Bucknell naturalist researches treacherous terrain — the world of insects and their host plants
March 31, 2004
By Gigi Marino
Biology professor Warren Abrahamson is a naturalist who lives on 30 acres of wooded property, but the terrain he inhabits is often treacherous, filled with fierce competition, invading forces, the very struggle to survive. And all of this taking place in small balled worlds that most of us ignore — the life and times of the solitary gall.
Galls, the tumorlike growths you often see on plants or leaves, are actually the homes for insects like flies or wasps. Abrahamson studies both and says that although there are 1,700 species of gall insects known in North America, these parasites live in mostly three plant families: the oak/beech family, the rose family, and the sunflower/goldenrod family. His specialty is the latter. He's recently celebrated his 30-year mark at Bucknell and says, "My tenure at the university is synonymous with goldenrod ball galls."
Goldenrod galls are inhabited by the goldenrod gall fly, which mates in the spring. The female lands on the plant, and using her ovipositor, deposits an egg into the pliable young plant. Five days later, the egg hatches, and the larva burrows into the stem, eating its house. In response to larval secretions, the gall grows. In the fall, the single-minded larva tunnels a passage through the gall for escape in the spring, then falls into a deep-freeze sleep over the winter. The cold is essential, Abrahamson says. "It keeps their respiration low." When spring arrives, the pupa forms, and a few weeks later, an adult fly breaks through the gall. That is if the larva survives the winter. He says that birds, especially downy woodpeckers, love little larva. And he also studies wasps that attack the goldenrod gall fly, invade its home, and deposit their own egg, which hatches and eats the fly larva.
Most recently, Abrahamson's work focuses on how species are formed. Goldenrod fields containing tall goldenrod and late goldenrod throughout most of the eastern United States have galls only on tall goldenrod. But walking through fields of tall goldenrod and late goldenrod in New England, Michigan, and Minnesota, he noticed something odd — galls appearing on both tall and late goldenrod. "This insect covers a range from Canada to Texas, and throughout the eastern States it was only living on tall goldenrod. I was puzzled by this pattern," he says. He wondered if the fly had multiple host plants (no) or if the fly had changed (yes). Studies showed genetic differences in the flies. "Not enough to be a different species," he says, "but the late goldenrod fly is definitely another race derived from the tall goldenrod fly. We're observing speciation in operation. We often see the end result but not the transition from one species to another."
Once Abrahamson and his students realized that they were looking at two races, they noticed something else. The late goldenrod fly occurred only in northern climes. Kim Weihrer Long MS'01 had done a population study of late goldenrod gall flies and found thousands in the Syracuse area. When Abrahamson had returned to the same field two years later, he only found 13. "This was the trigger," he says. "Syracuse had a record warm winter." Currently, the nearest sizeable late goldenrod fly population occurs 150 miles north of Syracuse.
Abrahamson and Jason Irwin, Burpee post-doctoral fellow, are studying respiration rates to understand the new fly with its smaller mass, which make surviving warm winters more difficult. Their research is funded by a $312,000 National Science Foundation grant. He says, "My revelation was seeing that the range of the organism I worked on could be so impacted by warm weather."
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Gigi Marino is editor of Bucknell World.
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