Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station

FIELD SCOUTING OF BLUE ALFALFA APHID AND PEA APHID
Although mixed infestations of blue aphids and pea aphids may have fairly equal proportions of the two species during early March, research has shown that the blue aphid becomes the predominant species by late March and April. During April, sampling for aphids can be done most effectively by use of a sweep net, because height of alfalfa and density of the plant canopy make stem sampling quite difficult. As large numbers of aphids may be collected in just 10-20 sweeps, it may not be possible to count all aphids collected, and only rough estimates of numbers per sweep can be made. It is important to identify which species is most abundant.    Blue aphids are somewhat smaller than pea aphids and have darker green or blue-green coloration. A magnifying glass is needed to more accurately separate these species by examining the antennae. Antennae of the pea aphid are light in color with a dark ring at the end of each of four segments. The antennae of blue aphids do not have dark rings, but gradually become black in color near the tips. The economic threshold for blue aphid in alfalfa >10" in height is about 300 per sweep, while for pea aphid the threshold is about 400 per sweep. Additional information is available on sampling and control of aphid in OSU Extension Facts No. 7150 and OSU Extension Facts No. 7184.
Richard Berberet, Alfalfa Entomologist  Department of Entomology and Plant Pathology  Oklahoma State University

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