When soybean aphids first invaded North America about seven years ago, the pests had a field day: environmental conditions similar to their Asian homeland, millions of acres of soybeans and few natural enemies. As farmers know all too well, the aphid population exploded, spreading rapidly across the Midwest, inflicting millions of dollars of crop damage.

In China, by contrast, soybean aphids rarely reach damaging numbers, says University of Minnesota entomologist George Heimpel. That's because beneficial natural enemies keep them in check.

Now Midwest entomologists are hoping to combat soybean aphids by introducing an effective enemy from its native land. This approach, known as classical biological control, has been used successfully to manage other foreign crop pests in the U.S., including cereal leaf beetle and alfalfa weevil, says Christine DiFonzo, a Michigan State University entomologist.

“The idea behind classical biological control is to explore the native range of an introduced pest and identify natural enemies for importation into the U.S.,” DiFonzo explains.

Beginning in 2001, entomologists from the University of Minnesota, Purdue University, the Illinois Natural History Survey, and the USDA-ARS went to China, Japan and Korea to collect insects that kill soybean aphids. Researchers brought back a number of species, including Binodoxys communis, a stingless parasitic wasp. The wasp, which is the size of a pinhead, lays its eggs inside soybean aphids. When the tiny larvae hatch, they eat the aphid's inner organs, then pupate inside the aphid's hollowed-out body.

This summer, the wasp was released in field cages in Illinois, Indiana, Iowa, Michigan, Minnesota, South Dakota and Wisconsin. The field tests will allow entomologists to assess the insect's efficacy and winter survival.

The release of Binodoxys communis comes after four years of rigorous testing at the University of Minnesota's quarantine laboratory in St. Paul, says Heimpel, who is heading up the Binodoxys communis investigations. Researchers had to make sure the wasps would not harm plants or insects other than soybean aphids.

“There has been a long process of lab work to ensure that the insects are safe” to release into the environment, Heimpel says. The lab testing was followed by an 18-month safety review by the U.S., Canadian and Mexican governments.

USDA release permits were issued this spring. “This is an important milestone for researchers and producers in the development of a long-term management strategy for the soybean aphid,” according to a statement from the North Central Soybean Research Program, which is funding the research.