As people scout for alfalfa weevils, they usually encounter many other species of insects and insect relatives. As a perennial crop, alfalfa is an excellent habitat for the survival and development of many arthropod fauna. One of the most frequently encountered insects early in the season is the pea aphid. Occasionally, people encounter rather high numbers of this insect. Dale Burmester with Gateway FS in Redbud found large numbers of pea aphids in some alfalfa fields in Monroe County. Matt Montgomery, Extension Unit Assistant in Sangamon County, also has found pea aphids.|
Pea aphids rarely cause economic damage in alfalfa. In fact, their presence probably supports several predators and parasitoids that may regulate other pest insects. Nevertheless, because they are common and occasionally numerous, you might want to take note of the characteristics for their identification. Pea aphids are about 1/8 inch long with light-green bodies, long antennae with dark bands, and dark eyes. They have piercing-sucking mouthparts, long legs, a pointed "tail," and two long cornicles ("tailpipes" projected from the rear of the abdomen). The pea aphid can be confused with the blue alfalfa aphid, a species known to cause damage in western states but not known to occur in Illinois. The blue alfalfa aphid is slightly smaller (1/10 inch) and lacks the characteristic dark bands on the antennal segments.
Pea aphids in alfalfa, and one pea aphid "mummy."
Aphid "mummies" also are common in pea-aphid colonies. These swollen, tan-to-copper aphid skins are the result of parasitism, suggesting that natural control is at work.
We do not recommend application of insecticides to control pea aphids in alfalfa. As I recall, one of the suggested economic thresholds is 1/2 cup of aphids per sweep of a sweep net. That's a lot of aphids, probably more than most people ever encounter in Illinois.--Kevin Steffey