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Yield and Quality Losses Due to Alfalfa Weevil¹

Evaluation of management practices to control the alfalfa weevil (Hypera postica Gyllenhal) requires quantitative knowledge of the effects of insect defoliation on yield and quality of alfalfa (Medicago sativa L.). Measurements were made in 1971 when the uncontrolled population peaked at 2.7 larvae stem^–1 and in 1972 when the peak was 1.1 larvae stem^–1. With weevils chemically controlled, a three-cut system gave seasonal dry matter yields of 10.1 and 10.5 metric tons ha^–1 in 1971 and 1972, respectively, whereas weevil infested plots yielded 8.4 and 10.0 metric tons ha^–1, respectively. Corresponding yields for two-cut systems were lower at 6.4 and 9.1 metric tons ha^–1 for non-infested plots and 7.7 and 8.6 metric tons ha^–1 for infested plots. In vitro true digestibility (IVTD) and crude protein (CP) percentages of leaves were reduced during peak larval feeding with the higher population of 1971. Weevil defoliation did not have a significant influence on leaf percentage or on the quality of the total herbage, apparently because stem growth was also retarded. However, yield of quality components was reduced. The greatest effect for a single harvest was in the second cut of the three-cut system where feeding occurred in the early stages of regrowth. Total seasonal yield reductions in 1971 were highly significant for IVTD and CP, and significant for leaves. Losses for these components were 1.4, 0.3, and 0.8 metric tons ha^–1, respectively,in the three-cut system. Yields and quality tended to be higher in the two-cut system when weevils were not controlled because of reduced lodging in late cut alfalfa and renewed growth when larval activity subsided. With the lower weevil population of 1972, there was no significant effect of weevil feeding on any of the measured yield and quality parameters.

Key Words: Medicago sativa L. • Hypera postica Gyllenhal • Defoliation • Digestibility • Crude protein • Leaf yields

² Graduate research assistant and assistant professor of agronomy, Agronomy Dep., Cornell Univ., Ithaca, NY 14855.

Received for publication December 11, 1974.