Alfalfa harvest timing impacts across pest management, agronomic and economic contexts.

Authors: HERREID, JUDITH - US Department Of Agriculture (USDA); LEE, BRIAN - University Of Wyoming; ISLAM, ANOWAR - New Mexico State University; RITTEN, JOHN - Colorado State University; JABBOUR, RANDA - University Of Wyoming

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: he alfalfa weevil (Hypera postica Gyllenhal) is a highly destructive pest of alfalfa (Medicago sativa L.). They can cause significant economic damage to alfalfa plants both before and after harvest. Insecticide use against the alfalfa weevil is less that ideal due to environmental impacts and high costs. Additionally, this pest has recently developed resistance to insecticides. In this study we explored an alternative control method, early harvest, by staggering the initial alfalfa harvest of the season. This experiment took place over 2 years at a research farm in Wyoming. We tested how different harvest timings and insecticides compare. Earlier harvests were similarly effective to insecticide use for alfalfa weevil control. Alfalfa quality was higher in early harvests compared to later harvests. This resulted in a higher potential sale price if alfalfa was harvested earlier. Overall, weevil densities were higher the second year of the study and when alfalfa was treated with insecticides the later harvest crop was higher in quality compared to untreated alfalfa. Although this could lead to increased revenue the slight boost in revenue that insecticides provided was roughly the cost of the insecticides. If growers are targeting higher yields and price is a priority when alfalfa weevil pressure is strong, insecticides may need to be considered to allow for later harvest. Overall, an integrated pest management approach should be based on producer goals and crop needs for the best possible alfalfa weevil control.

Technical Abstract: Alfalfa weevil (Hypera postica Gyllenhal) is a highly destructive pest of alfalfa (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.