Check Alfalfa For Fall Pests

LEXINGTON, KY.
   Investigation of a recent alfalfa seeding that was developing bare spots turned up the black legume aphid, also called the cowpea aphid. It is the only black aphid found infesting the crop. Adults are usually shiny black while the nymphs are slate gray. In alfalfa, these aphids primarily feed on young terminal growth but can be found infesting leaves, blooms, and stems.
   Damage symptoms can include yellowing, wilting, and dieback. This aphid injects a toxin into plants as it feeds that can stunt or kill plants. It also produces lots of liquid waste (honeydew) that supports sooty mold growth. It is easy to check for this insect by looking for colonies of the aphids at plant tips at scattered locations over a field.
   Infestations occur when winged migrant aphids fly into fields and begin to deposit live young on suitable host plants. Aphid populations can build rapidly the fall and again in the spring. The fall flight, as seen thru trap catches, showed winged forms active from mid-August thru early October of this year with a peak in early September.
    No monitoring guidelines or economic thresholds have been developed for this aphid on alfalfa. However, Oklahoma State University recommendations base treatments on evident yellowing and stunting. On alfalfa less than 10 inches tall, 50 aphids/stem should be used as a threshold. In general control measures should be considered when the alfalfa is heavily infested with cowpea aphids and is not growing properly (stunted, yellowing, and sticky with sooty mold).
   What if the aphids are present in very low numbers? Note that they are there. Survival can be good during mild winters so check in the spring for population levels as you begin to watch for alfalfa weevils.
   The cowpea aphid is generally distributed across North America and has been reported in at least 28 states and three Canadian provinces. It 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 females ready to produce live offspring. Δ
Dr. Lee Townsend is Extension Professor of Entomology with the University of Kentucky at Lexington.