Peak Alfalfa Weevil Activity

We took another jump in growing degree days (GDD) this week and are seeing the response from alfalfa and alfalfa weevils. The entirety of the state is within or just past the peak alfalfa weevil feeding and damage window of 325 to 575 GGDs (accumulation from a base of 48°F starting January 1^st). As of writing this (Jan. 1 – May 5), heat units range from 618 in southcentral Ohio to 337 in northeast Ohio.

Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – May 5, 2025 at CFAES Ag Weather stations across the state (https://weather.cfaes.osu.edu//) and additional NOAA stations around Ohio (Midwestern Regional Climate Center (https://www.mrcc.purdue.edu))

Scouting is still important at this stage, as alfalfa weevil population numbers vary from field to field. We are also getting to the point in the season where the more economic method of control may be harvesting early, based on the maturity of the stand. If a field with a heavy alfalfa weevil population is harvested early, be sure to scout the regrowth one week after harvest to ensure the alfalfa weevil larvae don’t persist and impact the second cutting.  Two scouting resources can be found here: Alfalfa Weevil Scouting and GDD and Alfalfa Weevil Scouting Video.

County educators from across the state continue to scout fields in their respective counties, as shown in Table 1.

Table 1: County alfalfa weevil scouting reports for April 29- May 5

For more information on control methods, consult the OSU and MSU “Field Crops Insect Pest Management Guide” (https://aginsects.osu.edu/extension-publications/msuosu-ipm-guide)