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Just lookin' for a home

Natural History, Sept, 1998 by Arthur E. Weis, Warren G. Abrahamson

Like the boll weevil in the old folk song, a mother goldenrod gall fly is determined to give her young a good start in life.

A parent struggling to stay awake for the 2:00 A.M. feeding of her month-old baby or a tree swallow staring into the gaping mouths of his five featherless, chicks might be tempted to envy insects, whose parenting duties generally end with the laying of the last egg. But the life of an insect parent, or rather parent-to-be, is not carefree. Ecologists have found that for many insects, actions taken by the mother before the birth of her young are as vital to their survival (and ultimately their reproductive success) as is postnatal care in mammals and birds. Over the last twenty years, in the course of studying the goldenrod gall fly, Eurosta solidaginis--one of North America's most abundant but least-noticed insects--we have observed just how important these maternal preparations can be. Along the way, we have uncovered a complex set of relationships involving the insect, the plant on which it lays its eggs, and its several predators and parasites.

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Perhaps it is misleading to say the goldenrod gall fly goes unnoticed. The quarter-inch-long fly itself is rarely seen, but any observant person driving through the countryside of the eastern or midwestern United States or walking through a weedy vacant lot in winter is likely to spot the insect's eponymous trademark: a gall on the stem of its host plant, goldenrod. A curiosity of nature, galls are abnormal growths of plant tissue induced by the larvae of various species of aphids, wasps, and flies. The insect's relationship to the plant is parasitic, for while the tumorlike growth supplies the larva with food and shelter, the plant receives no benefit in return; in fact, it produces slightly fewer seeds than an unaflicted plant and grows more slowly.

The bright green gall induced by Eurosta is a prominent spherical swelling, typically about an inch in diameter, halfway up the goldenrod stem. Inside, a single fly larva feeds on the gall's inner tissues and bides its time. It pupates in early spring and emerges in May as a tawny, speckle-winged adult, quite handsome as flies go. The male seeks out the tip of a nearby, newly growing goldenrod stem, where he flicks his wings in a way females find irresistible. After mating, the female goes off in search of her own goldenrod plant. When she finds a suitable stem, she inserts her ovipositor (an egg-laying structure that works much like a hypodermic needle) into the stem's terminal bud and injects a single egg. Over the course of her adult life--which lasts, at most, two weeks--a female goes without food and may lay a hundred eggs. Each egg hatches four to seven days later, and the tiny larva burrows its way down through the bud and into the stem. There it stimulates the plant to begin producing a gall, which consists of an inner tissue rich in protein and starch and an outer, protective tissue that soon becomes dry and corky. (Just how the fly induces a gall is unknown.) As the larva grazes on the nutritious inner tissue, the host produces more of it, guaranteeing a steady food supply. The developing insect remains in the gall's small central chamber for the next fifty weeks, emerging as an adult fly to begin the cycle over again.

An emerging adult female Eurosta fly has some crucial decisions to make. First, she must find a goldenrod plant, and not just any one will do. Throughout the eastern United States, the fly lays its eggs on Solidago altissima, or tall goldenrod. Four related, similar-looking plants--the Canadian, rough, early, and late golden-rods--also grow in this region. A gall fly mother, as we learned from a series of experiments, will land on other species of goldenrod, but a quick walk over the developing flower is enough for the "taste buds" on her feet to persuade her to move on. We found that when we wrapped leaves from S. altissima around the bud of another species of goldenrod, we could trick a female into inserting her ovipositor into the bud, but she would quickly realize her mistake; microscopic receptors on the tip of her ovipositor enable her to "taste" potential hosts with her reproductive organs as well as with her feet. Additional experiments showed why it is adaptive to be choosy: when we fooled a few females into injecting eggs into the wrong goldenrod, nearly all the resultant larvae failed to induce galls.

Not all tall goldenrods are created equal, however, so mother gall flies must be even more discriminating. Some plants in our study area never carry more than a few galls, year after year, while others only yards away consistently carry many. We could tell that females had investigated the seemingly resistant plants; their exploratory probes left small scars on the buds. To find out how these plants managed to discourage the flies, we first cloned equal numbers of resistant and susceptible plants. Experiments showed that newly hatched larvae stimulate both types of plants to begin producing galls, but the resistant plants soon kill off the abnormal tissue. The larvae, left without food, die. A mother gall fly can detect--at least some of the time--the plants that are likely to starve her young. In our experiments, females probed resistant plants less often than they did susceptible ones. And the probes into resistant plants were less likely to end with the injection of an egg.

Just lookin' for a home

Natural History, Sept, 1998 by Arthur E. Weis, Warren G. Abrahamson

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