Unusually quiet year for soybean aphids, but don't stop scouting

August 11, 2009

(PhysOrg.com) -- Entomologists across the Midwest expected to see soybean aphid outbreaks this year, but aphids have been slow in coming out, according to a Purdue University field crops entomologist.

Soybean aphid outbreaks typically occur during odd-numbered years, which means that there would typically be an increase in aphid numbers across the Midwest this year, said Christian Krupke.

Not only is it an odd year, but last fall researchers also observed large numbers of winged aphids overwintering on host sites like buckthorn, Krupke said. This also would indicate increased aphid numbers.

Normally the aphids start to attack soybeans in June and July, but activity was scarce until just last week, Krupke said. Soybean aphids did manage to reach threshold levels in LaPorte County in Indiana and required treatment in some fields, he said. The threshold level for treatment is 250 soybean aphids per plant in Indiana.

When scouting, Krupke saw several winged aphids in northwest Indiana fields, which means that some of those aphids soon will migrate and infest other fields.

"If you divide the state into four quadrants, the northwestern corner will always see heavy aphid activity first, at least historically," Krupke said. "It's the area closest to the prevailing winds that carry the soybean aphids in."

Despite the scarcity of activity throughout most of the state, growers should continue to scout their soybean fields at least once a week, Krupke said. It's cheap insurance, and aphids still have the potential to be a problem. With winged aphids on the move, fields can become infested very rapidly, he said.

"We could still see a late push from aphids, however, time is on our side," Krupke said. "Once soybeans reach full seed, also known as the R6 growth stage, there is no need to worry about yield loss. It's too late for aphids to cause economic damage."

The late appearance of soybean aphids could be caused by several factors, including late planting, Krupke said.

"There were no soybeans for the aphids to feed on until two or three weeks later than usual in Indiana," he said. "We may also be coming into equilibrium with the aphid even/odd year cycle, which may have been mostly a product of how new the aphid was. Those cycles rarely persist very long in any agricultural pest system."

Soybean aphids, a native of Asia, have been a serious pest in the Midwest since 2000. Aphids are small, yellow-bodied insects with distinct black cornicles, and they may be either winged or wingless. They are typically slow-moving and often found on the underside of the soybean plant's newest growth.

Producers also should check the nationwide soybean aphid map at http://www.sbrusa.net . Once at the Web site, select " Aphid" from the dropdown menu at the top right.

Provided by Purdue University (news : web)

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