Three inch long Hornworms

Three inch long hornworms can scare the life out of an unsuspecting gardener when they don't know what they are.

I wonder if you’d be a little freaked out to find a big, fat, 3 inch long black or green caterpillar with a “horn” sitting on the leaves of your tomatoes or flowers? I think it would be safe to say that the majority of people would be.

I received a call from a lady a couple of weeks ago who said her landscape maintenance guy found a bunch of these menacing horned creatures among her plants. Needless to say, they were both a little more than concerned and wondered what would happen if a person was to get stung by that big horn.

So let me put you at ease should you happen to find one of these monsters in your landscape. First, the horn is on their hind end, so I guess that makes it more like a tail. The horn on a tobacco hornworm is red but that’s not because it’s dripping with blood. They don’t use it to sting or attack. As far as I know it’s used simply to frighten predators, or people, away. In fact it works so well that I gave a few of these “caterpillar treats” to my chickens, and my poor chickens ran around in circles discussing the strange new danger before deciding to run to the back of the coop and scream until I removed these monsters. That was fine with me though. I wasn’t sure I wanted to eat the eggs after they feasted on these little sausage size caterpillars.

So let me tell you a little more about these caterpillars. There’s actually very little difference between tomato hornworms and tobacco hornworms. They’re both larva of a family of sphinx moths or sometimes called hawk moths. You may have seen a sphinx moth and thought it was a hummingbird because it exhibits the behavior of a hummingbird, and in a way it looks like the little birds.

You probably guessed by now that tobacco hornworms feed on a lot more than just tobacco plants. And both caterpillars have the capability of turning a plant into a skeleton almost overnight, so I’m not sure if it really makes a difference which worm it is especially since they look so similar, but the little red spots on their sides and the white V shaped stripes are one way to distinguish the two.

Even though the larva is large, they can be hard to see because of their green or dark coloring. They like to hide on the inside of the plant or on the underside of the little branches and tend to move towards the outer portion of the plant later in the day when hungry birds are thinking about going to bed. I’ve been in the garden and detected a slight movement of the plant and discovered larva happily chewing way. Something that eats this much will leave signs, so you might check the ground around the plant for large, black droppings (frass) that accumulate on the ground beneath the affected plants. Once the larva have chewed off the leaves of the plant they are definitely easier to see, but that’s often too late to save some plants. A friend told me you can go into the garden when it’s slightly dark and shine a black light onto the plants. The tomato hornworms are supposed to really show up in the light, making them easy to spot. (I’d like to see that!) While you’re checking plants, you might want to check your potatoes, eggplants, peppers and other members of the nightshade family. I don’t usually see the larva on them, but they are on the menu.

So I’m sure you want to know how to get rid of the caterpillars and I’ve discovered several ways to rid my plants of these pests. One of the first “go to” treatments is to simply pick them off and drop them into a bucket of soapy water. Or, the organic, environmentally safe product Bt (Bacillus thuringiensis subsp. kurstaki) could be shaken or sprayed onto target plants. Bt only kills caterpillars making it a great product. It will not harm humans, pets, beneficial insects or birds that may occasionally be feeding on the caterpillars. Because there are a variety of Bt products, each designed to control specific insects, make sure the Bt that you choose is labeled for caterpillars.

Here’s another way to rid your garden of them. One of the master gardeners says she cuts the caterpillars in half with a pair of scissors, but that’s a little too gross even for me. I find making a little grave in the dirt with my foot, dropping the critter into it and covering it with a little dirt and smooshing it isn’t quite as traumatic. Of course products such as Spinosad, Carbaryl (Sevin) or Pyrethrum will give great control, but I would apply it late in the evening so birds won’t eat a recently treated caterpillar.

Braconid wasp, Cotesia congregatus are great beneficial insects for caterpillar control. The wasp will lay eggs on the caterpillar and when the larvae hatch they feed on the inside of the hornworm until the wasp is ready to pupate. The cocoons on the caterpillar look like white rice protruding from the hornworm’s body, so if you see this just let them be. After all, it’s a bug eat bug world out there.

So whatever your method of control is, I hope your flowers remain beautiful and you have a bountiful harvest. And don’t feel bad about not sharing your tomatoes with these critters.

Linda Corwine McIntosh is a licensed pesticide applicator, advanced master gardener, and ISA Certified Arborist.

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