Alfalfa Weevil is Active Across Ohio

Despite the cold temperatures experienced across the state this past week, alfalfa weevil larvae have hit peak feeding activity in nearly every part of the state. Alfalfa weevil feeding activity is driven by the accumulation of growing degree days (GDD) peaking when GDD is between 325 and 575 (accumulation from a base of 48°F starting January 1^st). As of writing this (Jan. 1 – April 28), heat units range from 587 in southwest Ohio to 294 in northeast Ohio.

Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – April 28, 2024 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))

With much of the state at its peak activity, it is important to scout alfalfa fields diligently, until first cutting or just after to make timely decisions and to maintain a quality alfalfa crop. Scouting for alfalfa weevil is a simple process that only requires a bucket and tape measure. Full details on scouting can be found here: Scouting Early Alfalfa Weevil, as well as in the video resource here: https://forages.osu.edu/video.

Alfalfa weevil larvae can be identified by their wrinkled green body, black head capsule, and the presence of a white strip that runs lengthwise along their back. They are approximately ¼ inch long or smaller (Figure 2).

Figure 2. Alfalfa leaf with pinhole feeding damage, green alfalfa weevil larvae in different development stages (instars), and brown adults. Photo Credit – Julie Peterson, University of Nebraska

Control thresholds are determined by a combination of larvae per stem and the stand height of the alfalfa, as outlined in the table below. As the alfalfa matures, harvesting early becomes the recommended economic control. If early harvest is utilized, be sure to scout for larval activity on the regrowth one week after cutting. Heavy infestations of alfalfa weevil can persist past the first cutting and can stunt regrowth, so a second control treatment could be necessary.

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)