The UK Ag Weather Center’s degree day (DD) model for alfalfa weevil indicates that many counties in Kentucky are likely to exceed 190 DD (used as a starting point to begin scouting) by the third week of March. Once temperature accumulations reach 190 DD, growers are advised to look at their alfalfa fields and begin alfalfa weevil larval counts. So far, degree day accumulations for this year are trending close to average for the last 10 years.
Fall-laid alfalfa weevil eggs are the first to hatch in the spring. These eggs hatch earlier than those laid in the spring and 190 DD approximates when the first leaf feeding damage becomes noticeable. Temperature extremes during the winter help to limit the survival of alfalfa weevil eggs that were laid in stems in the fall. Damage by the young larvae will first appear as tiny pinholes in the leaves.
Scouting
To scout for alfalfa weevil, use the stem sampling method. While walking in a “U” or “Z” pattern through a field, collect 30 alfalfa stems. Carefully cup the top of each stem in one hand and break it off the crown with your other hand; place it bud-end downward in a plastic bucket. Be sure your samples are at least 20 feet from the edge of a field so that they are representative of the entire interior of a field. Knock the stems in groups of 4 or 5 stems at a time against the inside of the bucket to dislodge the larvae. Count the number of larvae. Measure the length of 10 random alfalfa stems. If the field is close to harvest then harvest can be an alternative to spraying, but producers need to watch for damage to the regrowth. There are similar scouting tables for regrowth after the first cutting.
Alfalfa Weevil Larvae Thresholds for Spraying 190 to 225 Degree Days (Check your degree days)
| Average stem height (inches) | Number of alfalfa weevil larvae on 30 stems |
| 2 | 27 |
| 4 | 67 |
| 6 | 100 |
| 8 | 130 |
- Apply a long residual insecticide if the number of larvae is greater than the number in the table for the average height of alfalfa sampled.
- Sample again in 2 days if the number is above 15 but less than the number in the table.
- Sample in 7 days if the number is less than 15 in your sample of 30 stems.
Alfalfa Weevil Larvae Thresholds for Spraying 226 to 275 Degree Days
| Average stem height (inches) | Number of alfalfa weevil larvae on 30 stems |
| 2 | 15 |
| 4 | 19 |
| 6 | 20 |
- Apply a long residual insecticide if the larval number per 30 stems is greater than the number in the table above for the height of alfalfa sampled.
- Sample again in 7 days if you find less than the number of larvae for the appropriate alfalfa height.
For degree day accumulations above 275 DD, use the economic threshold tables in
- Alfalfa Weevil Field Sampling Program (ENTFACT 127) or
- Insecticide Recommendations for Alfalfa, Clover, And Pastures – 2022 (ENT-17)
to determine the need to spray the field for alfalfa weevil.
Rotate Insecticide Modes of Action
If you do need to treat for alfalfa weevil larvae, keep in mind that insecticide resistance has been an issue in some areas. The best strategy to manage resistance is to use an insecticide only when necessary and to rotate modes of action each year. For many other pests, you would rotate insecticides more often, but alfalfa weevil has only one generation per year. To rotate modes of action, select insecticides that have a different IRAC group number on the label.
By Ric Bessin, Entomology Extension Specialist
Kentucky Pest News 