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

WINTER INFESTATIONS OF SPOTTED ALFALFA APHID The spotted alfalfa aphid is typically most abundant during times of warm temperatures and limited rainfall, and infestations often occur during late summer and fall. This species has limited tolerance for freezing temperatures and usually does not survive the winter in Oklahoma. Alfalfa is reinfested by winged adults migrating from the south as weather warms each spring. However, in years when dry weather in fall allows the population level of spotted aphids to increase and relatively warm, dry conditions in winter allow continued survival and population increase, serious damage may be done to alfalfa stands due to feeding on crown growth. The most serious infestations may result in the virtual loss of stands in the period from December through March.      The threat of stand loss is greater in new, fall-planted alfalfa than in established stands, and is less severe in improved cultivars with resistance to spotted alfalfa aphid than in susceptible cultivars. Newly planted fields should be sampled in fall and winter by carefully brushing small plants so that aphids that drop to the soil surface can be counted. Whereas seedlings can withstand only 1aphid/plant, if numbers average 10/plant under mild and dry weather conditions after plants have formed trifoliolate leaves, an insecticide should be applied. In established stands, sampling should be conducted by brushing crown growth to dislodge aphids, then carefully examining the soil surface beneath plants to estimate numbers. When numbers exceed 30/crown, spraying may be profitable. Additional information on sampling and control measures can be found 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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