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With cooling temperatures, soybean aphids may be on the rise

With lower temperatures forecast in parts of the Corn Belt over the next few days, soybean aphids that have been previously stifled by heat this summer may be poised to develop into "threatening numbers."

As a result, University of Illinois Extension entomologist Kevin Steffey says it's now time to scout for yet another pest in soybean fields. To date, Steffey says aphid numbers have been steadily climbing throughout Illinois, and with projected weather conditions more conducive to the pest's development, that climb could sharpen soon.

"Depending on temperature and some other factors, soybean aphid population densities can double in two to three days," Steffey says. "Densities of approximately 20 or more aphids per plant on Aug. 1 or 2 could build to economic threshold levels within 7 to 10 days after the last sampling date. It is possible that numbers of soybean aphids could reach economic threshold levels in some fields in northern Illinois before the soybeans reach the R5 development stage."

According to the latest research, the economic threshold for soybean aphids is 250 aphids per plant, with 80% or more plants infested. This figure, Steffey says, is well below the economic injury level -- the aphid density that causes yield loss equal to control costs -- of 1,000 aphids per plant.

"Given the amount of time required for a soybean aphid population to double in size, the economic threshold of 250 aphids per plant allows for about seven days before numbers of soybean aphids reach the economic injury level," Steffey says.

If aphid numbers above the economic threshold are observed -- which Steffey says is happening in some parts of Illinois -- insecticide applications should come only after consideration of both growth stage and pre-harvest interval for the selected insecticide.

"Pay particular attention to the growth stage of the soybean plant. A return on investment for an insecticide application is unlikely after the soybeans reach the R6 stage of development," Steffey says. "Consider the pre-harvest interval when selecting an insecticide and follow all directions and precautions."

Insecticide products suggested for soybean aphids, according to Steffey, include Asana XL, Furadan 4F, Lorsban 4E, Mustang Max, Orthene 90S, Penncap-M, Proaxis and Warrior. With the exception of Orthene 90S, all are restricted-use products approved for application by certified applicators only.

With lower temperatures forecast in parts of the Corn Belt over the next few days, soybean aphids that have been previously stifled by heat this summer may be poised to develop into "threatening numbers."

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