2016 Scouting For Pea Aphids In Alfalfa

Jun 03, 2016

By Adam J. Varenhorst

SDSU Extension Field Crop Entomologist SDSU Plant Science Department

Several species of aphids can be found when scouting alfalfa including the pea aphid, blue alfalfa aphid, cowpea aphid, and spotted alfalfa aphid. This season in South Dakota, the pea aphid is the most common, being present at varying populations in every field scouted.

About Aphids

Aphids are small soft bodied insects that have pear or tear-drop shaped bodies. Depending on the species they may vary in color from green to black, and both winged and wingless forms may be observed in a single colony. Aphids have piercing-sucking mouthparts that are often compared to tiny needles. Aphids use these mouthparts to feed on the phloem of the plant, which is composed primarily of sugar. This high sugar diet results in the production of excrement called honeydew, which is also mostly sugar. Honeydew promotes the growth of black sooty mold, which reduces photosynthesis and further decreases plant health. Fields with large aphid populations may appear shiny (due to honeydew) or darker than normal (due to black sooty mold). Large aphid populations will often attract natural enemies such as lady beetles, minute pirate bugs, green lacewing larvae, damsel bugs, and hoverfly larvae. These predators can be efficient at managing the aphid populations. However, during optimal weather conditions aphid populations can increase rapidly making it necessary to manage them using insecticides.

Pea Aphids

Pea aphid adults are approximately ¼ of an inch in length making them noticeably larger than other aphids species present on alfalfa. Both the nymphs and adults may be light green to yellow in color, or occasionally pale pink. Pea aphids have long cornicles (tailpipes) that are noticeably darker at the tips. Pea aphids also have characteristic dark bands on their antennae (Figure 1). Pea aphids tend to colonize alfalfa throughout the entire season, but their population growth is hindered when temperatures exceed 90° F. A sweep net can be used to scout for pea aphids in alfalfa; however, it is often easier to count the 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 nutrient loss caused by feeding. This type of injury is generally associated with populations of pea aphids that exceed 50 aphids per stem, which would be very noticeable.

Figure 1. Pea aphid. Photo by Salvador Vitanza.

Blue Alfalfa Aphids

Blue alfalfa aphids get their name due to their blue-green coloration. Although these aphids may be found in South Dakota, their presence on alfalfa is generally rare. These aphids are often found in association with pea aphids, but they are slightly smaller at 3/16 of an inch in length. Blue alfalfa aphids also have elongate cornicles that are darker at the tips. Their antennae are uniformly black (Figure 2). These aphids tend to congregate on newly developed leaves, and winged or wingless adults may be observed in a single colony. Like pea aphids, the blue alfalfa aphids experience decreased population growth when temperatures exceed 90° F.