Backyard Naturalist – Attack of the Caterpillars

I am a desultory vegetable gardener. Abetted by friends’ and neighbors’ prolific generation of tomatoes, peppers, potatoes, even the more exotic sorts of veggies, I get to turn my garden into a wanton aesthetic paradise of impractical or inedible blooms and foliage, or just grow whatever the pollinators might like best.

One year at work, my boss brought in a caterpillar she’d found on her tomato plants. It was a tobacco hornworm, Manduca sexta, and she didn’t want it eating her tomato plants, but neither of us wanted it dead, either. I had, that year, made a feeble attempt at growing a couple of tomato plants in tubs in my yard, so I volunteered them as tribute to this caterpillar and brought them into work to be his food, making him our office pet.

Caterpillars grow through several stages, called instars, and our pet was likely in his third when he arrived at his new home. He munched away at the first plant we gave him at a manageable pace, but soon he was nearly thumb thick, undulating across the stems of my last tomato plant. He would rear his round head up, searching for more leaves, and then set off, stretching, the back half of his body slightly delayed, as if he were two tiny men inside a Chinese dragon suit at a parade.

We were out of food for him at that point, and one of us was headed away on vacation, and we discussed what to do. We worked at a wildlife clinic, and in the lobby was a big tank with two sunfish in it. The fish had to eat, and the hornworm was a meaty morsel at that point, so I solemnly dropped him in. One fish lunged at him and gulped him down, and that was that, I figured, until I saw a bloom of whitish flecks emerge from under the fish’s gill covers. It turns out the caterpillar’s body had been host to dozens of wasp larvae, injected as eggs by their mother along with an immunosuppressing virus to keep the caterpillar’s body from destroying them. While we’d been growing the hornworm on tomato plants, he’d been a mobile nursery for these young wasps. Now, all of them, tomato, wasp, hornworm, had been sacrificed to the appetite of a sunfish.

To gardeners, caterpillars often appear as a plague, skeletonizing leaves in hours, descending in hordes as they hatch in huge batches and seek enough food, in a short summer, to make it. They can be discomfiting in such large numbers, or when, like the hornworms, they get so disconcertingly big.

I attended a program last summer put on by the Caterpillar Lab in Keene, N.H. Someone asked about tobacco hornworms and the threat they posed to tomatoes. Rising up on his toes a little, the presenter told us, with some urgency, “Here’s the thing, those guys will eat anything in the tomato family, so if you can find some nightshade, give them that instead.”

Nightshade is not all that difficult to come by. It’s an introduced, sometimes invasive plant most of us have in the untended corners of our yards. So now, if I do try growing tomatoes or peppers at all, and I find hornworms, I escort them to a nightshade vine. Many of them probably harbor wasp eggs in the darkness of their bodies. Some hornworms will escape that fate and drop into the dirt and pupate, emerging in their adult form as sphinx moths. I’ll wish them well, because worm, wasp, fish and person, we’re all at the same work, filling our bellies and racing the brief season.