Alfalfa Scouting: Pea Aphids
While scouting alfalfa last week we observed populations of pea aphids throughout much of the field. We also received reports of fields that have a golden hue and large populations of pea aphids present in south central South Dakota. The golden hue that is being observed is due to pea aphid feeding, and is an indicator that a large population may be present. The presence of pea aphids in alfalfa is common, but if they reach large populations there is the potential for yield reductions to occur. Many beneficial insects that are commonly referred to as natural enemies are attracted to large aphid populations. In alfalfa, these include multiple species of lady beetles, minute pirate bugs, green lacewing larvae, damsel bugs, and hover fly larvae. These insects can be efficient at managing pea aphid populations. However, during optimal weather conditions aphid populations can increase rapidly making it necessary to manage them using insecticides.
Pea Aphid Profile
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 aphids species present on alfalfa. The nymphs and adults are generally light yellow-green or dark green in color. However, some pea aphids are a pale pink color. Pea aphids have long cornicles (tailpipes) that are noticeably darker at the tips. The dark bands present on their antennae are also characteristic of the species (Figure 1). Pea aphids have red eyes that can be useful for identification purposes (Figure 2). Pea aphids tend to colonize alfalfa throughout the entire season, but their population growth is reduced when temperatures exceed 90 °F.
Scouting for pea aphid populations in alfalfa can be done using a sweep net, or 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 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 of pea aphids that exceed 50 aphids per stem, which would be a very noticeable infestation.
Figure 1. Pea aphid top view. Notice the dark tips on the “tail pipes”.
Figure 2. Pea aphid side view. Notice the red eyes.
Generally, pea aphid populations are not much of a problem, though it is common to see them increase after the application of insecticides for other insect pests. 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, use a sweep net or direct stem counts.
Sweep Net Sampling
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 pea aphids present after every 30 swings, and calculate averages. Repeat this process in an additional area of the field to determine field infestation levels.
Direct Stem Counts
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.
Table 1. Economic thresholds for average pea aphids in alfalfa per 30 sweeps or 30 stems.
|Less than 10” tall||300||40|
|More than 10” tall||400||75|
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 2017 South Dakota Pest Management Guide: Sunflower & Oilseeds/Alfalfa & Range.
Reference: Hodgson, E. 2009. Getting to know aphids in alfalfa. ICM News, Iowa State University Extension.