Aphids in Alfalfa

Crop Injury: Aphids that feed in alfalfa include the blue alfalfa aphid (Acyrthosiphon kondoi), pea aphid (Acyrthosiphon pisum), spotted alfalfa aphid (Therioaphis maculata), and the cowpea aphid (Aphis craccivora). Aphids feed on plant sap using piercing-sucking mouthparts. They may also inject a plant toxin during feeding which can cause yellow, distorted leaves and stunting. Aphids produce sticky honeydew which covers alfalfa foliage. Fungi known as sooty molds grow on honeydew and reduce plant photosynthesis. Dry weather can exacerbate aphid feeding, and seedlings are at greater risk than established plants. Disease may also be transmitted through aphid feeding.

Pest Description: Aphids are small (usually about 1/16 to 3/16 inch) insects that have soft bodies and “tail-pipes” (cornicles) on the end of the abdomen. In general, aphid nymphs appear similar to adults, although some adults have wings. Blue alfalfa aphids are bluish-green and appear dull or waxy with dark antennae. Pea aphids are pear shaped and range from light green or yellow to pinkish. They have long legs and long, dark-banded antennae. Spotted alfalfa aphids are relatively small with light green or yellow coloration. They have red eyes and six rows of dark spots on their backs. Cowpea aphid nymphs are dull gray while adults are shiny black. They have white legs tipped with dark “feet” and antennae that start white but darken towards the end. Aphids form colonies composed of developing nymphs and adults. Aphid colonies are often interspersed with whitish cast skins and parasitized aphid mummies.

Life Cycle: Adults and/or eggs overwinter, depending on species and environment. Aphids produce many generations annually and can quickly form large colonies when conditions are favorable. Females give birth to live nymphs with new generations occurring asexually. Winged adults can spread colonies to new plants or fields.

Scouting: Most aphids stop or slow reproduction when hot temperatures occur, although the spotted alfalfa aphid can continue to reproduce at temperatures greater than 90°F. Sooty mold and insects such as ants and lady beetles can indicate the presence of aphids. Aphid numbers can be estimated through stem counts. Weekly scouting, especially during spring and early summer, will provide the best information on aphid populations in a field.

Management: The best way to manage aphids in alfalfa is to plant aphid-resistant alfalfa varieties. Resistance ratings are available from the National Alfalfa and Forage Alliance (www.alfalfa.org). Aphid populations can generally be kept at tolerable levels through cultivar resistance, timely harvest, and natural enemies. Insecticides can be used to manage aphids in alfalfa; care must be taken to ensure aphids on lower plant parts are contacted by insecticide. Thresholds differ by aphid species and plant size; check with local extension resources for the most up-to-date economic thresholds. Be aware that insecticide use can kill important natural enemies of aphids.

Pea Aphid Management in Alfalfa

Prepared and reviewed by the Alfalfa Pest Management Working Group.