Thresholds for the Alfalfa Seed Pea Aphid

Make a Sweep

• Alfalfa producers use a sweep net to gather and count insect pests. The sweep net is 15 inches in diameter and made of a fine mesh material mounted on a long handle. The net is swung in a 180-degree arc with the bottom edge just touching the tops of the alfalfa plants. Make a number of sweeps throughout the alfalfa field and average the insect count per sweep.

Aphids per Sweep

• Control becomes necessary when the pea aphid count averages about 100 insects per sweep. Confirm the species of the aphids. While the threshold for the pea aphid is about 100 insects per sweep, blue alfalfa aphids require treatment at about 30 per sweep. Pea aphids are light green in color and are usually found on the bottoms of the leaves near the tops of the plant.

Check the Stem

• Producers don’t need to use a sweep net. Check the stems directly for the aphids. The University of California at Davis suggests treatment thresholds of 50 aphids on alfalfa stems under 10 inches long, 80 aphids on stems up to 20 inches in length and 100 aphids on stems over 20 inches long.

Controls

• Biological controls include ladybugs or lady beetles and green lacewings, which both feed on the aphids. Chemical treatments vary from state-to-state, licensing different chemicals for aphid control. Some chemicals have limited windows of application and can kill beneficial insects, such as bees.