Posted 27 March 2012. Plant Health Progress.
Source: University of Illinois at Urbana-Champaign Press Release. aces.illinois.edu
Urbana-Champaign, Illinois (March 19, 2012)--Pea aphids are green-colored, 1/6-inch long insects that overwinter within the state. They spend their winter life either as an adult, safely nestled beneath debris, or as an egg pasted upon the leaves or stems of alfalfa. If the winter is fairly hard, adults will die and only the eggs will remain the following spring, but if the winter is reasonably mild, such as was the case this year, then adults will survive along with the eggs.
During the growing season, only the female is detectable in the field. Those females have the rare ability to reproduce without males and to give birth to live young rather than eggs for a portion of the year. Occasionally, if populations are somewhat high and the food is fairly sparse, winged adult females will be observed. The winged females ensure that some aphids can fly to another field with more food and continue the cycle. During the fall, males begin to increase in number, mating occurs, and eggs are produced. A single female may produce up to 199 young, each of which takes about a week and a half to go from egg stage to adult stage. A dozen or more generations may be observed each year, and large pea aphid numbers may be more likely when daytime temperatures hover around 50 to 60 degrees.
Their presence in the early spring is rather common, and during most years they do not harm alfalfa stands. However, when their feeding and reproduction surpasses the plant's ability to cope, the result can be yellowing and wilting of the plant as the insects insert a needle-like mouthpart into the plant and begin to suck away plant juices. That feeding can result in decreased yield and quality, but as was mentioned earlier, pea aphids typically do not reach a high enough population to cause such damage. In most years, a fungal disease, in combination with a parasitic wasp, keeps the population in check. Even at this early date, pea aphid mummies, swollen brown aphid bodies that house the larval stage of the parasitic wasp, have been found. This is an indication that the natural controls are already at work in the field.
Scouting for the insect, as with all insects, should occur previous to feeding symptoms and should include scouting for signs of natural control at work in the field. The producer should simply use a fifteen-inch sweep net, available in several agricultural catalogs, and make twenty sweeps in five separate areas of the field. If the producer averages about a half-cup of aphids per sweep, economic losses may occur.