper and potato, all in the same family. Tomato is far and away the preferred host, which is why the damage tends to be concentrated on one crop.

Is handpicking really enough?

In a home garden, yes, and both Clemson and Illinois say so. Checking twice a week from early July, looking under the leaves, and following the droppings upward will find them while the plant can still spare the foliage.