Abstract
In studies on the reaction of seedling alfalfa to feeding of the spotted alfalfa aphid, Therioaphis (Pterocallidium) maculata (Buckton), in southern Arizona, the rate of plant destruction was in inverse geometrical proportion to the number of aphids. One aphid per plant feeding and multiplying under uncontrolled conditions killed plants in approximately 12 days. The threshold period varied under both conditions, but the range was much greater when the population was controlled. The resistance differential was constant under uncontrolled conditions but varied greatly when the population was controlled. Damage to plants consisted of yellow vein clearing, chlorotic mottling, wilting, and finally death.

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