Alfalfa harvest timing impacts on alfalfa weevil with agronomic and economic contexts

Abstract

Alfalfa weevil (Hypera postica Gyllenhal) is a highly destructive alfalfa pest (Medicago sativa L.) in many parts of the world. Alfalfa weevil can cause significant economic damage via plant defoliation and slowed regrowth after harvest. Insecticide application is limited given developing resistance, environmental impacts, and high costs. We explored the impact of manipulating the timing of the first harvest of the season, an alternative control often referred to as “early harvest.” Impacts of harvest timing on alfalfa weevil remain poorly understood. We conducted an experiment for 2 years at a research farm in Wyoming, United States, to test how harvest timing affects damage to alfalfa, yield, quality, and alfalfa weevil densities, in comparison to treatment with insecticide. Earlier harvests were comparable in efficacy to insecticide treatment for managing alfalfa weevil. Alfalfa quality was higher in early harvests compared to later harvests resulting in a higher price per ton for alfalfa harvested earlier in the season. In the second year of the study, weevil densities were higher; alfalfa treated with insecticide had higher quality in later cuttings than untreated alfalfa, leading to increased revenue. The slight boost in revenues that insecticide treatment provided was roughly the cost of the insecticide. If higher yields and price are a priority for producers and alfalfa weevil pressure is strong, insecticides may need to be considered to allow for later harvest. Overall, an integrated pest management approach should be employed and determined based on producer goals and crop needs for optimal control of alfalfa weevil.