Ric Bessin, UK Extension Entomologist

The UK Ag Weather Center’s degree day model for alfalfa weevil indicates that many counties in Kentucky have exceed 190 DD used as a starting point to begin scouting by the third week of March. In fact alfalfa weevil damage has appeared in some fields. Once temperature accumulations reach 190 DD, growers are advised to look at their alfalfa fields and conduct weekly alfalfa weevil larval counts and compare those to the economic thresholds listed below.

Figure 1. When degree-day totals reach 190 Degree Days, it is time to begin scouting for alfalfa weevil larvae. Scouting continues on at least on at least 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 approximate when first leaf feeding damage becomes noticeable. Temperatures 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.

To scout for alfalfa weevil, we 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 and 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 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, 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)

• 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

• 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, use the economic threshold tables in ENTFACT 127 or ENT-17 to determine the need to spray the field for alfalfa weevil.

If you de 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 we would rotate more often, but alfalfa weevil has only one generation per years.  To rotate modes of action, select insecticides that have a different IRAC group number on the label.