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Study Shows Promise of Biological Weed Control
(March 20, 2003)

A University of Colorado study shows the success of using biological controls to manage weeds in regions of the western United States. The study, Effect of biocontrol insects on diffuse knapweed (Centaurea diffusa) in a Colorado grassland, published in the March-April 2003 issue of Weed Science, by T. R. Seastedt, Nathan Gregory and David Buckner, examines the effect of six biocontrol insects on knapweed over a period of five years (1997-2001). The researchers released four types of insects (knapweed root weevil, lesser knapweed flower weevil, spotted knapweed seedhead moth, and bronze knapweed root borer) at a diffuse knapweed site, where two other biocontrol species (banded gall fly and knapweed seed head fly) were already present.

Absolute cover of the diffuse knapweed declined from 8.3% to 1.9% from June 2000 to September 2001, the largest declines appearing in areas closest to where the insects were released. It is not known which specific species were responsible for most of the weed decline. However, the lesser knapweed flower weevil larvae seemed to be responsible for a reduction in diffuse knapweed seed production, which decreased from 5,000 seeds per m-2 in 1997 to less than 100 seeds per m-2. The adult insects, as opposed to the larvae, affected actual weed cover.

Diffuse knapweed, which covers about 3.2 million acres in the west, is a popular target for pesticide use. However, since the weed will always come back, simply killing it only treats the symptom, according to one of the study's authors, Tea Seastedt, Ph.D., a professor in the Department of Environmental, Population and Organismic Biology at the University of Colorado, Boulder. Using biocontrols instead of chemicals is practical, feasible and sustainable. Read more about Dr. Seastedt's work from his article Biological Control of Noxious Weeds: Using insects to manage invasive weeds, which appeared in the Winter 2000-2001 (volume 21, number 4) issue of Pesticides and You.

If you are interested in biological control, or any other non- or least-toxic pest management strategies, please contact Beyond Pesticides for more information and resources.