Alfalfa Weevil and Alfalfa Quality

Alfalfa weevils have been in the growing degree days (GDD) window for peak feeding for at least a week and a week or two longer for some regions across the state. 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 12), heat units range from 698 in southcentral Ohio to 388 in northeast Ohio.

Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – May 12, 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))

As part of our state-wide scouting effort, many county educators are seeing alfalfa weevil larvae pressure rise in the fields that have yet to be treated. The following table shows the average state of alfalfa fields scouted in each county. Some counties have zero alfalfa weevil larvae or lower numbers than the previous week due to the scouted field being treated. Be sure to scout your field to gauge your situation specifically.

Table 1: County alfalfa weevil scouting reports for May 6 – May 12

For a lot of the fields that are right around the treatment threshold, the decision this time of the season is whether to treat with an insecticide or harvest early. Many fields have reached the height at which it is generally recommended that an early harvest be made, as shown in Table 2. The goal for Dairy quality alfalfa hay is below 42% NDF; however, feed value declines quickly between 35-42% NDF. The goal of harvesting at an NDF of 35% in the spring can quickly lead to 42% NDF with one rain. Over the past week, alfalfa grew 2-4 inches across the state. The tallest reported alfalfa was 25 inches. Using the Alfalfa Predictive Equations for Alfalfa Quality (PEAQ) it would have an NDF of 34.7 in the vegetative stage or 35.9 in the bud stage. If this alfalfa only grew four more inches next week, it would have an NDF of 37.5 in the vegetative stage or 38.7 in the bud stage. Warmer weather this week should lead to faster growth than last week. Alfalfa under 16 inches should still be dairy quality, even if a treatment of alfalfa weevil is needed, as long as an insecticide with a short PHI is chosen, allowing you to still maximize tonnage and quality. If you use harvest to control alfalfa weevil, be sure to scout for alfalfa weevil the week following harvest to ensure the population doesn’t persist and impact the second cutting.

If utilizing an insecticide as a treatment, be sure to follow the label and take into account the harvest restriction that may impact the forage quality as a stand continues to mature during the waiting period.

Table 2. Alfalfa stand height and larvae pressure treatment recommendations

For more information on scouting practices, resources can be found here: Alfalfa Weevil Scouting and GDD and Alfalfa Weevil Scouting Video.

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)