Courtesy of Kansas State University

The following information has been largely taken from University of Texas, University of Arizona, and Oklahoma State University websites. During the past few years, this insect, Aphis craccivora Koch, has been extremely abundant in alfalfa fields throughout arid parts of the Southwest, including Arizona, California, and Texas. Commonly referred to as the “black aphid”, it has been around for many years in the south, usually present in low numbers on cotton, alfalfa, and weeds. In addition, outbreaks of cowpea aphid have been reported sporadically throughout Kansas and Oklahoma, all within the past year. Infestation levels in these areas were reported to be from 50-125 aphids per stem and there was noticeable yellowing and stunting. In Nebraska in 2002 the aphids were noticeable but were not quite at economic levels in most fields. Since many growers were in the process of harvesting the third cutting when the aphids popped up, we advised them to take the cutting and watch the regrowth.

Description - The cowpea aphid is easily distinguished from other aphids in alfalfa largely because it is the only black aphid found infesting the crop. In general, it is a relatively small aphid, less than 2 mm long. Non-winged and winged adults are usually shiny black while the smaller nymphs may appear to be a dull gray to black. The first half of the antennae is white, and the legs are usually a creamy white color with blackish tips. In alfalfa, these aphids obviously feed on young terminal growth, but can be found infesting leaves, blooms, and stems. Damage symptoms include yellowing, wilting, and dieback. In general, legumes can be seriously damaged, either by direct insect feeding or by the transmission of virus diseases.

Distribution - The cowpea aphid is generally distributed across North America and has been reported in at least 28 states and in three Canadian provinces. This aphid species also has an extensive host range with a marked preference for legumes. Other known host plants are apple, carrot, cotton, cowpea, dandelion, dock, goldenrod, kidney bean, lambsquarters, lettuce, lima bean, pinto bean, peanut, pepperweed, pigweed, red clover, shepherdspurse, vetch, wheat, white sweet clover, and yellow sweet clover. The aphid lives throughout the year without producing sexual forms and they are always parthenogenetic viviparous females (ready to produce offspring at birth).

Monitoring and Treatment Guidelines - Because the cowpea aphid has only recently become a problem in alfalfa, no monitoring guidelines or economic thresholds have been developed for this aphid. An Oklahoma State University Entomologist provided the following information. “Normally, we do not worry much about cowpea aphid, and if temperatures increase, predators will feast heavily on them; however, if damage (yellowing and stunting) is evident, then insecticide treatment may be appropriate.” Based on his observations, cowpea aphids damage alfalfa and feed on the plant similar to the pea aphid; therefore, thresholds are likely similar. OSU Cooperative Extension recommends that on alfalfa less than 10 inches tall, 50 aphids/stem should be used as a threshold. On alfalfa taller than 10 inches, 100 aphids/stem may be used. On the other hand, the Texas website suggests a threshold near or below that of blue alfalfa aphid: Height less than 10 inches: 10-12 aphids per stem or 50 per sweep. Height greater than 10 inches: 40 - 50 aphids per stem or 200 per sweep. These thresholds have not been verified locally, but may be helpful in making treatment decisions.

Control - Very little information is available on insecticide efficacy against cowpea aphids. We suggest you use pea aphid management guidelines until more cowpea-specific information is available. A glance at a University of Arizona insecticide trial showed that Warrior at 2.6 oz/acre provided the best overall control. We established an insecticide trial at the Haskell Ag Lab (Concord, NE) in 2002, but heavy rains caused the aphid populations to crash right after insecticide treatment. Rain and possibly an abundance of lady beetles, a major aphid predator, appeared to have solved the aphid problem for us in our production fields. Early harvest is also suggested as a control method. Time will tell if this aphid will become a more common pest of alfalfa in Nebraska.

More information on the cowpea aphid can be found through the UNL Entomology Department Website at: http://entomology.unl.edu/fldcrops/pestipm.htm

Tom Hunt
Extension Entomology Specialist
Haskell Ag Lab, NEREC

Keith Jarvi
Integrated Pest Management Assistant
NEREC