Behavior of the Spotted Alfalfa Aphid on Resistant and Susceptible Alfalfas12

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Abstract
As the number of adult Therioaphis maculata (Buckton) decreased on resistant alfalfa clones, the total production of nymphs also decreased, but the rate of production probably did not decrease. Although adult survival on resistant clones was similar to that of starved aphids and of aphids on broadbean, total production of nymphs was about 4 times greater on resistant clones than on the nonhost (broad bean). Thus, both resistant and susceptible alfalfa apparently contain something that increases the production of nymphs. Leaves of resistant alfalfa seedlings were more resistant than petioles or stems. The aerial parts of all mature alfalfa plants, except the flower petals, were highly resistant; the flower petals were not resistant. Although differences in the stage of growth of the plant did not affect aphid survival on resistant alfalfa, survival on susceptible alfalfa was reduced on mature shoots compared with that on rapidly elongating shoots. It is indicated by the results obtained when aphids were offered a choice between resistant and susceptible plants or were transferred to susceptible plants after either starvation or feeding on resistant plants that aphids are driven from resistant plants by a repellent in the phloem, or by the possibility that such plants do not satisfy dietary needs.

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