Tomato Caterpillars Identification and Guide
Tomato Hornworm Caterpillars
Caterpillars eating your tomatoes are most likely the larvae of a large group of moths, family Sphingidae. The group is distributed throughout the world, and some species are stunningly beautiful. But the caterpillars eating your tomatoes are not particularly beautiful, and they are exceptionally destructive. These are "tomato hornworms," and once these caterpillars get a grip on your plants, they can devour every leaf and immature fruit they find.
There are some effective ways to kill the caterpillars on your tomatoes, but good gardeners are thoughtful and proceed with caution. This guide will help you identify the caterpillars on your tomatoes to make sure that they're actually a serious threat. In some cases, you don't need to kill all the caterpillars on your tomato plants.
Is It a Tomato or a Tobacco Hornworm?
Big green caterpillars on your tomatoes are most likely a species of sphinx moth larvae. There are two closely related species in the genus Manduca, the tobacco and the tomato hornworm. The caterpillars are almost identical, and they both eat tomatoes, which can make telling them apart a little tricky. But from a gardener's point of view, the difference is moot -- there are great big green caterpillars on your tomato plant. Whether they're tomato hornworms or tobacco hornworms is not really the issue.
Manduca genus caterpillars are typically big and green with a curving "horn" on the tail end. For this reason, they are generally called hornworms. These big, tomato-eating larvae grow up to be a big brown moth called a "hawk moth" because of their strong flight.
Left alone, tomato caterpillars can eat your plants down to the stick. They eat the leaves and young fruit of tomato plants, and they eat a LOT. A few of these big boys can pretty much wipe out an entire tomato plant. These monsters grow to five inches long and weigh a ton, and all they do all day is eat tomato leaves and immature fruit. The horn protruding from their rear end gives them their name, but that horn is completely harmless, and so is the caterpillar. It cannot bite, sting, or otherwise harm you or anything else. Except for tomatoes.
Tobacco Hornworm: Manduca sexta
Tomato Hornworm: Manduca quinquemaculata
Tomato Caterpillars Can Be Hard to Find
As big as they are, you would think that finding them would be easy, but it's not. It sounds strange, but it's true -- You can be looking straight at a tomato hornworm and not even see it. It's almost an optical illusion: the pale stripes on the insect's sides mimic the pale veins of tomato leaves, and the color is a precise match.
So take a close look at your tomato plants. Caterpillars almost always leave evidence: If there are caterpillars eating your tomato plants, you will see damage to the leaves. Look for fresh "gouges" taken out of the edges of the leaves. If the eaten edges are brown, the caterpillar has likely moved on a few days ago and could be anywhere on the plant. If the edges are green and fresh, the culprit is probably nearby. You are likely no more than a foot or two away from a very large caterpillar. They are silent, still, and beautifully camouflaged, and you will need to spend some time finding them.
Caterpillar Droppings Can Give Away Their Location
If you look under your tomato plants, you may see the droppings from the caterpillars up above. This means that you have a infestation, even if you can't yet see the caterpillars themselves. Caterpillar poop looks like little hand grenades. Some species fling their poop a few feet from the plant to throw predators off their trail, but tomato hornworms typically just let them drop. It's a good trick for finding them, because otherwise they blend in so well with the leaves you may never even see them.
Time-Lapse of a Tomato Hornworm Egg Developing and Hatching
Finding Them Can be Difficult, Despite Their Size
Despite their size, tomato plant caterpillars are almost impossible to see among the leaves and stems of tomato plants. Their camouflage is so complete that you may find one, pick it off, and miss another half-dozen resting on the same branch. Keep going over places you think you have already searched. Where there is one, there are almost certainly more. Get other people involved in the effort if you can, since it is often true that other people will see ones you miss, and vice versa.
Killing Tomato Caterpillars
Decide how to deal with your problem. If your plants are healthy and you have all the tomatoes you want, you should probably just live and let live. Birds drawn to your tomatoes may fill up on caterpillars instead, and that will save part of your harvest. Tomato hornworms are part of nature's grand plan, and they fill niches that you and I couldn't even guess at.
One Way to Get Rid of Tomato Caterpillars -- Pick Them Off By Hand
This method is clean and natural, but time-consuming: Pick them off and smoosh them into the ground or your compost pile, where the scavengers and microorganisms that depend on dead animals can get their meal. Chemical insecticides are notoriously inadequate when dealing with tomato horn worms, and this method is both time-honored and, for some people, satisfying. My dad used to offer us kids a nickel for every hornworm we could find and kill. We could make several dollars some weeks -- maybe you could enlist the locals.
Poison Is NOT a Good Option for Caterpillars on Tomato Plants!
There are a few substances, like diatomaceous earth, that does help control horn worms -- but many will survive, and they will just keep on eating. More toxic substances are notoriously insufficient in dealing with horn worms in particular and caterpillars in general.
Natural Tomato Hornworm Caterpillar Control
The chief predator on tomato hornworms is a tiny wasp. This parasite -- actually a parasitoid, since true parasites rarely kill their host -- lands on the back of the caterpillar and glues on a batch of tiny eggs. These soon hatch out, and the baby wasp larvae, which are basically caterpillars themselves, burrow into the skin of the tomato hornworm. Once inside they begin to eat the fatty tissues and other non-essential parts of the caterpillar. The wasp larvae molt and grow larger, and after a couple of weeks they're full grown. The caterpillar, surprisingly, looks and acts normal while all this is happeneing, eating and growing more or less as usual. But when the larvae burrow back out and spin cocoons on the hornworm's back, it's all over. The caterpillar invariably dies; the little wasps hatch out and fly away to look for more hornworms.
Another Option: Soapy Water
Non-Toxic and Surprisingly Effective
My mom swore by this method, though most sources recommend it for aphid infestations, not horn worms. Still, you have little to lose by trying this method, and you may kill off a few aphids in the bargain.
How to: simply fill up a spray bottle with warm water and about a teaspoon of plain dish detergent. Spray liberally on and around your tomato plants. The dilute mixture won't hurt your plants, but it may drive away the caterpillars.
What Do You Think?
You can choose to kill as many as you can get your hands on, or you can let nature take its course. After all, some farmers think of their crop as divided by thirds: one third for animals, one third for insects, one third for the table.