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Field & Street

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The summer heat has gotten sullen. Zucchini and tomato plants thrive in the sweating air. My zucchini explode at dawn with gigantic yellow blossoms, and before you know it inflate their fruit to the size of dirigibles. I walk from neighbor to neighbor offering these fine logs of food, good only for zucchini Parmesan and zucchini bread. My brother Bob, who lives on a farm in Wisconsin, warns me to lock my car if I drive to Kenosha this time of year or people will stuff it full of swollen orphaned zucchini.

Tomato plants throw out endless new vines, and the wire frames and posts that once towered over the plants heave and groan under the weight of ripening fruit. Tomato hornworms are here, joining in the feast, working the succulent new vines and transmuting tomato stuff into hornworm stuff. I found one, big as a plumber's thumb, working ceaselessly to clear the foliage from a beefsteak vine.

I snipped off a good-size branch, intending to bring it indoors to watch the hornworm pupate. But first I showed it off to neighbors. I rang Louise's doorbell, and she came hauling Annie on her hip. Katie and Julian came running. And Eli showed up to greet this magnificent visitor. And the Malones. They saw a giant caterpillar, brilliant green integument (skin) with white diagonal stripes on its sides, dotted spiracles (air holes) decorated with concentric black and red rings on each segment, and a pointed, black, whiplike tail. The head was enlarged, curled upward in a sphinxlike attitude to intimidate enemies.

But the children weren't intimidated. They stroked the velvety skin, shrieked when it reared back, aggravated by too many grubby fingers. Finally I took it and the vine back home and set them in a vase on the screened-in back porch.

The next day at noon I sat on the back porch having tea. I looked for the hornworm and found it still feeding on a reduced plant, but sick somehow. Over the tapestry of its flesh was a pox, dozens of yellowish brown marks. As I watched over the next few minutes they deepened in color and distinctness, now hundreds of spots. And then, to my horror, they swelled. First a few and then scores at a time came to a head, like ripe pimples--translucent yellowish lumps.

And then they erupted. The hornworm was utterly still except for an occasional spastic twitch. But the eruptions were alive with the reaching white flesh of maggots. Segmented grubs were pushing and pulling their way out of the green flesh like a child shedding a wet bathing suit.

I didn't breathe. Right there on my table a great mystery was being worked. A mystery and horror like birth itself, like sleep and dreams. The alchemy of transmutation. I looked at my arms, my hands for spots. But they were more or less normal. Just goose bumps. The hornworm was now completely covered with writhing, contorted maggots. Their mass seemed nearly equal to the mass of their host. I kept thinking of the internal vacuum created by their exit, the collapse and emptiness of the host shell. Hundreds of pearly white segmented worms were dancing now, waving their bodies around and out, covering the hornworm with a medusa coat.

Then they stopped. I had been ready for the grubs to start dropping off onto the table, but in unison they began to hula. Hula around and around, bending their snouts down and whirling around themselves spidery threads of white silk. On and on it went, this dance. My tea was cold, my stomach high up. They danced and spun and wove until each had clothed itself in a furry white cocoon of its own. The hornworm now wore a wig that looked like the silver white Afro my brother Bob wore to my house after his visit to Hollywood.

What now? I waited and watched the clock. I had to go on shift. Time was running out. But I was breathless and dazed by what had happened. I thought about calling in sick. I didn't want to leave.

But peace had come to the hornworm and the grubs. Nothing moved. I waited, trying to remember what these cocoons might hold.

Stephen Jay Gould, Harvard biologist and well-known author, wrote in an essay called "Nonmoral Nature" about this general phenomenon and the quandary it posed for early naturalists. They saw in the splendor, diversity, and exquisite architecture of nature the hand of the master engineer, God himself. But parasitism looked like the horrifying work of some other great force. Why would a just and righteous creator produce such an evil relationship?

There is a whole group of parasitic wasps, mainly of the families Braconidae and Ichneumonidae, that deposit their eggs in spiders, caterpillars, and other succulent hosts or their eggs. Often, but not always, this results in the host's death. Some wasps paralyze their hosts with a neurotoxin and drag them into their lairs to feed their broods. Others lay their eggs in the hosts and fly on to other business, oblivious to the fate of their offspring.

