A pin-head-sized parasitic wasp can find, parasitize and kill the soybean aphid, College entomologists have found.

Crop scientists have been working hard to find ways to control the soybean aphid since it made its U.S. debut in a southern Wisconsin field in 2000. This aphid and the viruses it transmits now cost Wisconsin growers as much as a million dollars each year in crop losses. Farmers planted more than 1.4 million acres of soybeans in Wisconsin last year.

Although only time will tell, the researchers hope the wasp will control aphids year after year without the need for insecticides. Field releases during summer 2002 showed that the wasp can establish populations in soybean fields. However, they will know the wasp
is truly effective when it shows it can survive Wisconsin”s winter weather and can reproduce and spread fast enough to keep pace with the aphid.

A plague of aphids
“The aphids overwinter in Wisconsin and appear in soybean fields about mid-June. Aphids reproduce rapidly and the soybean aphid can spread across an entire field within two weeks,” says David Hogg, an entomologist who is also the College”s executive associate dean. By late July, aphid numbers may exceed 5,000 per plant and the population then produces winged adults that carry viruses from diseased plants to healthy ones.

“Last summer, we released tens of thousands of these tiny wasps in three fields in Columbia, Dane and Racine Counties,” Hogg says.

You won’t get stung
If the idea of scientists releasing clouds of wasps makes you nervous, relax and put away the can of Raid.

“When people see the word ”wasp” they think of bald-faced hornets – something that”s big and mean and is going to sting you. We”re talking about something much different,” Hogg advises. “These wasps are really, really small and they”re not going to be going around stinging people.”
Hogg and UW-Madison entomologists John Wedberg and Bob Ellingson have been looking for ways to manage the aphid without pesticides. “We want to see if the wasp will become established and can control this aphid,” Hogg says.

The USDA”s Animal and Plant Health Inspection Service laboratory in Niles, Michigan raised the wasps used in the experiments. This wasp is now found in 11 western states, where its populations help control an aphid that attacks wheat, Hogg says.

Where infestations are severe, the aphid-virus combination reduces soybean yields by 10 to 15 percent. The soy<strongbean aphid also feeds on red clover, snap beans and other plants according to Hogg. The viruses it transmits also threaten late-season snap bean production in Wisconsin.
Using insecticide to control aphids would be costly for farmers and may have unwanted environmental effects.

The UW-Madison entomologists are collaborating with researchers from the Wisconsin Department of Agriculture, Trade and Consumer Protection and with the Animal and Plant Health Inspection Service. Scientists at the University of Minnesota conducted a similar trial there last summer.
The wasp, whose scientific name is Aphelinus albipodus, is native to Asia and Eastern Europe. It was introduced into the United States in 1990 to control the Russian wheat aphid and helped limit damage from that pest. Since 1990, the wasp has become established in states from California to Texas and north to Washington and Montana.

"Parasitoids are generally considered to have the greatest potential for reliable aphid control," Hogg says. Each wasp lays her eggs in aphids. As the immature wasp develops, it kills the aphid. A single wasp can lay enough eggs to kill several hundred aphids.

The research was supported by state funding to the College and a grant from the Wisconsin Soybean Marketing Board.