Monitor Alfalfa for Pea Aphid Activity

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.

Originally Submitted: May 24, 2024

Although the forecast looks like it is going to be warming up, it’s important to remember that pea aphids can be an issue during this time of year in alfalfa. When weather conditions are right, pea aphid populations can rapidly increase and result in yield reductions, but even with less-than-ideal weather their populations can still increase but at a slower rate. When large pea aphid populations are present in a field, they cause the alfalfa fields to take on a “golden” color due to feeding injury. There are many natural enemies or predators that feed on pea aphid infestations, but these predators are unable to keep pea aphid populations in check when the pea aphid populations are rapidly increasing. Temperatures are around 70 F provide the optimal conditions for pea aphids. Pea aphids are able to colonize alfalfa throughout the entire season, but their population growth is greatly reduced when temperatures exceed 90 F.

Identification

Pea aphid nymphs and adults vary in size, but adults are approximately 1/8 to 1/4 of an inch in length making them noticeably larger than other aphid species present on alfalfa. The nymphs and adults are generally light green-yellow or dark green in color. However, there is the possibility to observe pea aphids that are a pale pink color. Pea aphids have long cornicles (tailpipes) that are noticeably darker at the tips (Figure 1). A characteristic that is useful for identifying pea aphids is the dark bands present on their antennae (Figure 2). Pea aphids also have dark red eyes (Figure 2).

When scouting for pea aphids, their presence can be determined by using a sweep net. The populations present can also be determined by counting aphids on individual stems. Pea aphid colonies tend to occur on alfalfa stems and the newest leaves. When large populations are present on a plant, the leaves of the alfalfa may turn yellow, and the plant may also be stunted due to the nutrient loss caused by pea aphid feeding. This type of injury is generally associated with populations that exceed 50 aphids per stem, which would be a very noticeable infestation.

Scouting and Management

Generally, pea aphid populations are not much of a problem, though it is common to see growth after the application of insecticides for a different insect pest. To stay ahead of pea aphid populations, alfalfa should be scouted on a weekly basis throughout the growing season to ensure that populations do not exceed the recommended economic thresholds (Table 1). To scout for pea aphids in alfalfa, either a sweep net or direct stem counts can be used. 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 pea aphids present after every 30 swings and calculate the average. 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 pea 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 and daily average air temperatures. 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.

Reference

Hodgson, E. 2009. Getting to know aphids in alfalfa. ICM News, Iowa State University Extension.