Alfalfa Insects
Blue Alfalfa Aphid, Acyrthosiphon kondoi.
Description: The blue alfalfa aphid looks very much like the pea aphid. However, the blue alfalfa aphid is slightly smaller, more of a blue-green in color, and may have a waxy appearance. In contrast, the pea aphid is lighter green in color and shinier than the blue alfalfa aphid. The third antennal segment of the blue alfalfa aphid is uniformly brown; that of the pea aphid has a narrow dark band at the tip. The thoracic area on the winged forms of the blue alfalfa aphid is dark brown as opposed to light brown on the pea aphid.
Light infestations have been recorded from most counties of Kansas, but serious damage has been rare. Heavy aphid infestations, particularly on pea aphid-resistant varieties, should be identified by an entomologist.
Alfalfa stunting caused by blue alfalfa aphid feeding
Distribution: The blue alfalfa aphid was discovered in California in 1975. Since then it has become well established in that state and has spread to Arizona, Nevada, New Mexico, and at least as far east as Kansas and Oklahoma.
Life History: The blue alfalfa aphid overwinters in the egg stage in the Midwest. The tiny black eggs are glued to the stems and fallen leaves of alfalfa and clover in the fall. Hatching occurs in the early spring, and the young nymphs feed on new growth of alfalfa or clover. They molt four times before becoming adults. After one or two generations on alfalfa, a large proportion of the next generation will develop wings. These winged females disperse with the aid of the wind into adjacent alfalfa fields where they start new colonies, producing nymphs parthenogenetically. The entire life cycle takes about 7 to 10 days at an average temperature of 70°F, and since each female can produce 50 to 100 nymphs, large populations can result. Blue alfalfa aphid populations decrease rapidly when the daily high temperature reaches 85° to 90°F.
Damage: The blue alfalfa aphid prefers to feed on the tender succulent parts of the alfalfa plant. These aphids cluster on the terminal growth, but as populations increase, they will spread over the entire plant. Heavily infested alfalfa is characterized by severe stunting of the stems, which have shortened internodes and smaller leaves. Leaf curling and eventual leaf drop are also common symptoms in severely infested fields. Large blue alfalfa aphid populations have delayed cutting schedules or have caused the loss of a full cutting. Usually, only a patch of the field is affected.
Scouting Methods: Randomly select complete stems throughout the field, count the number of blue alfalfa aphids per stem, plus all parasitized (mummified) or diseased (brown and flattened) aphids. Continue this procedure until you have sampled at least 20 to 30 stems. Calculate the average number of healthy aphids per stem and the average of diseased or parasitized aphids per stem. Then measure each stem and calculate the average stem length.
Treatment Thresholds: Stunting of plant growth is evident at lower infestation densities than with pea aphid feeding. For instance, 20 blue alfalfa aphids per stem on 10-inch-tall alfalfa, or 50 blue alfalfa aphids on 20-inch-tall alfalfa, may justify insecticide treatment. if diseased and parasitized aphids are scarce.
Please refer to the most recent Alfalfa Insect Management Guide for specific control options. Materials registered for pea aphids may be used.
Page last updated 10/3/2013 by J.P. Michaud.