Non-Preference as a Mechanism of Sweetclover and Alfalfa Resistance to the Sweetclover Aphid and the Spotted Alfalfa Aphid1

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Abstract
Aphids, Therioaphis maculata (Buck ton) and T. riehmi (Börner), had high mortality rates when confined to various resistant host plants and nonhost plants but these rates were seldom higher than a starvation treatment. When given a free choice among resistant host, nonhost, and susceptible hosts, no preference was manifested initially but migration from the resistant and nonhost plants onto the susceptible plants commenced within a few hours. After 12-hours it was evident that aphids preferred the susceptible plants the most, the resistant plants next, and the nonhost plants the least. Migration of the aphids occurred without appreciable mortality. Tests in which aphids were transferred from resistant to susceptible plants produced no evidence of carryover effects of a toxic substance in the resistant plants. Thus it was concluded that nonpreference is the primary mechanism for resistance to both species of aphids in the alfalfa and sweetclover selections that were studied.

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