Soybean aphid counts highest in northeastern Iowa

Scouting reports indicate that soybean aphid numbers are increasing in a few Iowa locations--most notably in northeastern Iowa. Brian Lang, extension field specialist, Decorah, notes that a field near Waukon averaged 30 to 40 aphids per plant with some plants at more than 200 aphids per plant, but less than 50 percent of the plants were infested.

Brian notes that he has observed two general patterns so far. First, earlier emerged fields have a higher percent aphid infestation. Observations that help support this thought are that since the first week in June, he hasn't found any winged aphids on any soybeans, which suggests the aphids moved to the fields early. Second, fields in Allamakee and Winneshiek counties have a higher incidence of aphids then fields in counties south and west.

In central Iowa, John Holmes, extension field specialist, Clarion, has scouted soybean fields in Greene, Hamilton, Hardin, Humboldt, Marshall, Tama, Webster, and Wright counties. He has not found soybean aphids in any of these counties. However, soybean aphids have been found at the Field Extension Education Laboratory in Boone County, so they do occur in central Iowa. In eastern Iowa, Virgil Schmitt, extension field specialist, Muscatine, also reports finding aphids in Muscatine County.

Field scouting for soybean aphids should begin either this week or the next. A seasonal timeline for assessing the risk of soybean aphids during a crop season, developed with Matt O'Neal, assistant professor in entomology, can be found on page 125. Additional information on scouting for soybean aphids can be found at www.soybeanaphid.info.

Winged soybean aphid
Soybean aphids clustering on a soybean stem.

Seasonal Timeline

March-May

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.

June

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.

July-August

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.

September-October

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.

This article originally appeared on pages 124-125 of the IC-494(15) -- June 20, 2005 issue.

Updated 06/28/2005 - 1:42pm