CHAMPAIGN, Ill. - Are insects always just pests, or do they play a co-evolutionary role with the plants they eat? According to a chemical analysis of parsnip webworms and wild parsnips in the Midwest, pests and plants can and do adjust to each other to survive.
The idea of co-evolution is not new, but it is far from universally accepted. In 1964, Paul R. Ehrlich, now president of Stanford University's Center for Conservation Biology, and Peter H. Raven, now director of the Missouri Botanical Garden and botany professor at Washington University in St. Louis, founded the study of co-evolution. They defined it as the process of reciprocal evolutionary change in interacting species.
They noted that plants produce chemicals that are toxic to insects, and that insects produce mechanisms to detoxify many plant defenses. However, while most scientists accept the notion that insects must adapt to their host plants, some have debated whether plants tailor their toxins specifically against insects.
With the help of museum specimens of parsnip plants, including some that predate the webworms' post-1900 entry into the Midwest, University of Illinois entomologists May R. Berenbaum and Arthur R. Zangerl say they have strong evidence for both sides of the adaptation scenario. They published their findings in the Nov. 10 issue of the Proceedings of the National Academy of Sciences.
Today's webworms and parsnips in Minnesota and in northern and central Illinois have adapted to each other, the scientists report. Webworms feed on the reproductive structures of parsnips, so resistance to the insect is vital to parsnip survival. At each site, levels in the plants of four defensive compounds, called furanocoumarins, match remarkably to the levels of detoxification enzymes in the insects.
"If 30 percent of the plants have a particular chemical phenotype, then roughly
30 percent of the insects have the
phenotype that permits them to live on those plants," Zangerl sa
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Contact: Jim Barlow, Life Sciences Editor
b-james3@uiuc.edu
217-333-5802
University of Illinois at Urbana-Champaign
3-Dec-1998