And this thing was happening in front of me. I was late, but I didn't want to leave. I didn't know if the cocoons would hatch, filling the house with hundreds of demon wasps.

I brought the vase and afflicted caterpillar outside and set them on a tree stump. Almost immediately the hornworm, which had seemed dead, was attacked by large wasps. They hovered over the creature's back, then landed and shaved off bundles of cocoons, which they carried off somewhere. The hornworm writhed, reared back, and energetically defended itself, snapping with its mandibles at the marauding wasps. On and on the attack went. I was paralyzed. Finally I ran inside, wrote a note to my wife asking her to check the worm when she got home, and went off to work.

Later she called me to say that the hornworm, cocoon, and wasps were all gone, the whole drama perhaps ended by a hungry robin. Only the vine and vase remained.

The tomato hornworm moth, Manduca quinquemaculata, is a member of the Sphingidae family of sphinx moths and hawkmoths. They are powerful fliers that migrate and forage over many miles. They hover before their host plant with broad wings buzzing like hummingbirds or bees. Their proboscis is characteristically long, sometimes three times the length of the body as it unfurls toward an awaiting blossom. These traits make hawkmoths and their kin crucial pollinators for plants in the orchid family. The long proboscis is matched to an equally long orchid flower column, at the bottom of which the hawkmoth collects its nectar reward for inadvertently carrying away pollinia to the stigma of a neighboring plant.

The hawkmoths of our region are especially interesting because of the relationship they have with one of our local orchids, Platanthera leucophaea, the eastern prairie fringed orchid. Leucophaea is on the federal threatened-species list and on the state endangered list. Ninety-nine percent of the prairie habitat that sustained it before the Europeans came has been destroyed--converted to cornfields, pastureland, industrial parks, roads, parking lots. Less prairie meant less habitat for our native hawkmoths and fewer colonies of leucophaea. So the chances of the two meeting up have shrunk to the point where few flowering leucophaea are setting seed. Persistence of this trend would doom the species.

Marlin Bowles, research associate at the Morton Arboretum, has been studying this relationship over the past 15 years. He wrote the recovery plan for the orchid commissioned by the U.S. Fish and Wildlife Service. Together with volunteers from the Nature Conservancy's Volunteer Stewardship Network, he has been doing the work of the delinquent hawkmoths, collecting pollen and carrying it to neighboring plants.

I suggested collecting the small army of hornworms at work in my yard and relocating them to restoration sites. But quinquemaculata, my backyard hawkmoth, though endowed with an overly long proboscis, is closely associated with plants of the Solanum genus--tomatoes and potatoes--plants not well represented in the native prairie. The recovery plan hopes to stimulate native populations of hawkmoths to do the job by restoring the orchid's prairie community to the kind of health and diversity that would attract and sustain them.

The prairie fringed orchid is endangered, but tomato hornworms are thriving in backyard gardens, even though they're routinely innoculated by braconid wasps. There are dozens of strategies these wasps use to parasitize caterpillars, but they all have the same aim: to transmute caterpillar tissue into wasp tissue. Judging by the size of the maggot brood I watched, my hornworm might have been parasitized by a wasp whose reproductive strategy was polyembryony, in which several eggs are inserted into the tissue of the host. The embryos subsequently divide, like identical human twins, but into numerous identical wasp larvae, sometimes up to 50 per egg.

The wasps that harvested the cocoons from the hornworm's back may have been practicing hyperparasitism, that is, parasitism on parasites. The cocoons may have been used to pack the egg dens of their own offspring with a bounty of food. Or they could have been inoculated with parasitic eggs that would eventually bore out from within their host, just as the larvae had done with the hornworm.

The 19th-century naturalists Gould wrote about were blinkered by their need to describe noble and good intent in every act of creation. But these eddying, infolding acts of transmutation are full of wonder all by themselves, without elaboration. Tomato vine to hornworm, to wasp, to wasp, to robin: ingredients jumbled and recombined in a kaleidoscopic array of life.

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