Tomato Hornworms, Hawkmoths and Parasitic Wasps
Early last spring, my sister gave me an heirloom grape tomato plant. I hadn’t done anything about a real garden by then, but had a space next to the front porch that I thought would do. I planted it, and it began to stretch and grow in all directions. What was this? I thought tomato plants were good citizens, growing up into their cages. Not this one. It outgrew its cage, sprawled onto the porch, blocked the sidewalk, and took over everything, including some nearby basil. Its one redeeming quality, the thing that kept me from cutting it down, was that it was loaded with lovely pop-in-your-mouth-and-taste-sweet-summer tomatoes. They kept coming on as the plant climbed through the railing and rested its load on the floor of the porch. Soon the frost will put an end to this bullying behavior, but until then I’ll collect the tomatoes.
Meanwhile, as I was trying to corral the plant, I discovered something about four inches long that moved when I touched it. The monster tomato had monsters of its own—tomato hornworms. And the hornworms, in turn, had their own terrors—wasp larvae. This will not surprise any of you gardeners, but it both surprised and fascinated us.
The tomato hornworm, it turns out, is the larval stage of a five-spotted hawkmoth. When mature, these moths have relatively heavy bodies and are mottled in gray and brown, with yellow spots on their abdomens. Their front wings are narrow but may span up to five inches. You’d notice if one landed on your head! They’re as big as a hummingbird. In fact, people call them hummingbird moths because they fly and feed similarly. The first batch of them emerges from their underground pupa stage as adults after over-wintering in the soil. Their plan is to happily go about feeding on nectar of honeysuckle, petunias, phlox, bouncing bet and tobacco from dusk to dawn. They will mate, as you would expect, and then lay eggs on tomato plants, peppers, potatoes, tobacco and eggplants. The eggs hatch; the larvae (hungry little hornworms) eat and eat, threatening to defoliate the plants. They will grow and grow for three to four weeks. When they’re big enough caterpillars, about four inches long, they will drop off, burrow into the soil and pupate. After a couple of weeks, they will emerge as the second generation of moths for the year and start the second half of the cycle. They will drink nectar with their amazingly long tongues, mate, and lay eggs on the plants. Eggs will hatch, little hornworms will eat and grow, and drop off. But this time they’ll burrow down into the soil and stay there through the winter, pupate in the spring, and emerge next year as adult moths in search of next year’s tomato plants.
The moths don’t hurt anything. In fact, they’re good pollinators. It’s their larvae, those bright green caterpillars with black-edged white stripes on their sides. Each stripe has a yellow spot, and the caterpillar has a black horn sticking out at the back end. These are the critters that are gobbling up the leaves of my tomato plant.
The hornworms weren’t having much effect on my monster plant, which could be due to the little white dots on their backs. The hornworm’s worst nightmare, these dots are cocoons made by the larvae of parasitic wasps. The wasps lay about 65 eggs apiece under the skin of the hornworm. Along with the eggs, they inject a venom and a bacteria that stops the hornworm from walling off the eggs and processing protein. This leaves more food for the wasp larvae. The hatchlings burrow into the host hornworm and start eating it from the inside out. When they’re full, they emerge, spin their little white cocoons, and pupate. As adults, they are about an eighth of an inch long, black with clear wings and yellow legs. They fly off to find small flowers like yarrow for lunch. They don’t usually sting, so are considered good insects for the garden. Meanwhile, the hornworm dies, never to become that amazing five-spotted hawkmoth. But what’s this? Another hornworm, this time with no cocoons on its back. And the tomato plant is losing the battle after all.
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