A Eurasian weevil imported by U.S. and Canadian scientists to help kill invading thistles also threatens native thistles. Ecologists say the finding, reported today in Science  , highlights the ecological risks posed by biological control programs--often hailed as environmentally friendly alternatives to pesticides.
Inflorescence of Cirsium canescens showing (A) the adult R. conicus on the stem and eggs beneath the flower. (B) Adult R. conicus. (C) A pair of R. conicus larvae inside the flower head, having consumed developing seeds of C. canescens. (D) Larva of R. conicus.
Over the last century, scientists and farmers have intentionally imported hundreds of kinds of insects and parasitic microbes to North America to fight unwanted species. Some of the killer bugs, which are considered safer and cheaper than toxic chemicals, have helped dramatically reduce the economic damage wrought by some agricultural pests and weeds, such as alligator weed in Florida waterways.
But these benefits may be accompanied by some unexpectedly high and often ignored ecological costs, says biologist Savata Louda of the University of Nebraska, Lincoln. Louda and three colleagues have documented the misadventures of a Eurasian weevil, Rhinocyllus conicus, which was widely released in North America beginning in 1968 to control several kinds of introduced thistles. But the weevil also has a taste for native thistles, Louda and colleagues have found. In 1992, weevils infested fewer than 10% of the flower heads per thistle in national parks in Colorado and South Dakota. Last year, up to 70% of the flower heads were infested, and seed production had dropped--possibly threatening the future of the five native plants, the researchers say.
Donald Strong, an ecologist at the University of California, Davis, calls the study "startling news of a mainstream biocontrol project gone haywire." He says it emphasizes the need for more careful scrutiny of the economic and ecological issues surrounding the purposeful introduction of exotic insects.
But L. T. Kok, an entomologist at Virginia Tech in Blacksburg, says that very few of the pest-control insects approved for release in North America each year will pose a serious threat to native plants. He says that an international panel of scientists scrutinizes all proposed releases and that "it is very difficult to get approval."