How to Fight Hornworms
By Pam Dawling
Photo by Getty Images/Lawrence Glass
One July day, in two 80-foot rows of tomatoes in our hoophouse at Twin Oaks Community, I found 42 hornworms varying in length from 1 to 4 inches — a collective total of 85 inches of pests! They were stripping the tomato leaves and munching on the green fruit.
Hornworms are large caterpillars capable of doing serious damage to tomato crops. The most common hornworm species in the garden is the tomato hornworm (Manduca quinquemaculata), but both the tomato hornworm and its cousin the tobacco hornworm (M. sexta) are bad news, and both attack tomato plants. Before Twin Oaks was established here in Virginia (Zone 7a) in 1967, the land had been a tobacco farm, so we have a lot more tobacco than tomato hornworms on our property.
Hornworms in their caterpillar forms feed on plants exclusively in the nightshade family, and have a strong affinity for tomato plants. Their adult forms are moths — the five-spotted hawk moth for the tomato hornworm, and the sphinx moth for the tobacco hornworm. This year, I caught one of the moths and killed it before it could lay too many eggs, but we still have plenty of caterpillars. Even our most vigilant caterpillar hunting seems to miss some.
Though it may feel impossible to get rid of these tomato terrors once they hatch, you can catch them before they take a strong hold by recognizing the signs of hornworm damage, knowing where to look for the caterpillars, and utilizing some tools and insect allies to rid your garden of a most unwelcome pest.
Searching for Signs
The first step to claiming victory over hornworms is to know the signs of their damage. We conduct hunting raids every morning. If you can’t check daily, look for them at least twice a week in the summertime. A good first place to check is the tomato plants’ upper leaves. If they’re stripped bare down to the ribs, hornworms are probably nearby; the caterpillars only like the tender upper leaves. If the older leaves are damaged but the newer, younger leaves are intact, it may mean there was a hornworm, but it’s been removed already, and the plant is recovering.
Photo by Flickr/Dyogi
Another sign of hornworms is chewed fruit, typically caused by the larger caterpillars. Hornworms also leave “pineapple poop” in their wake: miniature brown droppings, shaped like pineapples or hand grenades (see photo, bottom left). If you find fresh poop, look directly upward for the culprit or culprits. The size of the poop is, naturally enough, proportional to the size of the hornworm.
The Hunt is On
Once you’ve determined that there are hornworms in the vicinity, your next task is to find them. Tomato hornworms sport a green body with white Vs running along the body, and a black horn on the back. Tobacco hornworms have a red (not black) horn, and diagonal white lines instead of Vs. You may think it’d be easy to spot these caterpillars, but not so! They’re the same shade of green as tomato leaves. Their shape can be remarkably similar to curled tomato leaves, and their white stripes mimic the veins on the undersides of the leaves.
Photo by Getty Images/BackyardProduction
Knowing the signs of hornworm grazing can save you time. Be sure to focus your attention where you’re most likely to find them. I do my hornworm hunting when it’s warm but not too hot, on the theory that the caterpillars are more likely to be active rather than snoozing in a sheltered spot. I walk along the row looking for damaged leaves. When I find some, I gaze at the area, looking for discrepancies in the pattern — in other words, bare stems with lumps on them. If I still can’t see the worm, I stand still and sway a bit from side to side, viewing the plant from different perspectives. It helps if the top of the plant is backlit, but I always check both sides of the row, no matter where the sun is. Usually, the caterpillars are on the underside of a chewed stem, and often (but not always), they have their heads raised. When you find a hornworm, get a firm grip on its body with your fingers, pull it off the plant (they have strong legs that hold on tight), drop it on the ground, and stomp on it. Its skin is quite thick.
You can try combating hornworms by other means. Bt (Bacillus thuringiensis) is an organically approved pesticide spray that kills small caterpillars without killing other insects. I don’t expect it to work on large hornworms, though. Hunting them seems to be the way to go.
Another way to battle hornworm infestation is by breaking the moths’ lifecycles of laying eggs. You can close your hoophouse (if you have one) at dusk every night, which is the time the moths become active, and open it every morning before it gets too hot.
Know Your Garden Friends
You likely have an insect ally nearby. Hornworms often get parasitized by a tiny braconid wasp (Cotesia congregata) that lays eggs in the caterpillars. The larvae develop inside the caterpillars, and then the pupae develop as white, rice-grain-like cocoons sticking out of the back of the hornworms.
Photo by Isabelle Betancourt
The parasitic wasp will naturally aid you in an outdoor garden, but it doesn’t usually venture inside a hoophouse. To get parasites into the hoophouse, we at Twin Oaks have to bring in parasitized hornworms from outdoors. This doesn’t work so well, because the hoophouse tomatoes are a month earlier in their development than the outdoor ones, and the hornworm cycle is well underway in the hoophouse by the time the parasitic wasps are in action outdoors. This year, however, we’ve found several parasitized hornworms indoors, and we’re very happy. If you observe parasitized hornworms, leave them be. They’ll stop eating, shrivel up, and die. Meanwhile, the wasps (harmless to us) will hatch and go forth and multiply.
Photo by Getty Images/Jeremy Christensen
The tomato hornworm’s lifecycle brings forth two generations of tomato pests in one year. In spring, the five-spotted hawk moths emerge from overwintering and lay their eggs, usually on the surface of tomato plant leaves. The caterpillars hatch and begin feeding, reaching their full size in a quick 3 to 4 weeks. And the bigger they get, the more they’ll consume. Once the hornworms are fully grown, they drop from the plants, dig into the soil, and transform into pupae. About two weeks later, around midsummer, the new moths emerge. These moths also mate and deposit eggs, and those caterpillars again feed and pupate. This time, the pupae overwinter in the soil, and the moths will emerge the following spring, essentially resetting the cycle for the next year.
Photo by Getty Images/sgoodwin4813
Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at the Mother Earth News Fairs, as well as at sustainable agriculture conferences. Her books Sustainable Market Farming and The Year-Round Hoophouse are available at the Mother Earth Gardener store.
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