A soybean pest that has the potential to rob soybean yield has again been sighted in Iowa. The soybean aphid has been found across the northern half of the state in recent days. While the pests have not reached the level where treatment is required, Iowa State University Extension entomologists urge farmers to begin scouting their fields.
"Soybean aphids are returning to soybean fields around the state," says Matt O'Neal, a soybean insect specialist and professor of entomology at ISU. "Growers should scout now and into August to determine if they will need to manage this pest with a application of insecticide."
Use 250 threshold for treatment decisions
The economic threshold for treating soybean aphids across the Midwest is 250 aphids per plant and increasing. In other words, if you have an average of 250 aphids per plant in a field, and weather conditions favor an increase in that population, it's time to spray an insecticide.
Despite the significant increase in the value of soybeans, entomologists at ISU and other universities are not altering their recommendation. "We have no evidence that soybean aphid populations below 250 per plant reduce yield," says O'Neal. "The density of this pest from year to year, and even from field to field, is remarkably variable. By scouting and using the threshold, growers can avoid an unnecessary application of insecticide."
Following this recommendation also avoids killing the beneficial insects which are predators of the aphids. "The predators can keep populations down until insecticide control is needed, if it is needed at all," he explains. "In the long run, following this 250 threshold recommendation helps reduce the risk of aphids becoming resistant to insecticides."
Tools to help you manage soybean aphid
Since the arrival of soybean aphid to Iowa, significant advances have been made in understanding this invasive insect's biology. The tools available to farmers for managing this pest have expanded as well. The Iowa Soybean Association and ISU are offering an informational resource to help growers manage this pest.
The pocket-sized Soybean Aphid Management Field Guide is a portable, durable tool farmers can keep in the glove box of their pickup truck and take to the field. This decision aid is designed to answer questions about the specifics of managing this potentially yield-robbing insect.
The guide also includes information on selecting insecticides and timing of application, the use of seed treatments, and information on tank mixing insecticides with fungicides and herbicides.
Speed scouting for aphids is new method
Also available to soybean growers is a companion publication titled "Speed Scouting Soybean Aphids," a new method that offers farmers and crop professionals information on how to determine whether or not they need to spray insecticides to control the aphid population in their fields. Developed by scientists in Minnesota, it helps soybean growers quickly and accurately determine if the number aphids in a field have reached the economic threshold.
"Speed Scouting Soybean Aphids is an excellent tool for soybean farmers who don't like the idea of spraying indiscriminately. Research has shown little or no yield loss when aphid populations are below the economic threshold," says David Wright, ISA director of contract research. "The pocket-sized Soybean Aphid Management Guide is a unique publication that includes the most up-to-date information on management. Both of these tools are designed to take the uncertainty out of the decision of whether or not to spray for soybean aphids."
Either aphid publication is available by calling ISA at (800) 383-1423. Also available is the Soybean Disease and Pest Management Field Guide. This publication is loaded with high quality images to help the user easily identify yield-robbing soybean diseases and insects as well as the latest soybean production information from ISU. The publications are offered at no cost by ISU through a partnership with ISA and the soybean checkoff.