Pea Aphid
Acyrthosiphon pisum (Harris), Aphididae, HEMIPTERA


DESCRIPTION

Adult - The long-legged pea aphid adult is light to deep green with reddish eyes. It has a body length of 2.0 to 4.0 mm and in North Carolina, most adults are wingless. The cornicles (a pair of tailpipe-like structures projecting from the abdomen) of this aphid are characteristically long and slender.

Egg - Approximately 0.85 mm long, the light green egg turns a shiny black before hatching. Eggs are very rare in North Carolina.

Nymph - The immature aphid is smaller than, but similar to, the larger wingless adult. It requires four molts to reach the adult stage.


BIOLOGY

Distribution - The pea aphid is found throughout the United States and Canada wherever peas and alfalfa are grown.

Host Plants - Pea aphids infest garden and field peas, sweet peas, sweet clover, alfalfa, and some leguminous weeds. Vetch and crimson clover are important overwintering hosts.

Damage - Pea aphids extract sap from the terminal leaves and stem of the host plant. Their feeding can result in deformation, wilting, or death of the host depending upon the infestation level. Plants that survive heavy infestations are short and bunchy with more lightly colored tops than those of healthy plants. Wilted plants appear as brownish spots in the field. Moreover, plants are often coated with shiny honeydew secreted by the aphids, and cast skins may give the leaves and ground a whitish appearance. These aphids also transmit the pea enation mosaic and the yellow bean mosaic viruses. The first of these viruses, the pea enation mosaic, has been a problem in New York but has not been reported in North Carolina.

Life History - In North Carolina, wingless, female pea aphids continue to feed and breed throughout the winter months. In spring, feeding activity increases. At this time, some winged aphids develop and migrate, usually to peas. Most of the progeny of these winged females develop into wingless females. Whenever overcrowding occurs, more winged aphids appear, migrate to different areas, and establish new colonies.

Each adult female gives birth to 6 to 8 nymphs each day until she has produced about 100 offspring. Nymphs mature into adults in 10 to 14 days. Since generations overlap and reproduction continues all year, the number of annual generations is difficult to determine. The pea aphid thrives best and reproduces most quickly at temperatures around 65 degrees F (18 degrees C) and humidities near 80 percent.


CONTROL

When the aphids infest alfalfa or clover, proper timing of cutting (during the early-bloom stage) reduces the aphid population by exposing it to the direct sun. In addition, proper and effective crop management results in a vigorous stand which resists aphid attack. Predators and parasites usually control this pest, to some extent, and resistant varieties are available.

A series of economic threshold based on the height of the plant and the number of aphids per stem are helpful to determining when chemical control is necessary. They are as follows: 40 to 50 aphids per 25 cm (10 inch) or shorter stem; 70 to 80 aphids per 25 to 38 cm (10 to 15 inch) stem; 100 aphids per 50 cm (20 inch) stem. For specific insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.