Begin Scouting Fields for Alfalfa Weevil

The UK Ag Weather Center’s degree day model for alfalfa weevil indicates that many counties in Kentucky have recently exceeded the 190 Degree Day (190 DD) used as a starting point to begin scouting for alfalfa weevil. Most other counties will exceed that total within the next few days.  Growers are advised to scout alfalfa fields and begin their alfalfa weevil larval counts.

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 a weekly schedule until regrowth after the first cutting.

Following the relatively mild winter, it is likely that more winter-laid alfalfa weevil eggs will hatch this year. These eggs hatch earlier and the 190 DD target estimates when their first leaf feeding damage becomes noticeable. Damage by the young larvae will first appear as tiny pin holes in the leaves.

Scouting for Alfalfa Weevil Larvae

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. 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.
  • Carefully cup the top of each stem in one hand and break off the crown with your other hand
  • Place stems bud-end downward in a plastic bucket.
  • 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 ten 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)

Average stem height (inches)Number of alfalfa weevil larvae on 30 stems
227
467
6100
8130
  • 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
215
419
620
  • 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.

By Ric Bessin, Entomology Extension Specialist

Posted in Forages