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Earlier this week cowpea aphids (Aphis craccivora) were reported in a Douglas County alfalfa field. Cowpea aphids were first reported in Missouri in the mid 1990′s and are usually observed in low number; however, occasionally the insect is found in large enough number to cause concern.
Cowpea aphids are very dark gray to black, and are the only black aphids to affect alfalfa crops. Adult cowpea aphids are usually shiny black, where as nymphs are smoky gray. This insect is relatively small, 1.5 mm to 2.5 mm long. Cowpea aphids can be found in 28 states and feed on legume crops.
In alfalfa, cowpea aphids feed on young, succulent growth by piercing plant tissue. Cowpea aphids tend to feed in clusters of newly expanding leaves, blooms, and stems. Infested plants become yellow, stunted, and photosynthesis will become inhibited resulting in low production and yield loss. Cowpea aphids are vectors of several important viral diseases, including alfalfa mosaic virus. The insects also secrete large amounts of honeydew, a sugary substance that may result in the formation of sooty mold.
Cowpea aphids are a sporadic pest; therefore, no treatment threshold or economic injury levels have been established for Missouri. Cowpea aphids have similar feeding habits to pea aphid (Acyrthosiphon pisum), so damage potential and treatment options are similar. Missouri thresholds for pea aphids in alfalfa indicate that treatment is justified if 50 or more pea aphids are present per alfalfa plant 20-inches in height and 100 or more aphids on plants greater than 20-inches in height. If plants are under drought stress or growing slowly due to cold weather, then the threshold number would be reduced. Treatment will also be warranted if plants are yellowing and showing symptoms of aphid damage. Cowpea aphid has become a major alfalfa pest in California. Threshold number for cowpea aphid in California suggest 10-12 cowpea aphids on stems of alfalfa plants 10-inches or less in height or 20-40 or more aphids on stems of alfalfa greater than 10-inches in height justify treatment.
Infestations of cowpea aphids are not uniform throughout the field. Spot or border treatments with insecticides are preferable to total field treatments when feasible; spot treatment will give beneficial insects a refuge. Many beneficial insects are present that are usually sufficient to keep aphid populations below damaging thresholds.
Begin scouting legume fields now for cowpea aphids. For questions about what insecticides to use to treat large infestations, contact Agronomy Specialist Sarah Kenyon at 417-778-7490 or KenyonS@missouri.edu.