Abstract
In counts over a 3-year period, 1959-61, in 15-to 80-acre fields in warm, low-elevation areas of south-central Arizona, average populations of the spotted alfalfa aphid (Therioaphis maculata (Buckton)) were 5 to 13 times higher on susceptible alfalfa varieties Chilean 21-5 and African, than on Moapa alfalfa, a recently developed resistant variety. Foliage damage by the aphid was 15 to 22 times greater on the susceptible varieties than on Moapa. The resistance of Moapa to the aphid was evident in each of the 3 years and in all seasons of the year.

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