Effect of Temperature on Spotted Alfalfa Aphid Reaction to Resistance in Alfalfa1

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Abstract
The effect of different temperatures on the response of spotted nlfulfa aphids (Therioaphis maculate (Buckton)) was not the same on plants varying in aphid resistance. Aphid survival and reproduction were retarded relatively more at high than at low temperatures on a resistant as compared with a susecptible plant. It appeared that a resistant plant on which an aphid population could not he maintained at 80° would support a limited population at 60° F. Temperature influenced the reaction of adults and nymphs on resistant, intermediate, and susceptible plants in numerous ways. Optimum temperature for reproductive period, total reproduction of adults, and numbers of nymphs which matured and reproduced decreased with an increase in resistance of the plants. Rates of reproduction deer cased with a decrease in temperature and with an increase in resistance of plants, and differences among plants were evident at all except the lowest temperature. A post reproduction period occurred on the susceptible plant at temperatures from 60° to 90° F. but was not evident on the resistant and intermediate plants. The time required for nymphs to mature and reproduce was longer at 60° than at 80° F. and appeared to be longer with an increase in resistance of the plants. Where resistance limited survival, adults appeared to survive longer than nymphs. Infested seedlings from both resistant and susceptible lines were killed more rapidly at high than at low temperatures, although final survival did not appear affected by temperature, indicating that resistant seedlings would perform satisfactorily under aphid infestation at cool temperatures.

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