One of the insects that should be monitored for in alfalfa during the spring and early summer are pea aphids. Although we haven’t spotted any yet in 2019, the presence of pea aphids in alfalfa is common. If they reach large populations, there is the potential for yield reductions to occur. There are many natural enemies or predators that are attracted to fields with pea aphid infestations. In alfalfa, these include multiple species of lady beetles, minute pirate bugs, green lacewing larvae, damsel bugs, and hoverfly larvae. These insects can be efficient at managing pea aphid populations; however, during optimal weather conditions (around 70o F), pea aphid populations can increase rapidly and make it necessary to manage them using insecticides.
Pea aphid description
Pea aphid nymphs and adults vary in size, but adults are approximately 1/8 to 1/4 of an inch in length. Their larger size makes them more easily noticeable 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) on the end of their abdomen that are darker at the tips. A characteristic that is useful for identifying pea aphids is the dark bands present on their antennae (Image 1).
Pea aphids also have red eyes that can be useful for identification purposes (Image 2). Pea aphids tend to colonize alfalfa throughout the entire season, but their population growth is reduced when temperatures exceed 90o F.
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 be stunted due to the nutrient loss caused by pea aphid feeding. This type of injury is generally associated with populations of pea aphids that exceed 50 aphids per stem, which would be a very noticeable infestation.
Pea aphid scouting and management guidelines
Generally, pea aphid populations are not much of a problem, though it is common to see pea aphid numbers increase 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 season to ensure that populations do not exceed the recommended thresholds (Table 1). To scout for pea aphids in alfalfa, either a sweep net or direct stem counts can be used.
| Average Number of Pea Aphids Found | ||
| Growth Stage | Sweep | Stem |
| Seedling | 5 | |
| Less than 10” tall | 300 | 40 |
| More than 10” tall | 400 | 75 |
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 averages. Repeat this process in an another seperate 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. Next, count the total number of aphids per stem and calculate an average for the field.
No matter which method you use, pay attention to the presence and abundance of natural enemy species while walking through the field. 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 2019 South Dakota Pest Management Guide: Alfalfa and Oilseeds.
References:
Hodgson, E. 2009. Getting to know aphids in alfalfa. ICM News, Iowa State University Extension.
