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Corn+Soybean Digest

Soybean aphids found overwintering in Missouri for first time

The first over-wintering soybean aphids and their immature nymphs have been found in Missouri. This raises the risk of having earlier infestations that could cause soybean losses in the coming season, said entomologists at the University of Missouri.

"We've avoided problems for two years," said Wayne Bailey, MU extension entomologist. "This is not the time to let down on our scouting."

Soybean aphids were first found in the state in 2000, but have not built harmful populations, unlike in soybean-producing states to the north. "Mostly soybean aphids have been a problem in northern Iowa and beyond," Bailey said.

"In the past, we thought our populations were wind dispersed from infestations in the north," said Ben Puttler, MU entomologist. "It now appears the aphid has potential to overwinter in the state."

In the first year, the pests, which suck juice from soybean leaves, were not found until Sept. 9 in Missouri. Last year the first aphids were detected Aug. 13.

With late starts, the populations did not build to economic infestations.

In Missouri, the aphids have been a threat mainly to second-crop soybeans, those planted after wheat harvest, Bailey said. "If they are overwintering, they can get an earlier start."

Puttler said, "If they became established by June, they could cause serious problems."

Puttler first found the pests in October on buckthorn trees and shrubs, the alternate host to soybean aphids. Soybean aphids originated in China and Japan. The buckthorn, an exotic ornamental, was imported from the Far East.

The MU entomologists said there is much unknown about the pest. But in China, the aphids build huge populations and go through several generations in a season.

The reason for the slow-growth in populations here is not known, Puttler said.

Some speculate that Missouri's warmer temperatures and higher humidity may deter the population explosion. "With large populations and multi-generations, the insect could adapt to our climate fairly quickly," Bailey said. "We just don't know what to expect."

The aphid was found in 18 counties of northeast Missouri in 2000. In 2001 it spread south and westward to another 14 counties, mainly along the Missouri River westward to Kansas City.

Soybean aphids do have several predators. "The most common observed with the aphids so far are the minute pirate bug and the green lacewing," Puttler said. Lady beetles and several native species of parasitic wasps also attack the aphids.

"We don't know yet the impact those predators are having," Bailey said.

Soybean producers should start scouting their fields soon after the crop comes up this year, Bailey said. The aphids are lime green to yellow, with black tips, front and rear. Individual aphids are so tiny they are difficult to identify without a 10-power hand lens.

If there is a large colony of aphids on the undersides of soybean leaves and the lower stems, it will be the soybean aphid, Bailey said. Black splotches show up on pant legs after walking in an infected field. The smudges are caused by honeydew produced by the aphid.

Work still needs to be done to determine an economic threshold for pest control, Bailey said.

The soybean aphid is included in the Soybean Diagnostic Guide just released by the United Soybean Board. It can be viewed on the Internet at:

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