Despite the current cold snap, alfalfa weevil is still a pest to watch for this week. With temperatures returning to the 70s this week, larval feeding and growth will accelerate. All of Ohio has reached the window for peak damage of 325 to 575 GDDs (accumulation from a base of 48°F starting January 1st).
Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – April 20, 2026 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)).
This past week county educators from across the state scouted fields in their county to gauge the current progress of alfalfa weevil. Results are summarized in table 1, with a wide variation of alfalfa weevil infestations, some fields warrant more attention. When scouting many of the larvae are still in the earlier growth stages and are very small and hard to see. That said, some fields where the larvae are hard to observe, still have a notable amount of tip feeding. Another observation was that the fields that had alfalfa weevil problems last year, seem to have a high chance of problems this year. Those fields may be a good starting point for scouting on your own operation.
Table 1: County alfalfa weevil scouting reports for April 13 – April 19
|
County |
Alfalfa Height (inches) |
Larvae Count per Stem |
|
Defiance |
5.5 |
0.05 |
|
Defiance |
4.5 |
1.75 |
|
Defiance |
5.5 |
0.5 |
|
Huron/ Erie |
6 |
0.5 |
|
Mercer |
15.7 |
0 |
|
Mercer |
13.1 |
0.5 |
|
Miami |
6 |
2.2 |
|
Paulding |
5.5 |
0.5 |
|
Perry |
9 |
0.2 |
|
Ross |
8 |
2.1 |
|
Ross |
7 |
2.4 |
|
Ross |
9.5 |
1 |
|
Wayne |
8 |
0.2 |
Table 2. Alfalfa Weevil Threshold Recommendations
|
Stand Height (Inches) |
Indication of Problem (% Tip Feeding) |
Problem Confirmation (Larvae per Stem) |
Recommended Action |
|
6 |
25 |
1 |
Recheck in a week |
|
9 |
50 |
>1 |
Spray |
|
12 |
75 |
>2 |
Spray |
|
16 |
100 |
>4 |
Harvest early |
With variation in alfalfa weevil populations from field to field, scouting is the best course of action to understand what is needed for your operation. Scouting for alfalfa weevil is quite simple only needing a bucket and bucket and measuring tape. For details on scouting see our article from two weeks ago “Alfalfa Weevils Are Active Earlier than Usual this Spring” or this ~4 minute video “Alfalfa Weevil Scouting”.
If a pesticide application is necessary the OSU/ MSU IPM Guide is a great resource for insecticide decisions. Prior to application be sure to read the label. After the insecticide application and the reentry interval is complete a scouting pass is a good practice to ensure sufficient control.