Pea Aphids

Macrosiphum pisi

The pea aphid is a pea-green, soft-bodied insect about one-sixth inch long. The adult may be winged or wingless, while the smaller nymphs, which resemble the adults, are wingless. The nymphs shed their shins several times in the normal growth process; these cast-off skins are white and quite noticeable on plants and on the ground. Aphids produce a sugary, sticky material called "honey dew," which is visible on plants in moderate to severe infestations. Alfalfa, clover, and garden peas are the plants on which the aphids usually feed.

Pea Aphid Adults, Nymphs, and Mummy (parasitized) on Pea Plant
Figure 2. Pea Aphid Adults,
Nymphs, and Mummy
(parasitized) on Pea Plant

Life Cycle
Pea aphids spend the winter in both the adult and egg stages. Although in mild winters the adults may survive in the crown of the plant or in debris on the ground, during a severe winter most of the adults die and only the eggs survive. The overwintering eggs are located in the stems of alfalfa and clover. During the summer all individuals are females, the males appearing only in the fall. Each female gives birth to 50 to 100 living young at the rate of 6 or 7 a day. There are 7 to 20 or more generations a year. Most of the aphids are wingless, but some winged forms are usually present. A large number of winged forms may suddenly appear as a result of overcrowding or climatic conditions. In this form they fly to nearby fields.

Alfalfa and red clover that is severely infested may be stunted and occasionally may die. The tops of the plants are generally damaged first. If the damage continues, the leaves may drop from severely infested plants. Thus both quality and quantity of the hay crop are affected. Pea aphids can almost always be found in alfalfa and clover fields each year in the spring. Occasionally, however, severe infestations do develop. If the infestation occurs early and plant growth is slow, the plants will be severely stunted and may die. This aphid is also suspected of transmitting certain virus diseases of alfalfa.

A fungus that infects the aphids kills them and thus prevents widespread outbreaks. When the humidity is high and temperatures are moderate, diseased aphids shrivel and turn.brovn. This disease may develop into epidemic proportions and almost eliminate the pest in a short time; if temperatures are low, however, the disease does not develop. Several kinds of lady beetles and certain other insects prey on these aphids and consume them in quantity. There is at least one wasp that parasitizes aphids. At first the aphid body appears swollen; then the wasp grub tnside cuts a hole in the back of the aphid to escape. When the weather does not favor these natural enemies, chemical control may become necessary. But no control measures should be used if lady beetles are abundant.