In theory, biological pest control is a clean and green technology, friendly to the environment and to the farmer alike. In place of a manufactured pesticide, a natural enemy attacks the pest, usually by eating it.
In practice, that's often how biological control works. The right control organism with a selective appetite can generally disperse itself and suppress damaging populations of a weed or a crop-feeding insect, even on a large scale (SN: 7/26/97, p. 56).
Yet ecologists have worried about what the numerous new control organisms may be doing to native plants and insects. Control organisms, which typically come from other parts of the world, have traditionally been screened primarily for their impact on agriculture. The landscape is littered with reminders -- from the gypsy moth to kudzu and purple loosestrife -- that organisms let loose in new environments can turn into menacing invaders.
Now, Svata M. Louda of the University of Nebraska in Lincoln and her colleagues have documented what ecologists have been warning about. They found that a weevil imported to control exotic weeds has been attacking native plants -- to the point where extinction threatens.
The possibility of such effects has been known, but Louda's work "is the most complete assessment of nontarget effects in biological control to date," says ecologist Peter B. McEvoy of Oregon State University in Corvallis. Her data fill a void, he adds, in a hotly debated issue.
Louda reported the findings this week at the Ecological Society of America meeting in Albuquerque, N.M.
She and her coworkers happened upon the weevil and its effects unexpectedly. As an ecologist, Louda studies the long-term dynamics of native plants and insects at two prairie reserves in the Nebraska sandhills.
In 1993, the researchers were surprised to find a new insect they had not seen in their previous 9 years of monitoring the prairies. A handful of the insects turned up on native thistles, whose large seed heads are food for goldfinches and other creatures. One of the plants, the Platte thistle (Cirsium canescens), is unique to the sandhills.
The insect did not disappear. "In 1994, we started getting 20 instead of 3. Then in 1995, we got hundreds of them," Louda recalls.
Curious about the newcomer, they had the insect identified. It turned out to be the flowerhead weevil (Rhinocyllus conicus), a European species imported by the U.S. Department of Agriculture to control European weed thistles in pasture and rangelands. Louda says it was released in Nebraska in 1972. None of its target thistles grows on the prairie sites.
"In 1996, the numbers went up even more," says Louda. At that point, the researchers' curiosity turned to concern, and they reanalyzed their monitoring data. "We found exponential growth of this weevil on the Platte thistle at both sites."
The weevil is not just an innocent bystander, says Louda. "There's a significant impact on seed production," a decrease of about 80 percent. In earlier work, Louda says, she has shown how such decreases have dire consequences for the thistle population.
"The thing that really hit me hard here was the magnitude of the effect," says Louda. Given its only habitat is the sandhills, the Platte thistle could be facing extinction, she says. A related plant in the Great Lakes region, pitcher's thistle (Cirsium pitcheri), is already on the federal endangered species list and could be at greater risk if the weevil expands its range. Louda has found that a native insect -- a pictured wing fly -- that feeds on thistle seeds has been affected as well. "Its population on the Platte thistle crashed at the same time that the weevils increased."
Ray Carruthers, who oversees the Agricultural Research Service's biological control program in Beltsville, Md., says the agency does screen control organisms to avoid unwanted ecological effects. "There are always some side effects" of any form of pest control, he notes, but the benefits may be worth it.
The weed thistles, says P. Charles Quimby Jr. of the USDA in Sidney, Mont., can overwhelm native species. "If you've ever seen a stand of musk thistle, you have no biodiversity there."
Louda says such dense invasions are rare. Both she and McEvoy think the weevil, which was known to feed on a wide range of thistles, should not have been released in the first place and that there is much room for improving other biological control programs.
"Biocontrol can work," says Louda, "but we have to be more judicious about the selection of agents that are released."
Louda, S.M. 1997. Ecological effects of a host range expansion by a weevil imported for the biological control of weeds. Annual meeting of the Ecological Society of America. Albuquerque, N.M.
McEvoy, P.B. 1996. Host specificity and biological control. BioScience 46(June):401.
Mlot, C. 1997. Cassava pest biologically suppressed. Science News 152(July 26):56.
U.S. Congress Office of Technological Assessment. 1993. Harmful non-indigenous species in the United States. OTA-F-565. Washington, D.C.: United States Government Printing Office. The report can also be obtained at http://www.wws.princeton.edu/~ota/.
United States Department of Agriculture
Agriculture Research Service
Building 005, Room 220
Beltsville, MD 20705
Svata M. Louda
Department of Ecology
University of Nebraska
Lincoln, NE 68588
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