Small Grains and Forage

Pea Aphid

Acyrthosiphon pisum Harris

Appearance and Life History

Adult Aphids Adult Aphids
Photo by J. Obermeyer

The pea aphid is a pest of legume crops including, peas, clovers, and alfalfa. Heavy populations may develop when insecticides used to kill other alfalfa pests and with little to no activity against aphids kill the predators and parasites of this aphid.

Pea aphid is about 1/6 inch (4 mm) long and ranges in color from light to dark green. Prolonged periods of cool temperatures [50°F to 60°F (10°C to 15.5°C)] and dry conditions are conducive to the development of pea aphid populations.

Up to twenty generations develop during the season. The entire life cycle takes about 12 days. Populations can increase rapidly since each female can produce 50 to 100 nymphs.

Time of Attack to Alfalfa - Pea Aphid


Pea aphids feeding Pea aphids feeding
Photo by W. Buhler

Both adults and nymphs suck juices from alfalfa plants. Under heavy infestations, this feeding can cause alfalfa to turn yellow and wilt. The top of such infested plants will die if subjected to excessive feeding. If large numbers of pea aphids develop in the spring, they can cause extensive damage to the first cutting and reduce the vigor of subsequent cuttings.

Sampling Method

If pea aphid populations appear to be large while sampling for alfalfa weevil, resample to estimate the extent of the pea aphid infestation. Take 20 sweeps in each of 5 locations throughout the field and estimate the volume of aphids in the net after each set of sweeps. Also note and record aphid predators, such as lady beetles and larvae and lacewing larvae, in net samples and while walking through the field. Use the following rating system to rank pea aphid abundance.

Average Vol./20 Sweeps Description of Population

If "moderate" to "heavy" populations are encountered, examine 25 randomly selected stems and count the number of pea aphids on each stem. Count all parasitized (gray to brown and balloon- like) aphids separately. Determine the number of healthy and parasitized aphids per stem. Also examine each stem and record length and growth stage.

Management Guidelines

Forage Insect Control Recommendations: E-series 220-W (PDF)

Treatment may be advisable if:

  1. the pea aphid population is moderate to heavy
  2. less than 10% of the aphids in the field are parasitized
  3. few aphid predators are present
  4. the average stem length in the field is less than 14 inches (35.5 cm)

If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.