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Tomato Hornworms and Other Late Summer Pests

          Sometimes it’s easy to get discouraged about the garden: Early in the spring there are seeds that don’t germinate well in cold, wet soil. Then the weeds germinate too well and try to take over the garden when things warm up. Next Colorado potato beetles attack the potatoes. By mid-summer early blight and other diseases start killing leaves and slowing growth on tomatoes, and now the tomato hornworms come along to browse the tomatoes and their vines. Gee whiz, is there no mercy for gardeners? I thought gardening was supposed to be easy!

          But there is some good news. Mother Nature is doing her best to control some pests, tomato hornworms among them. Tomato hornworms are green larvae that get huge (up to 3-5 inches long), and have voracious appetites.  Bad infestations can defoliate tomato plants in just a day or two. Because of their green color and markings, they are often very hard to notice, too. This year they seem worse than normal, according to some of my readers – though not in my garden. This is the season when they are most prevalent.

          If you seen a tomato hornworm with little white attachments something like small grains of uncooked rice, do not kill the hornworm as it is being attacked by the larvae of a parasitic wasp. These white projections are cocoons containing the pupae of a braconid wasp. They indicate that the good bugs have found the bad bugs, and will destroy them.

Hornworm with cocoons attached

I watched a hornworm that was covered with these tiny white cocoons, and over a 24 hour period it ate nothing. It was slowly dying, and appeared to have no appetite. I called Dr. Alan Eaton, NH State Entomologist, who told me that the wasps kill their prey slowly, allowing the larvae to feed long enough to develop. At a certain point, he said, the hornworms stop feeding. When he encounters a hornworm with parasitic wasp cocoons, he takes no chances – he moves it to another location away from his tomatoes where the braconid wasps can develop.

If you’ve seen a moth that resembles a hummingbird, that is the adult form of the tomato hornworm. According to Dr. Eaton, an adult moth of the tomato hornworm is about the same size as a ruby throated hummingbird. They are very strong fliers, and actually overwinter in the places where they can survive without freezing. Then they fly north in the spring, finding new feeding and mating territories.

One last hornworm tidbit: according to Dr. Eaton, there are 2 species, the tomato hornworm and the tobacco hornworm, and the tobacco hornworm is much more prevalent. So if you have been cussing out those nasty tomato hornworms, maybe they’re laughing and saying, “We’re actually tobacco hornworms!”

According to Dr. Eaton – and several readers who have sent me questions – this is a bad year for green stinkbugs. These triangular-shaped green bugs are feeding on everything from grasses and veggies to peaches and apples. Perhaps the mild winter resulted in higher numbers of them. They stink if crushed, so Dr. Eaton recommends picking them and dropping them into soapy water. The brown marmorated stinkbug is often a pest, but the green one is not usually as prevalent as it is this year.

          This is also the season that fall webworms appear. You may call them tent caterpillars, but they are of a different species of pest. The tent caterpillars form webs, or tents in crotches of trees early in the summer, but the webworms create their nests near the tips of branches now, and into the fall. With time, the nests get bigger, enclosing more and more leaves as they grow. If the tent is down low you can physically remove the teeming mass of caterpillars by clipping off the branch and dropping it into a bucket of soapy water.

Fall Webworm

If the fall webworms are close enough to the ground that you can spray the leaves near them, you can apply a biological control. There is a product called Dipel, which contains a bacterium (Bt kurstaki) that will kill the worms if they eat leaves sprayed with it. This bacterium will not hurt us, our pets, fish or birds. It is specific to caterpillars. And Dipel lasts for years in the container, so you can invest in a package of it and know that it will be good in the future.

          I understand the urge to “nuke” the bad bugs. Hornworms eating my tomatoes? Nuke ‘em. But I don’t. And since I let the braconid wasps feed on the hornworms years ago, I rarely see one of those bad boys. Chemical pesticides change the garden environment. It may please you in the short run, but in the long run, let Mother Nature – and your fingers – take control. Pick’em, don’t nuke ‘em.

Henry Homeyer’s new children’s book, a fantasy-adventure for 8-12 year-olds called Wobar and the Quest for the Magic Calumet, is coming out in October. His Web site is www.Gardening-Guy.com; his e-mail is henry.homeyer@comcast.net.