Relationships Between Acyrthosiphon kondoi and Damage in Three Varieties of Alfalfa1

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Abstract
Relationships were established between Acyrthosiphon kondoi Shinji stem length and stem dry weight at harvest and the weekly rate of change in stem dry weight of alfalfa, Medicago sativa L., varieties Condura 73, CUF 101, and Hunter River. These were expressed as linear functions. Aphid infestation levels were best expressed as aphid days per stem, and values equating to arbitrary yield reductions of 10, 20, and 30% were calculated from the pest-yield relationships. The resulting aphid day per stem values were converted by definition to damage thresholds given as a range of possible aphid numbers and feeding times. Limitations were imposed on the derived thresholds by the experimental procedure and by the data. Damage thresholds varied with the innate capacity of varieties to tolerate aphid attack, and tolerance increased with increasing plant size.

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