Our entomological colleagues to the north and northeast are reporting the first soybean aphids of the summer on seedling soybean plants.
Eileen Cullen, extension entomologist, University of Wisconsin, reported that in a research plot near Madison on May 24, 8 of 60 plants were infested with 1-13 aphids per plant. Similar populations were found near Arlington on May 30. David Ragsdale, research entomologist, University of Minnesota, reported soybean aphids on V1-stage soybeans in east-central Minnesota during the week of May 28. Some fields had more than 25 percent of the plants infested with an average density of 5 aphids per plant. Furthermore, Chris DiFonzo, extension entomologist, Michigan State University, reported three research farms with infestations of soybean aphids. In Michigan, all sites had less than 1 percent of plants infested with aphids.
In Iowa in recent years, soybean aphids have been found during the last week of May in central or northeastern Iowa, but as of June 4, we have not yet received any reports of aphids in Iowa soybeans. Our aphid suction trap network with traps located near Ames, Chariton, Nashua, and Sutherland did not collect any winged soybean aphids the weeks preceding May 18 and May 25. This may suggest that the Iowa population is small this year or maybe not because the traps won't necessarily represent the entire state. Also, it may take us several years to be able to interpret what the spring soybean aphid trap captures mean in relationship to early summer infestations in soybean fields.
The important point to remember is that soybean aphids will appear somewhere in Iowa in damaging numbers this summer--and field scouting will be necessary to determine the need for chemical control.
As of yet, no damaging populations occur in Iowa, but field scouting should start in 2-3 weeks so that informed decisions can be made in a timely manner.
Marlin E. Rice is a professor of entomology with extension and research responsibilities. Matt O' Neal is an assistant professor of entomology with research and extension responsibilities in field crops.
This article originally appeared on page 167 of the IC-498(13) -- June 4, 2007 issue.