By Erin Hodgson, Department of Entomology
Aphids are common insects to see in field crops, especially in alfalfa. In Iowa, there are at least four different aphid species that can develop on alfalfa (Table 1). A recent report of cowpea aphid near Gibson, Iowa from extension field agronomist Mark Carlton prompted me to write this article. Many of you are quite familiar with pea aphid, but other species occasionally show up alone or in combination. Learning to distinguish the aphids takes a little practice with a hand lens, but is worth knowing in order to make sound treatment decisions.
General Description. In general, aphids are soft-bodied and pear-shaped insects with walking legs. The main diagnostic feature of aphids is a pair of cornicles that resemble tailpipes, located towards the end of the abdomen. Sometimes the cornicles are highly reduced, making identification more difficult (e.g., spotted alfalfa aphid). In addition, all aphids have a piercing-sucking stylet and feed on plant phloem. Aphids excrete a sugar-rich honeydew that can promote a sooty mold that reduces photosynthesis. Some aphids are capable of vectoring plant diseases via persistent or non-persistent (i.e., dirty needle) transmission. Those species that vector disease are considered more economically important because low aphid densities can reduce quality and yield. Heavily infested plants will be discolored and stunted.
Table 1. Common aphids in Iowa alfalfa.
Pea aphid. The pea aphid is found throughout North America and is the most common species in Iowa alfalfa. Adults are one-fourth inches in length, and body color ranges from light green to yellow, or pale pink. In addition to their large size, pea aphids can be distinguished from other aphids by the dark bands of color on the antennae. Pea aphids are in alfalfa the entire summer, but reproduction is dramatically slowed down when temperatures exceed 90 degrees F. Colonies prefer to feed on stems and newly expanding leaves. Pea aphids may turn leaves yellow and stunt overall plant growth when present in moderate numbers (50-100 per stem).
Blue alfalfa aphid. The blue alfalfa aphid can be found throughout the United States, but is not commonly found in Iowa alfalfa. Often blue alfalfa aphids and pea aphids are intermingled on plant stems, but can be distinguished with a few common characters. Adults are three-sixteenth inches in length and bluish-green in color. Blue alfalfa aphids can be dull or waxy and have uniformly dark antennae, compared to shiny pea aphids with dark antennal bands. Blue alfalfa aphids are most productive during spring and early summer due to mild conditions; these aphids begin to decline when temperatures exceed 90 degrees F. Colonies prefer to cluster and feed on newly expanding leaves, but will move down to stems as the leaves mature and become crowded.
Spotted alfalfa aphid. The spotted alfalfa aphid is found throughout the United States and is occasionally found in Iowa alfalfa. The spotted alfalfa aphid is smaller than most aphids in alfalfa, reaching one-eighth inches in length. This aphid is also distinguished because it is pale yellow with dark spots covering the abdomen. Unlike the pea aphid or blue alfalfa aphid, the spotted alfalfa aphid can successfully reproduce in warm temperatures (above 90 degrees F). Colonies prefer to feed on the lower portions of alfalfa, including stems, petioles and leaves. Spotted alfalfa aphids transmit a plant toxin while feeding and can cause early leaf drop, distinctive vein-banding or chlorosis.
Cowpea aphid. The cowpea aphid is common throughout the United States and Mexico and is becoming more common in Iowa alfalfa. This small aphid is less than one-eighth inches in length, and is easily distinguished from other aphids in alfalfa because adults are shiny black and nymphs are dull grey. The base of cowpea aphid antennae are white, but gradually darken towards the tip, and the legs are white with dark "feet." Colonies prefer feeding on newly expanding leaves, but cluster on leaves, blooms and stems. These aphids are most successful during early spring or late fall, and begin to decline when temperatures exceed 75 degrees F. Cowpea aphids transmit a plant toxin while feeding; moderate infestations can cause wilting and discoloration, and heavy infestations (more than100 per stem) can cause severe stunting, dieback or death.
Scouting and Thresholds. Although aphids are considered secondary pests in alfalfa, sometimes they surpass treatment guidelines (Table 2). Scouting for aphids in alfalfa is relatively easy, and can be estimated by sweep netting or direct stem counts. Fields should be scouted weekly, especially in the spring and early summer. Count aphids on at least 30 stems or take at least 20 sweeps per field, and average the number of aphids per stem or per sweep. For large fields, consider sampling multiple areas to ensure coverage.
Aphid Management. There are options to consider before using insecticides. Biological control, the use of resistant cultivars, and harvesting will often minimize aphids to tolerable levels in most cases. Fortunately, there are many different natural enemies to aphids. For those fields with consistent aphids, consider cultivars with at least moderate resistance to pea aphid.
Besides cost, there are two negative consequences for the overuse of insecticides on aphids: resurgence and resistance. Resurgence happens as a result of killing the primary insect pest (e.g., alfalfa weevil or potato leafhopper) along with the biological control agents in the field. Aphids on the undersides of leaves and lower stems will survive and the colony thrives without predators or other competition. Historically, aphids can build up genetic resistance to insecticides when complete coverage is not achieved over multiple applications. Any aphid survivors and their offspring are considered genetically resistant to that class of insecticides at any dosage. Because aphids have multiple generations with clonal reproduction, genetic resistance can build up faster than in other insects.
Insecticides should only be applied if they exceed treatment guidelines (Table 2). Several products are registered in Iowa for aphid control in alfalfa: beta-cyfluthrin, chlorpyrifos, cyfluthrin, gamma-cyhalothrin, lambda-cyhalothrin, methomyl, permethrin, pymetrozine, and zeta-cypermethrin. Use sufficient volume and pressure to ensure contact with aphids on the lower parts of the plant.
Table 2. Treatment guidelines for aphids (per stem) in alfalfa.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at firstname.lastname@example.org or phone (515) 294-2847.