Eileen Cullen, Extension Entomologist
Regular scouting for alfalfa weevil larvae should be underway at this time. Larvae are active and the May 23 Wisconsin Pest Bulletin state survey reports that larval populations and leaf tip damage have exceeded economic levels in a few fields in southwestern WI alfalfa fields. The state survey is showing quite a bit of variability in weevil pressure. Most fields north of Dane County still have low numbers of larvae and are well below the threshold for alfalfa leaf tip feeding.
Regular scouting for larval feeding is critical at this time during first crop growth. Scouting should also continue after first crop harvest, particularly along strips of the field where windrows dried, for large larvae and adult feeding on in these areas.
To scout for alfalfa weevil, walk an “M” – shaped pattern in the alfalfa field and randomly pick 50 stems throughout the field. Examine each of the 50 stems for any amount of tip feeding from the alfalfa weevil larvae. Count the number of stems that have weevil feeding at the tip.
Control recommendations are to treat with insecticide or harvest early when 40% of the sampled stems have defoliation. That is 40% of the 50-stem sample with any defoliation (not 40% defoliation leaf loss on a particular stem).
If threshold is reached during first crop, the most economical recommendation is to harvest early as long as you are within 7 to 10 days of planned harvest. If you are farther from harvest, a treatment would be warranted.
Bryan Jensen, UW IPM, visits an alfalfa field to demonstrate and discuss alfalfa weevil scouting, light versus heavy feeding, and treatment recommendations. To view the videos click on the images below:
[Video 1] “Alfalfa Weevil Scouting in Alfalfa Fields”
[Video 2] “Alfalfa Weevil Scouting After First Cut”