Soybean aphids increase in Plymouth County

Soybean aphid numbers have increased dramatically in northwest Iowa soybean fields, and many fields are requiring treatment.

"There are several fields in the county above treatment threshold so people should be scouting their fields," ISU Extension Grain Specialist Joel DeJong said Wednesday. The treatment threshold is 250 aphids per plant.

"The 250 level tells us we're likely going to have that population increase but we want to treat it before it gets to that damaging level," DeJong said.

The injury threshold is the number of aphids per plant that actually starts causing yield loss. DeJong said that is 400-500 aphids per plant at today's soybean prices.

People in Plymouth County should be scouting their beans, DeJong said.

If aphids are found, there is time to apply an insecticide before much damage is done, DeJong said.

"At trigger level, it's probably going to take about five days, depending on the weather. It gives you a little window to apply insecticide without significant yield loss," he explained.

If aphids are present, sample them by counting the aphids on the plants. If there is an average of more than 250 aphids per plant in a field, DeJong said it's time to 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. DeJong said aphids are most likely to concentrate in the plant terminal early in the growing season.

He recommends scouting 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. DeJong said 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; then use that as a mental reference for gauging populations on other plants.

A quicker scouting method, called speed scouting, has been developed at the University of Minnesota. 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 aphids per plant threshold.

DeJong said a publication that describes how to use the speed scouting method is available at the ISU Extension office in Le Mars. The method is described online at http://www.soybeans.umn.edu/crop/insects...