Identifying the Caterpillars Eating Your Tomatoes
Tomato Caterpillars—What Are They?
Do you have big green caterpillars on your tomato plants? That's a sure sign that your garden is playing unwilling host to one of the most common tomato pests in North America, the tomato hornworm caterpillar.
The scientific name for this insect is Manduca quinquemaculata. They are the larval form of a big brown moth known as a "hawkmoth" for its powerful, swooping flight. You will likely never encounter the adult moth, since it hides during the day and is pretty inconspicuous despite its size, but if you grow tomatoes, then you have probably come across the big green caterpillar of this species at one time or another.
This guide will help you identify the big green caterpillars that are eating your tomato plants, and also offer some ideas for controlling the damage.
How to Identify a Hornworm Caterpillar
- Leaf-green in color. This shade of green blends in perfectly with the leaves of your taomato plant, so even though they can be huge, they can also be very hard to see, even when you look very closely.
- Eight V-shaped white stripes along the side. These markings match the veins on the leaves of the tomato plant. Added to the green background color, they give the tomato hornworm one the best camouflage jobs in all of nature.
- Eight black and yellow spiracles on each side of the body. These are tiny airholes, and it's how the caterpillar breathes. If you clog them up, the caterpillar dies, and this is one way to control them (more on this in a bit).
- Curved horn on the tail-end. This is called a "caudal horn," and it often has a red, yellow, or blue color to it. This of course if how the tomato hornworm gets its common name. The horn looks scary, and it's supposed to, but it's not a stinger or even sharp. It's just for show -- the tomato hornworm is completely defenseless against a predator as big as you!
- Grow as large as 4 inches long, and this is not a skinny four inches, either. They're fat and heavy. If you hold one in your hands you'll be surprised at how massive they are. They got that way by eating one thing and one thing only: lots and lots of tomato leaves and fruit.
- Specific to tomatoes, but will also occasionally eat eggplant, peppers, and related vines.
How Can I Tell if Caterpillars Are Eating My Tomatoes?
- Examine your plants. The first sign of a caterpillar infestation is missing leaves. These guys can eat a leaf right down to the stem, and if enough of them attack a plant they can turn it into a cluster of bare green stems and stalks. It's probably the big green larva of the hawkmoth, and once you have them, it can be hard to get rid of them.
- Check for young tomatoes that have been partly (or completely) eaten. If you have leaves with big chunks missing, and baby tomatoes with even bigger chunks eaten away, you probably have hornworms in your garden.
- If you still don't see evidence, have a look around the ground under your plants. You are looking for poop, and these big caterpillars make big poops. They look a little like tiny hand-grenades. A pile of these under a tomato plant means someone up above is doing some serious eating.
- Where there's one, there's more. If your eyes are sharp and you do find one munching away on a leaf, don't stop! Keep looking—where there is one, there is almost certainly a whole bunch more close by.
- They hide in plain sight. Even when you know what you're looking for, these big caterpillars can be hard to find. They are unbelievably well-camouflaged on the bright green leaves, and the little slanted marks on their sides blends perfectly with the angled veins of the tomato plant leaves. So when you look, you may find some, but you may not—they are truly well-adapted to evade discovery by predators, which you are now.
How to Get Rid of Tomato Caterpillars
Tomato Caterpillar Control Option 1: Pick Them Off by Hand
The first method to eliminate them is to physically pick them off the plant. This is labor-intensive, but it's perfect for the young ones in your family or neighborhood -- when I was a kid it was a dime per caterpillar, and we made enough to buy a week's worth of candy. Offer the neighborhood scamps a a small bounty for every hornworm corpse they bring to you, and there's a pretty good chance your caterpillar problem will be over. You might be down a few dollars, but it's a better deal than paying for store-bought insecticide that may or may not work.
The drawback to this method is the same reason that hornworms and other prey insects are not extinct: it's really impossible to kill every member of a population, unless you also destroy their habitat (predator animals don't do that, while humans are sadly very good at). So yes, some caterpillars will survive and keep munching on your plants. But the problem will be a lot less serious—maybe you could even live with it?
Tomato Caterpillar Control Option 2: BT Dust
One of the most common ways to control tomato hornworm caterpillars is by dusting with a product containing Bacillus thuringiensis (BT), which is a naturally occurring bacteria that specifically targets caterpillars. While this organism does occur in nature, you need to be very careful if you decide to use it -- if you cover other plants, it will kill other species, including the caterpillars of butterflies and moths that are not causing you any problems. There have been studies showing that widespread use of BT in forests attacked by gypsy moth caterpillars killed not only the gypsy moths but hundred of other species as well.
BT Dust for Tomato Caterpillars
In my experience this product works very well to kill tomato hornworms. But be careful with it, because it will kill pther species as well.
Tomato Caterpillar Control Option 3: Organic Solutions
The half-way step between removing them with your bare hands and plain old poison is insect soap or dust. There are several recipes for this online, and they usually affordable and generally fairly effective. They work by blocking the spiracles (the holes along the side of the body that the insect uses to breathe).
Another product that I have used with success is pure cedar oil. It's safe, effective, and it smells good too! It's been used as an insect repellant for eons. If you're determined not to use chemical insecticides, cedar oil may well be an answer for you.
Ladybugs, green lacewings, and common wasps are its natural predators. They can also be reduced by planting marigold close by.
To Poison or Not to Poison?
Would you ever decide to use poison or chemicals to control tomato hornworms?
Are Hornworm Caterpillars Dangerous?
Although the tomato hornworm does have what looks like a stinger coming out of its tail, it's completely harmless. There are some other species, though, that are venomous and can give you a pretty nasty sting if you touch them or brush up against them. Handling caterpillars is fine if you know which one you're dealing with, but otherwise it's best to use gloves or avoid letting the caterpillar contact your skin, especially if it's furry or has spines.
Why Does It Have a Horn?
The curved tail or horn on the hornworms's rump is not a stinger, though scientists think it may look enough like one to provide protection to the caterpillar. There is no sting, or even a sharp tip to the horn, so its use as a protective device is debatable. It has also been suggested that the tail resembles a snake's tongue, or that it draws attention to the back end, sparing the head of the insect from attack.