Cowpea Aphids Infesting Alfalfa

Identification

Figure 1. Cowpea aphid colony. Courtesy: Philip Rozeboom.

Cowpea aphids vary in color based on their life stage. Nymphs tend to be a dull, dark-gray, while adults are a shiny black color (Figure 1). Both nymph and adults have tan-colored legs with darker tips (feet), tan antennae and black cornicles (tailpipes). The adults are approximately one-eighth of an inch long.

Scouting and Management

Generally, cowpea aphid populations are not much of a problem. However, it is common to see population growth after the application of insecticides for a different insect pest, like alfalfa weevils. To stay ahead of cowpea aphid populations, alfalfa should be scouted on a weekly basis throughout the season to ensure that populations do not exceed the recommended thresholds (Table 1). To scout for aphids in alfalfa, either use a sweep net or do direct stem counts.

If using a sweep net to sample, conduct 30 pendulum swings for each leg of a “W” or “Z” pattern while walking in the field. Count the number of each species present after every 30 swings and calculate averages. Repeat this process in an additional area of the field to determine field infestation levels.

If using the direct stem counts, collect a total of thirty stems while also walking in a “W” or “Z” pattern. For each stem, carefully cut it near the soil surface and shake it into a white bucket, then count the total number of aphids per stem and calculate an average for the field.

No matter which method you use, while walking through the field, pay attention to the presence and abundance of natural enemy species. If numerous natural enemies are also collected in the sweep net or observed while walking, insecticide management may not be necessary depending on the aphid populations.

However, if thresholds are exceeded, a list of insecticides that are currently labeled for aphid management in alfalfa can be found in the latest South Dakota Pest Management Guide – Alfalfa and Oilseeds.

Economic Thresholds