Soybean aphid life cycle: The soybean aphid overwinters as eggs on buckthorn. The eggs are deposited around the buds on buckthorn branches. Around April in Iowa the eggs hatch the resulting aphids will asexually reproduce between 1-4 generations. Winged aphids will develop during this time and migrate (the spring migration) to other hostplants (i.e., soybean). Once on soybean the aphid will produce 15-18 generations through the soybean growing season and frequently produce winged aphids that take part in "trivial flights" that move the population between soybean fields. As soybean begins to senesce aphids will produce winged individuals that migrate back to buckthorn (the fall migration) and sexually produce and deposit eggs near the buds on buckthorn.

Soybean aphid eggs: Soybean aphid eggs (small and yellow-green in color) are deposited in buckthorn at the base of the bud. (Photo Marlin E. Rice)

Winged soybean aphid: Winged soybean aphids are produced to move the population between its overwintering host and soybean and also for trivial flights between soybean fields. (Photo Marlin E. Rice)

The most dominant aphid in soybean is the soybean aphid [see soybean aphid life cycle on this page]. These aphids overwinter as eggs on buckthorn, Ramnus spp., [first photo from top of page] and the nymphs hatch in the spring, eventually producing winged migrant adults [second photo from top of page]. Migrating aphids will land and give birth to nymphs, which establish colonies on soybean [third image from top]. Depending on host plant and environmental cues, these colonies of wingless aphids may produce winged aphids on soybean [right -- forth image from top]. These aphid colonies in soybean may go through several cycles of winged-aphid production. At the end of the growing season, winged migrants are produced that fly from soybean to buckthorn where males and females mate and eggs are then laid.

Aphid-transmitted viruses can be an important part of the ecology of aphids (Hull 2002, Donaldson and Gratton 2007). This is particularly true for aphid behavior with some plant-virus infections, which attract aphids to some plants (Hull 2002) while some plant viruses are antagonistic to aphid population growth (Donaldson and Gratton 2007). The latter case apparently occurs with the soybean aphid and three RNA viruses (Soybean mosaic virus, Alfalfa mosaic virus, and Bean pod mottle virus) in soybean. Field plots infected with these three viruses have much lower in aphid abundance than uninfected plots (Donaldson and Gratton 2007).

Aphids in soybean are attacked by numerous species of predators. In Iowa, a majority (about 67-78%) of the predator community in soybean is composed of multicolored Asian ladybeetles (Harmonia axyridis), flower flies, (Toxomerus spp.), and insidious flower bugs, (Orius insidiosus) (Schmidt et al. 2008). These predator populations help suppress aphid populations to some extent; however, population buildup of predators generally lags behind that of aphid populations. This difference in population growth between predators and pests means that spraying an insecticide in the absence of economically damaging aphid populations can eliminate the predators from a field, which then allows aphids to recolonize a field in the absence of predators. In the absence of predators, aphid populations can increase dramatically in soybeans (Johnson et al. 2008).

For more information about the soybean aphid see, Soybean Aphids in Iowa–2007, Just for Growers and Soybean Aphids: Identifying, Confirming and Managing Soybean Aphids.

Soybean aphid mummy: A soybean aphid stung by a parasitic wasp is subsequently killed by the wasp larva, which then transforms the dead aphid into a mummy. (Photo Marlin E. Rice)

Predator attacking a soybean aphid: An adult multicolored Asian lady beetle, Harmonia axyridis, feeding on a winged soybean aphid. (Photo Marlin E. Rice)