Soybean aphid numbers are increasing in the Red River Valley area and other eastern counties of North Dakota. Most fields are approaching the economic threshold level and may be treated soon.

“This is the time to scout and monitor fields for soybean aphids,” says Janet Knodel, North Dakota State University Extension Service entomologist. “The economic threshold for soybean aphids in the upper Midwest is 250 per plant in 80 percent of a field.”

The economic threshold of 250 aphids per plant was arrived at through research conducted during a three-year period at 19 locations in the northern U.S., which included North Dakota. The economic threshold was established to give growers a seven-day lead time for insecticide application before aphid populations reach a level where economic losses actually begin.

“Even though soybean market prices are high, the 250-aphids-per-plant threshold is still valid,” Knodel says. “This is because 250 aphids per plant are lower than the population at which yield loss can be measured and attributed to aphid injury. What this means, in practical terms, is that the lead time for insecticide application is reduced from seven days to three or four days. Therefore, it is critical that growers monitor their fields closely.”

The NDSU Entomology Department has received a few calls from growers asking whether they should tank mix an insecticide with their last glyphosate application, even though soybean aphid numbers are not at the threshold.

“We do not recommend this practice, nor do we recommend any insecticide application when the economic threshold has not been reached,” says Patrick Beauzay, NDSU Entomology Department research specialist. “Applying insecticides too early to control soybean aphids very well may result in a second aphid invasion, which would require a second insecticide application. Insecticides also kill natural enemies of soybean aphids, such as lady beetles and damsel bugs, so reinvading aphid populations can increase very rapidly in the absence of natural enemies.”

Some vendors are offering guarantees on second insecticide applications. While the guarantee covers the chemical, it may not cover application costs. There also is the risk of spider mite flare-ups in fields that have been treated twice with pyrethroid insecticides because beneficial mites also have been killed. This may require a third application using an organophosphate insecticide to control the spider mites. Natural enemies keep aphid populations from growing rapidly and even may keep aphid populations from reaching the threshold.

“Conservation of natural enemies is very important in controlling soybean aphids, so don’t spray unless and until you have to,” Knodel says.

Most fields are in the R2 to R3 (end of flowering to beginning of pod development) growth stage, though some still are in late vegetative stages. The 250-aphids-per-plant threshold is valid from late vegetative through the R5 (early seed development) growth stage. Research indicates that insecticide applications at R6 (full seed) and beyond do not give a yield benefit. Therefore, insecticide applications for soybean aphids are not recommended after the R5 growth stage.

Pyrethroid insecticides offer good control of soybean aphids. Several pyrethroids are labeled for soybeans in North Dakota. Be sure to read and follow the label for application rates and preharvest interval restrictions.

NDSU Agriculture Communication