Heavy infestations of soybean aphids can result in severe yield losses. Routine scouting is necessary to determine aphid populations and if an insecticide application is necessary. Controlling soybean aphids is most critical during the reproductive stages of soybean (growth stages R1 through R6).
Soybean aphids are small (1/16 inch) insects with black cornicles, or tailpipes, at the rear of the abdomen. The soybean aphid has a complicated life cycle that is completed on two different plant hosts. Aphids overwinter in the egg stage on the leaves of buckthorn and then migrate to soybean. Aphids can have up to 12 generations per year, most being wingless. Aphid populations can grow quickly so infested fields should be scouted on a regular basis. During late summer or fall, the population will produce winged aphids which move back to buckthorn where overwintering eggs are laid.
Soybean aphids cause damage by sucking plant sap. Soybean aphid outbreaks are associated with a reduction in plant height, pod number, reduced seed size and quality, and reduced yield potential. Under heavy infestations, leaf edges may turn yellow, which can mimic potassium deficiency. Moisture stress also favors aphid population growth and adds to risk for yield loss. Symptoms of feeding damage may include plant stunting, leaves covered with honeydew (sticky substance excreted by aphids) and black sooty mold, a fungal growth on honeydew coated leaves. Aphids are also capable of transmitting soybean viruses such as soybean mosaic virus.
You should begin spot-checking for aphids in seedling soybean and continue through pod fill (Figure 1). Soybean aphid can be found on growing points and young leaves of early vegetative soybean. As soybean matures to the reproductive stages, aphids can be found on all plant parts including the leaves, stems, and petioles. Aphids tend to build up more heavily on late-planted soybean fields, so check late planted fields closely.
Scout more intensively beginning at the late vegetative growth stages and calculate average aphids per plant on 20 to 30 plants per field. Field edges can be hotspots for aphid infestations, resulting in overestimation of aphid populations, so be sure to cover all areas of the field. It is important to check the entire plant. At this stage, aphids move from the top of the plant to the middle and lower areas of the canopy. Take note of winged aphids, high predator activity, and diseased aphids. These are all signs that the population of aphids may be in decline. Scout these same fields again within a few days to note if populations are increasing or decreasing.
Treatment is justified when an average of 250 aphids per plant is found on over 80% of the plants in the field and populations appear to be increasing. Protecting plants with insecticides will have the greatest impact on yield potential from flowering through pod development. Applications made prior to flowering may not provide an economic benefit and generally reduce beneficial insect populations, which may result in a resurgence of aphids. Applications made at or beyond the full seed growth stage (R6) have not been found to improve yield potential unless plants are under additional stress such as from drought.
1Ragsdale, D., Voegtlin, D., and O’Neil, R. 2004. Soybean aphid biology in North America. Annals of the Entomological Society of America 97(2): 204-208. https//:academic.oup.com.
2Pedersen, P. 2007. Soybean Aphid. Iowa State University Soybean Extension and Research Program. http://extension.agron.iastate.edu.
3Soybean aphid. 2009. Purdue University. http://extension.entm.purdue.edu.
4Koch, R. and Potter, B. 2014. Scouting for soybean aphid. University of Minnesota Extension. www.extension.umn.edu.
5Tilmon, K. 2014. Soybean aphid scouting and management. South Dakota State University. http://igrow.org.
6Kaser, J., Heimpel, G., and Koch, R. 2015. Parasitic wasps attacking Minnesota soybean aphids: Summary of a collaborative statewide survey. University of Minnesota Extension. www.extension.umn.edu.
Web sites verified 4/18/18.