Soybean Aphid Numbers Have Increased Rapidly

Infestations in some Iowa areas are reaching levels where treatment with insecticides makes sense economically.

Soybean aphid numbers have increased dramatically in some Iowa soybean fields, especially in northern Iowa, and are reaching levels where there will be an economic benefit from the application of insecticides. A number of fields have been sprayed in the past week.

Iowa State University Extension entomologists say scouting is recommended to determine whether the population of this insect in your fields exceeds 250 aphids per soybean plant. That's the economic threshold to use to decide when to treat.

Jon Tollefson, a long-time ISU entomologist, says Iowa has been experiencing heavy aphid infestations in soybeans during odd numbered years such as 2005 and 2007. The even-numbered years have had lower aphid numbers. So why have soybean aphid populations increased in 2008?

This summer's weather favors aphids

He says "the weather this year has been somewhat atypical. There were frequent rains, with the totals now reaching the total typically that Iowa gets in a whole year. We also have been having a summer that has been rather cool. This cool weather is very favorable to soybean aphid reproduction and increased numbers in our soybean crop."

Minnesota experienced the increase the week before last and ISU Extension released an alert last week on the ISU Integrated Crop Management News website. That news alert encouraged growers to watch their soybean fields closely for aphid invasions and increases.

By this week extensive economic infestations of soybean aphids were reported in northwest Iowa with some economic infestations in northeast and north central Iowa. "There is the possibility with the several days of very hot and humid weather earlier this week that the aphid increases will slow down," says Tollefson. "However, you should not wait to see if the weather stops the aphid increase. Walk your beans now and look at the upper trifoliate leaves for aphids."

He advises scouting bean fields twice per week when populations are increasing so fast, like they have been recently. He provides the following guidelines.

How to scout for soybean aphids

If aphids are present, sample them by counting the aphids on the soybean plants. If there is an average of more than 250 aphids per plant in a field, then you should apply an insecticide to the field.

To scout for soybean aphids, look at the upper two or three trifoliolate leaves and stem for aphids first. Aphids are most likely to concentrate in the plant terminal early in the growing season. Scout five locations per 20 acres. Also, look for ants or lady beetles on the soybean plant--they are good indicators of the presence of aphids. Lady beetles feed on aphids while ants tend the aphids and "milk" them for honeydew. Field scouting needs to occur weekly until plants reach the mid-seed stage (R5.5) or the field is sprayed.

When aphids are found, estimate the population size per plant. Count all the aphids on several leaves and the plant terminal to establish what 100 or 250 aphids look like and then use this as a mental reference for gauging populations on other plants.

Speed scouting is another method

There is also a 'speed scouting' technique you can use that requires less time. Tollefson says this quicker scouting method, which is actually called "speed scouting," was developed at the University of Minnesota. The program is described at

Speed scouting uses the number of infested plants (40 or more aphids per plant equal an infested plant) as a guide for determining whether an insecticide application is justified. This is not a new threshold but rather a sampling tool that helps determine if the soybean aphid population within a field is above the 250 aphid per plant threshold.

Other resources to help you control aphids

ISU Extension offers several resources to assist soybean producers with aphid management. The ISU Entomology Soybean Aphid Web site,, is produced and maintained by Matt O'Neal, an ISU entomologist who specializes in soybean aphid management. The site includes links to local and regional soybean aphid information.

The Web site includes links to insecticide evaluation reports from 2005-2007 for soybean aphid management; a thorough description of soybean aphid biology and distribution; Integrated Crop Management Newsletter articles on soybean aphids; and other resources.

Two new publications directly addressing soybean aphid are now available from ISU Extension county offices or directly from the ISU Extension store, They are Soybean Aphid Management Field Guide, 2008, CSI-11, and Speed Scouting Soybean Aphids, CSI-15. Both are available free, supported by a grant from the Iowa Soybean Association.

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