By TIM ALEXANDER
Illinois Correspondent

URBANA, Ill. — While many soybean producers in Illinois are readying for an invasion of Asian soybean rust disease that may not arrive, some have been busy contending with a more familiar foe: soy aphids.

The Illinois Farm Bureau reported the first confirmation of aphids in a soybean field on May 30 when the destructive pests were found on the underside of an early-planted soybean plant in Kirkland (northern Illinois).

A University of Illinois (UOI) entomologist, however, told Farm World that an all-out infestation of the critters, such as in 2003, is unlikely to occur this year.

“Soybean aphids are almost always going to show up first in northern Illinois because their overwintering host, buck-thorn, is prevalent north of Interstate 80. Also, soybean aphid populations develop best when temperatures are in the 70’s and 80’s - so if it’s cooler in northern counties than in central or southern counties, economic infestations will show up first in northern Illinois,” said Kevin Steffey, UOI entomologist.

“Having said this, however, it is important to note that the numbers of soybean aphids being found in northern Illinois this year have been very low, due primarily to the high temperatures that have prevailed thus far. If temperatures drop back into the 80’s, we’ll have to sharpen our focus.”

Steffey explained that since their discovery in 2000, high soy aphid populations in the state have occurred on an every-other-year cycle, basically.
“In Illinois and elsewhere in the Midwest, economic infestations have occurred primarily in 2001, 2003 and 2005, so we expect economic populations in 2007,” Steffey reported.

“The worst year, by far, was 2003 although other states have had pockets of severe infestations in other years. In 2005 when we anticipated another outbreak, temperatures were too high for optimal development of soybean aphid populations in Illinois.
“The same has been true, thus far, in 2007.

An early-April freeze which held much of the state in its grip for several days caused significant mortality of soybean aphids, lessening the threat in 2007, Steffey said.

“Based upon numbers of overwintering aphids, we were anticipating significant soybean aphid numbers this year. The freeze and the recent hot temperatures have held them back.”

Though the threat posed by soybean aphids may not be severe at this point in time, Steffey recommended producers maintain diligence in scouting for the pests.

“Scout early and often, at least weekly, beginning no later than late June or early July,” Steffey said. “The primary threat posed by soybean aphids occurs when their numbers reach economic thresholds - 250 or more aphids per plant - during the reproductive stages of soybeans, R1 through R5 or R6.

“If numbers of soybean aphids reach economic thresholds, a timely insecticide application (within three to four days after the threshold is reached) should prevent economic yield loss.”