Tomato Hornworm Caterpillar Identification and Control
Tomato Hornworm Caterpillars: Identification and Control
If you have tomatoes in your garden, then chances are good at some point that you will encounter tomato hornworms, a serious pest of tomatoes and other plants. This guide will help you identify the caterpillars eating your tomatoes, and offer you some options for controlling them.
This guide will answer the following questions:
- What do tomato hornworms look like?
- What does tomato hornworm damage look like?
- What's the most effective way to find hornworms?
- What's the best way to to control tomato hornworms?
- What to tomato hornworms grow into?
Scroll down for quick and easy answers to these questions!
What Do Tomato Hornworms Look Like?
It's hard to mistake tomato hornworms for any other insect. Young caterpillars look essentially the same as full-grown ones: leaf-green with pale white diagonal stripes, and a curved, reddish horn on the hind end. They are phenomenally well-camougflaged on tomato plants: the color and diagonal stripes perfectly mirror the color and veins of the leaves where the caterpillars live. It's very possible that when you're working among your tomato vines to suddenly realize that there is a very large caterpillar inches from your face. Even up close, these big larvae are amazingly hard to see. And if you can find one, chances are very good that there are several more very close by.
Another way to find hornworms on your tomatoes is to look at the ground underneath the plant. Hornworm poops are very big, and they produce a lot of them, so if you have cateprillars you will easily see their droppings on the ground underneath the infested plant.
What Does Tomato Hornworm Damage Look Like?
Since tomato hornworms are so big, the damage they inflict is usually very obvious. The caterpillars eat both leaves and fruit, and they seem to prefer young, green tomatoes over red ripe ones (although as you can see from the photo, they aren't all that picky). The caterpillars tend to eat the entire leaf, rather than leave half-eaten ones, so you might not immediately realize how many leaves are missing. If there are more than one hornworms on your tomato plant, however, the damage will be obvious: entire vines without leaves, badly chewed young fruit, and your tomato crop clearly in serious trouble.
Tomato Hornworm: Manduca quinquemaculata
A Step-by-Step Guide to Finding the Caterpillars on Your Plants
1. 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. Hornworms tend to eat the entire leaf, so look for tendrils on which all of the leaves have been removed -- they'll look like they have been clipped off.
2. Look for fresh "gouges" taken out of the edges of some 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.
3. Examine the fruit for damage. Hornworms rarely eat the entire fruit, and they tend to prefer green tomatoes. If you see "bites" taken out of your tomatoes, you may well have a hornworm infestation.
4. Look on the ground for caterpillar droppings -- this can be a good way to tell that there are caterpillars presence, even when they're too well-camouflaged to find.
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.
What's the Best Way to to Control Tomato Hornworms?
In my experience, the most effective way to control hornworms is a naturally occurring pesticide called "diatomaceous earth." The is simply dirt scooped from old stream beds where microscopic animals called diatoms once lived. When these small creatures die, they leave behind their silicon-based skeletons, or shells. These shells are sharp, being basically tiny shards of glass. When you spread this dusty old earth over plants, caterpillars crawl over the dirt and the sharp shells make cuts in the caterpillars skin. The damage inflicted by the sharp shells of the diatoms add up, and the caterpillar is killed.
Diatomaceous earth is inexpensive, effective, and completely organic. Rain (or a thorough washing by hose) rinses the diatom shells off the plant and into the ground, where they are neutralized. There are several good manufacturers of this organic pesticide, but the one I recommend is Harris Diatomaceous Earth, which comes with its own power duster.
Harris Food-Grade Diatomaceous Earth
Why "Food Grade"?
The designation of "food grade" in diatomaceous earth simply means that it's refined enough to be considered a food additive. I have never added it to anything I eat, but theoretically you could without ill effect!
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!
With excellent non-toxic options like diatomaceous earth and good old-fashioned hand-picking, it's truly unnecessary to resort to toxic, inorganic pesticides for dealing with a tomato hornworm infestation.
Natural Tomato Hornworm Caterpillar Control: Parasitic Wasps
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.
Tomato Hornworms Showing Cocoons of Wasp Parasites
Once the tiny wasp larvae chew their way out of the host caterpillar, they spin little cocoons. After a week or so, they hatch out and the little wasps fly away to attack other caterpillars.
Another Option: Soapy Water
Soapy Water is 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.
Time-Lapse of a Tomato Hornworm Egg Developing and Hatching
What Do Tomato Hornworms Turn Into?
If your hornworms are not killed by control methods, or escape being parasitized by the tiny wasps that attack them, they will burrow under the ground, pupate, and eventually hatch out as a large moth that belongs to a large group of moths, family Sphingidae. The group is distributed throughout the world, and some species are stunningly beautiful. The tomato hornworm moth is not beautiful, but it is impressive -- about the size of a small mouse, with strong brown and gray wings. It flies at dusk and into the night, visiting flowers. When feeding, hawkmoths hover in front of the flower and extend their long thin tongue -- called a "proboscis" -- into the depth of the flower, where it sucks out the nectar. It's truly an amazing animal, and you can see one if you decide to raise of your hornworms to the adult.
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.