August 14, 2001

A soybean pest that first showed up in southeastern Minnesota is moving rapidly westward and northward across the state. Infestations of the soybean aphid are expanding and intensifying, says University of Minnesota entomologist Ken Ostlie.

The soybean aphid is an eastern Asian soybean pest that first showed up in southeastern Minnesota and eight other Midwestern states in the summer of 2000. It was detected in southeastern Minnesota earlier this summer and has spread rapidly into counties farther west and north. In southeastern Minnesota counties where it first showed up, infestations are common and severe, says Ostlie.

Soybean yield losses from aphid infestations can be significant. "China has reported up to 58% yield losses," says Ostlie. "In a Wisconsin study in 2000 there was a 13% yield loss. The soybean aphid also spreads soybean mosaic and other viral diseases."

Ostlie says soybean producers have many questions about how to manage the pest and whether to treat infested fields with insecticides. "We're playing catch-up in trying to answer questions, because there isn't a lot of information available," he points out.

Ostlie says no stage-specific thresholds for insecticide use have been developed for row or drilled soybeans, but several hundred aphids per plant during reproductive stages are likely to damage soybeans. Neither the short- nor long-term impacts of insecticide application have been studied in the U.S. Insecticides may disrupt natural control of soybean aphids by predators and parasitic wasps, he says. This could lead to aphid resurgence.

"As a result of these uncertainties, farmers and their agricultural advisors find it difficult to decide whether or not to spray, especially with low soybean prices," says Ostlie.

If producers do decide to apply an insecticide, Ostlie recommends recording pre-treatment infestations levels, leaving check strips, and checking yields to evaluate the effectiveness of treatment.

Soybean aphids are pale yellow and less than one-sixteenth of an inch long. They are the only aphids that form colonies on soybeans.

Ostlie is the author of a new four-color flyer, "Soybean Aphids," that's available from county offices of the University of Minnesota Extension Service. It's also on the Internet at www.soybeans.umn.edu . The publication contains an enlarged color photo of an aphid, a graphic showing the life cycle of the aphid, and information on scouting for aphids, insecticide options and how aphids damage soybeans.