Alfalfa aphids in northeast Colorado
By D. Bruce Bosley
Four aphids occur on alfalfa in northeast Colorado: pea aphid, blue alfalfa aphid, spotted alfalfa aphid and the cowpea aphid. All four of these aphid species have multiple generations each year. This and their rapid development allow aphid populations to build up quickly under favorable conditions. Weather conditions have a great effect on the likelihood of aphid outbreaks.
Pea aphids are comparatively large (3/16 inch long), bright green aphids, with long cornicles (paired "tails" near the end of the body). Spotted alfalfa aphid is smaller (1/10 inch long) than pea aphid. It is pale very light yellow with four to six rows of darker spots on the upper abdomen. Blue alfalfa aphids are very similar in appearance to pea aphids; they are slightly smaller and have a waxy, darker green color than pea aphid. 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 usually shiny black while the smaller nymphs may appear to be a dull gray to black.
Heavy populations of any of these aphids may damage stands directly by sucking plant juices or by injecting toxic secretions, causing yellowing, stunting and wilting of the plants. The spotted alfalfa aphid has the greatest damage potential of the three species because it injects a toxin during feeding that causes a severe discoloration yellowing of leaf veins. Heavily infested plants turn yellow, and some leaves often have reddish discoloration.
All four-aphid species suck photosynthetic fluids from the leaves and stems. Under low to moderate populations of aphids, there may be no visible yellowing of plants. However, after prolonged feeding, leaves will turn yellow, starting at the veins, curl and wilt, and will die. Seedling plants may be killed. Established plants may grow slowly and be stunted. Under high densities of any of these species, growth is retarded, and the weakened plants may grow slowly after cutting. Invasion by weeds and general susceptibility to stress is increased; stand longevity may decrease after damage by aphids.
Pea and blue alfalfa aphid feed in the new growth, at the tips and young leaves, while spotted alfalfa aphid feeds on older leaves. As leaves die, the aphids move higher into the plant; heavily damaged plants may become entirely defoliated except for a few leaves at the end of the stems. The spotted alfalfa aphid also secretes large amounts of honeydew that may interfere with cutting, curing and baling, and may lower the hay's quality from the growth of sooty mold.
Fields should be checked according to the aphid species biology, and current weather. Pea aphid and blue alfalfa aphid populations are favored by cool and dry conditions and economic populations are most likely to occur in the spring and fall. If the plants are not developing normally because of cool weather, fields should be checked to see whether aphids might be adding to the problem. Spotted alfalfa aphid populations are favored by hot and dry weather, typically peaking late in the summer. Aphid populations may buildup following insecticide treatments for alfalfa weevil.
If the crop is to be cut in less than one week, immediate cutting is recommended over an insecticide treatment. Weather has an important effect on aphid populations. Heavy rains and/or winds tend to dislodge aphids and result in some mortality. It is important to re-sample after such events prior to making a treatment decision.
In many situations, aphids on alfalfa are not economic pests in the High Plains, because of environmentally unfavorable factors, resistant varieties and natural enemy control. Parasitic wasps, syrphid flies and ladybird beetles are the most important natural enemies, and therefore, their populations should be considered when deciding whether to chemically control aphids. In the spring aphid, numbers may increase more rapidly than predator numbers. If natural enemy populations are present relative to aphid numbers, spraying may be not be justified, since it may disrupt long-term control.