Black aphids, like black squirrels, are striking in contrast to their leafy green backgrounds. But like squirrels, aphids are sometimes a nuisance, feeding on something we don’t want them to eat or damage. The black bean aphid, seen here on Euonymus, is fun to see but it can cause injury to the plant when its population is high.
It does have a pretty interesting life cycle. Like many aphids, the black bean aphid has both a winter host and a summer host. It overwinters on Euonymus or Viburnum in the egg stage. As temperatures start to warm in the spring, the eggs hatch and there are 2-3 generations of wingless, asexual females (produce young without mating) before winged asexual females appear. These winged females migrate to the summer hosts. Summer hosts of the black bean aphid include beans, corn, sugarbeets, lambsquarters, and pigweed. Multiple generations occur on these hosts and populations can build up very rapidly, consisting of wingless, asexual females. In the fall, winged asexual females and winged sexual males return to the winter host and have one generation of sexual females. Mating occurs and eggs are laid for overwintering.
The most effective management strategies for controlling aphids include preventing the population from increasing to damaging levels by monitoring, encouraging natural enemies, and the use of non-toxic or least toxic sprays. Monitor host plants beginning early in the season. Check the undersides of the leaves and hidden places on the plants. A sticky substance on the plant may indicate the presence of aphids. This is honeydew, excreted by aphids and other insects with piercing and sucking mouth parts. Sooty mold fungi may grow on this and cover surfaces with a black coating. Ants also like honeydew and may be found as well.
If aphids are present, they can be dislodged using a strong spray of water (not strong enough to injure the plant!). Encourage natural enemies by avoiding the use of chemical insecticides and by providing flowers to attract the adults of parasitoids.
Least toxic sprays include insecticidal soaps, horticultural oils and neem products. Test these on a small area of the plant to make sure plant injury does not occur. Plant injury is most likely when temperatures exceed 85°F.