Recent degree day accumulations for alfalfa weevils in western South Dakota would suggest that alfalfa weevil larvae activity should be wrapping up for the 2025 season (Alfalfa Weevil Activity Prediction Update: May 22, 2025). However, recent scouting of alfalfa suggests otherwise. Populations that greatly exceeded the recommend thresholds (Alfalfa Weevils Are Active in Some Parts of South Dakota) were detected this last week in western South Dakota (Figure 1).
Populations have now been detected in scouted fields to date in eastern South Dakota. In addition, the growth stages of the recently detected alfalfa weevil larvae indicate that they will be feeding for some time before pupating. Next week’s forecast of rainy and cooler weather may slow the development of these populations down, but they will still be present in the alfalfa fields.
As a reminder, alfalfa weevil larvae feed on the leaves and the terminal buds, which can result in stunting of the alfalfa plants. Although early harvesting may reduce the impact of the alfalfa weevil populations on the current cutting, observations of this tactic during previous years suggest that it can result in decreased regrowth for the next cutting. This is a result of the larvae still being present in the windrows and moving to feed on the alfalfa stubble. Routine scouting of alfalfa fields can reduce the risk of heavy defoliation and stunting caused by alfalfa weevils. When managing this pest, always read the label and ensure that the product’s preharvest interval matches the timeline for harvest.
