Corn+Soybean Digest

Keep an Eye Out for Aphids

If you haven't started thinking about scouting your soybean fields for aphids yet, Marlin Rice, Iowa State University (ISU) Extension entomologist, urges you to get ready to start checking your fields by the end of June.

"Aphids often occur in soybeans in northeastern Iowa around mid June, but they are difficult to find, and infestations rarely reach the economic threshold of 250 aphids per plant during June. However, this year aphids were found earlier than expected in northeastern Iowa. Therefore, I recommend that producers start scouting the last week of June, or the first week of July at the very latest. Scouting can provide information on the presence of aphids and by scouting for several weeks, it can be determined whether the population is increasing or decreasing," he says.

If you visit the Iowa Soybean Aphid Team Web site this summer, you will find a map showing the counties where there have been confirmed reports of soybean aphids.

"The soybean aphid map tells us when aphids were first found, and reported, in a county. It serves only as an early-warning system, and of course there is the possibility that aphids will not be found in a county when in actuality they are present," says Rice. "So the map could give a soybean producer a false sense of security of no aphids are reported from his county. It cannot be used to determine if fields across the reported county have reached the economic threshold and should be sprayed."

Rice and his colleague, Matt O'Neal, assistant professor in Entomology, developed this seasonal timeline for assessing the risk of soybean aphids during a crop season.

Risk to soybeans - None
Aphids that overwintered on buckthorn start producing winged aphids that will migrate to soybean plants when they emerge in the spring.

Risk to soybeans - Small
Aphids will migrate to soybeans shortly after the plants emerge. Once on the plants, females reproduce asexually (no male required). As many as 18 generations may occur on soybean plants in one crop season.

Growers are encouraged to begin preliminary scouting during the end of June, although it is unlikely that economically damaging populations will develop during this time. Insect predators (such as lady beetles) within the soybean field have been shown to suppress early season population growth. Insecticides should not be sprayed prophylactically, below-threshold insecticide treatments would remove these predators and possibly increase later aphid populations.

Risk to soybeans - High
Aphid populations usually increase significantly in late July and early August. Weekly scouting of five locations in each 20 acres of a field is recommended. In addition to looking for aphids, producers should look for ants and lady beetles on soybeans - they are good indicators of the presence of aphids.

When aphids are found, estimate the population size per plant. An insecticide application is recommended if the average number of aphids is 250 per plant and appears to be increasing. Stagnant or declining populations, such as those with parasitic wasps (indicated by dead aphid mummies), can occur and do not require immediate insecticide treatment.

Risk to soybeans - Small to none
As soybeans reach maturity, the aphid population will start to decline. Male and female winged aphids will return to buckthorn, mate and lay eggs. The eggs will remain on the buckthorn until they hatch during the following spring.

The Iowa Soybean Aphid Task Force members include faculty and staff from Iowa State University (ISU) Extension, the ISU Department of Entomology, and the Iowa Soybean Association. The group plans regular communication this summer to producers through ISU newsletters, the Web at, and through the news media.

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