The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year. It requires two species of host plant to complete its life cycle: common buckthorn and soybean.
Common buckthorn is a woody shrub or small tree and is the overwintering host plant of the aphid. Soybean aphids lay eggs on buckthorn in the fall. These eggs overwinter and hatch in the spring, giving rise to wingless females. These females reproduce without mating, producing more females.
After two or three generations on buckthorn, winged females are produced that migrate to soybean. Multiple generations of wingless female aphids are produced on soybeans until late summer and early fall, when winged females and males are produced that migrate back to buckthorn, where they mate.
The females then lay eggs on buckthorn, which overwinter, thus completing the seasonal cycle. Nebraska lacks significant and widespread buckthorn populations and early season soybean colonization by aphids migrating from buckthorn appears to be limited.
Soybean aphid populations can grow to extremely high levels under favorable environmental conditions (Figure 1). Reproduction and development is fastest when temperatures are between 70° and the mid 80°s. Aphid numbers can change rapidly and populations can double in just two to three days. The aphids do not do well when temperatures are in the 90°s, and are reported to begin to die when temperatures reach 95°. When temperatures drop below 48°, development stops.
When populations reach high levels during the summer, winged females are produced that migrate to other soybean fields. Like a number of other insect species (for example, potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin. These summer migrants are likely the major source of infestations in Nebraska during several of the past years.
Soybean Aphid Natural Enemies
There are many insect predators of soybean aphid. The most visible soybean aphid predator is the multicolored Asian lady beetle (Figures 2-3), but the tiny (1/10 inch long) insidious flower bug (or Orius) is the most commonly occurring and important predator. It feeds on a variety of small insects and spider mites.
Naturally occurring predators, primarily the insidious flower bug, can significantly slow soybean aphid population growth, particularly during our hot July weather. Resident populations of predators also help reduce the rate of successful colonization of soybeans by the soybean aphid. Other common predators include green lacewing, brown lacewing, damsel bugs or Nabids, and spined soldier bugs, among others.
Other groups of natural enemies include parasitoids and pathogens. The presence of aphid “mummies” (light brown, swollen aphids) indicates the presence of parasitoids. These mummies (Figure 4) harbor immature parasitoids, which will become adults, emerge from the mummy, and parasitize more aphids. The presence of “fuzzy” aphid carcasses indicates fungal pathogens are present, which occasionally can lead to dramatic reductions of aphid populations.