The soybean aphid (Aphis glycines Matsamura) is a relatively new pest of soybean in the United States. It was first discovered in southeast Wisconsin in July 2000. Today, it is found all over the upper Midwest and Ontario, Canada. Here are a few things that you need to take into consideration when managing this pest.
Description and identification
Soybean aphids are small soft-bodied insects, and may be winged or wingless, depending on the season and plant condition. Winged aphids are commonly seen in late summer or early spring as aphids migrate between soybean and buckthorn. Once on a soybean plant, overcrowding may force soybean aphid to produce winged forms, perhaps to enable it to colonize other less-infested soybean plants in the field. During the growing season, they are concentrated on leaves, petioles, and stems at the top of the plant.
Soybean aphids have a cold-hardy egge that allows them to overwinter in the upper Midwest. In the fall, soybean aphids lay eggs on buckthorn (Rhamnus spp.), the only known overwintering host in the United States. Buckthorn is a very common shrub in the Iowa landscape. They tend to grow underneath trees in shelterbelts as shrubs or as small trees along fence lines or open fields margins. Some homeowners grow buckthorn as ornamental or shelter plants around the house and buildings.
The life cycle of soybean aphid is heteroecious and holocyclic (host-alternating with sexual reproduction during part of its life cycle). In the spring, the eggs will hatch and the soybean aphid will have 2-3 generations with sexual reproduction on buckthorn before winged females (fundatrices) move into soybean in early June. The winged females are parthenogenetic (being fertile without mating) and bear living young. Several generations (up to 18) of wingless females can be produced in a short time frame because of parthenogenesis. The stem apices and young leaves of growing soybeans are colonized first. Later, the aphids appear on the underside of leaves of mature plants. Aphid development is favored in late June to early July with temperatures of 72-77ºF and relative humidity below 78%.
If the aphid population on crops becomes excessive or if plant quality deteriorates, the next generation will have winged females. The winged females will leave the field and migrate to new fields with better quality and lower aphid population. There will be no males until late summer, after the winged females have migrated back to the overwintering sites. Mating will occur there, and eggs will be laid for next year’s generation. Both winged and wingless aphids can be found in soybean at one time; both are yellow-green in color. However, the winged forms will be slightly darker, primarily a result of wings.
Soybean aphids have a very high reproductive potential
Soybean aphids pose a significant risk to soybean production because of their tremendous reproduction potential. In the summer, the population in soybean is comprised of females that essentially clone themselves. All offspring are female, born pregnant, and give live birth. Their birth rate is 3-8 aphids per day for 30 days. The generation time is 7-10 days. The result is an exponential growth rate, where populations can double in 2-3 days under favorable conditions.
The factors that influence this population dynamic in soybean are numerous: the size of colonizing population from buckthorn, soybean variety, mortality from predators, parasitic wasps, fungal outbreaks, effects of environmental conditions on reproductive rate and survival, local redistribution by winged aphids among fields, and dispersal of alates from fields.
For more information about soybean aphids in Iowa, go to the Soybean Entomology Lab website, which hosts a variety of publications.