The Economics of Aphids Infestations

Various ages of aphids on a soybean leaf
Various ages of aphids on a soybean leaf
Various ages of aphids on a soybean leaf.

"Soybean aphid populations remain small throughout most of Iowa except for the northeastern corner of the state. A few fields in Winneshiek County have reached the economic threshold of 250 plants and will soon be sprayed,” said Marlin Rice, Iowa State University (ISU) Extension entomologist.

Field scouting should occur weekly until plants reach the beginning seed stage (R5) or the field is sprayed. When aphids are found, estimate the population size per plant. Count all the aphids on several leaves and 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.

There are both wingless and winged forms, according to Matt O’Neal, assistant professor, Entomology. He says wingless soybean aphid adults are about 1⁄16 inch in length, pale yellow or green, and have dark-tipped cornicles (tail pipes) near the end of the abdomen. The winged form has a shiny black head and thorax with a dark green abdomen and black cornicles.

If producers find that aphids have infested their fields, there are a couple things they need to consider before having the field sprayed.

“Two concepts are very important in integrated pest management for understanding pest and yield loss relationships. These are the economic injury level and the economic threshold,” said Rice.

The economic injury level is the lowest number of insects that will cause economic damage, i.e., yield loss that equals the cost of control. Based on research from the University of Minnesota, the economic injury level is approximately 1,000 aphids per plant during the R1–R5 growth stages.

The economic threshold is a similar concept, but it is the pest density at which management action should be taken to prevent an increasing pest population from reaching the economic injury level. Again, University of Minnesota research suggests that the economic threshold is 250 aphids per plant.

“I recommend producers use the 250 aphids per plant economic threshold to justify an insecticide application to a soybean field,” said Rice.

“It incorporates a 5- to 7-day lead time before the aphid population would be expected to increase to economic damage level of 1,000 aphids per plants.

“Populations that average less than 250 aphids per plant should not be sprayed. Fields with small aphid populations should be scouted every 2–3 days to determine if they reach the economic threshold,” said O’Neal.

“Heavy rains and beneficial insects may reduce low or moderate populations. Insecticides are most likely the only option for control once the population reaches the economic threshold,” he said.

Visit the Iowa Soybean Aphid Task Force Web site for the latest on the spread of this insect in Iowa and more information on how to manage this pest.

Contacts :
Marlin Rice, Entomology, (515) 294-1101,
Matt O’Neal, Entomology. (515) 294-8622,
Keven Arrowsmith, Continuing Education and Communication Services, (515) 294-2405,
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,

The following high-resolution photos are available for use with this story:
Photo 1: leaflet115.jpg Caption: Small colonies of soybean aphids often occur on the undersides of leaves early in the season.  The aphid colony on the left leaflet contains about 115 aphids.  Photo by Marlin E. Rice
Photo 2: winged1.jpg Caption: Winged and wingless soybean aphids on a soybean leaf.  Photo by David Voegtlin
Photo 3: VariousAphids1.jpg Caption: A large colony of soybean aphids often consists of white, shed skins and brownish carcasses killed by fungal pathogens.  Photo by Marlin E. Rice