Critical time to Begin Sampling for Alfalfa Weevil

The University of Kentucky Ag Weather Center’s degree day model for alfalfa weevil indicates that many counties in Kentucky are likely to exceed the 190 Degree Days (DD) used as a starting point to begin scouting by early to mid-March. Once temperature accumulations reach 190 DD, growers are advised to look at their alfalfa fields and begin their alfalfa weevil larval counts. So far, degree day accumulations for this year are trending close to average for the last 10 years.

Figure 1. When degree day totals reach 190 Degree Days, it is time to begin scouting for alfalfa weevil larvae. Scouting continues at least on a weekly schedule until regrowth after the first cutting. Treat the DD totals for the counties as estimates.

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 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 pin holes in the leaves.

Scouting & Thresholds

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 off the crown with your other hand, then 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.

If the field is close to harvest, 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

Alfalfa Weevil Larvae Thresholds for Spraying 226 to 275 Degree Days

Average stem height (inches)Number of alfalfa weevil larvae on 30 stems

For degree day accumulations above 275, use the economic threshold tables in ENTFACT 127 or ENT-17 to determine the need to spray the field for alfalfa weevil.

Avoid Pesticide Resistance

If you 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 I would recommend rotating 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

Posted in Forages
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