Alfalfa weevils are currently mainly in their larger larval stages or beginning the process of pupation, transitioning to their adult forms. The growing degree days (GDD) took a leap from last week, putting much of the state above the window for peak damage of 325 to 575 GDDs (accumulation from a base of 48°F starting January 1st). As of writing this (Jan. 1 – May 19), heat units range from 842 in southcentral Ohio to 521 in northeast Ohio.
Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – May 19, 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))
Our county educators continued the state-wide scouting effort, and overall we saw a drop in the number of average larvae per stem throughout each county, with many counties seeing zero larvae do to treatment efforts. The following table shows the average state of alfalfa fields scouted in each county. Be sure to scout your field to gauge your situation specifically.
Table 1: County alfalfa weevil scouting reports for May 13 – May 19
|
County |
Alfalfa Height (inches) |
Larvae Count per Stem |
|
Allen |
16-24 |
0 |
|
Defiance |
19-30 |
0.9 |
|
Mercer |
22-33 |
1.2 |
|
Perry |
18-27 |
1.3 |
|
Ross |
14-30 |
0.5 |
A key topic to note is to watch for populations that may be presenting resistance to insecticide treatments, namely, pyrethroids. There have been confirmed cases of pyrethroid-resistant alfalfa weevil populations in the western US, but Penn State put out an article last week, where they have been getting reports of such resistance in the eastern part of Pennsylvania (Read that article here: Alfalfa Weevils in Eastern PA May Be Resistant to Pyrethroid Insecticides). Those cases of resistance are not confirmed, but it is a good reminder to scout fields that have been sprayed to ensure an effective treatment. It is also recommended to rotate the insecticide used, and the OSU and MSU “Field Crops Insect Pest Management Guide” is a great resource and can be found here: https://aginsects.osu.edu/extension-publications/msuosu-ipm-guide.
A final note, we are to the point in the season where the most economical choice for many of the affected fields is to harvest. Be sure to scout for alfalfa weevil the week following harvest to ensure the population doesn’t persist and impact the second cutting. If still looking to make an insecticide application, make sure to take into account the harvest restrictions associated with the application