Bryan Jensen, Dept of Entomology and IPM Program, UW-Madison
Southern Wisconsin has reached the benchmark for alfalfa weevil scouting. Northern Wisconsin will probably hit the magical 300 DD total (base 480) within a week or two. Of course, that all depends on temperatures. Regardless, keep an eye out for weevil feeding on first crop alfalfa.
Alfalfa weevils are monitored by looking for feeding signs, often described as pin hole feeding. At the 300DD scouting threshold, those signs will in the upper developing leaflets and may be difficult to see with casual observations. At 300 DD weevil larvae are in the early stages of development and feeding holes will be small. You may have to unfold the upper leaves to find damage. As larvae grow the amount of defoliation increases substantially. Therefore, looking for early signs of feeding can give you a look into the future. In recent years, weevil feeding has been rather light. However, just about every year I hear of hotspots showing up somewhere.
Economic threshold for first crop alfalfa is to treat when 40% of stems have signs of weevil feeding and you are more than 1 week from harvest. If possible, plan to harvest heavily infested fields first. Double check regrowth to make sure larvae are not feeding on second crop regrowth.
This year the timing of first crop harvest and peak weevil feeding may not be what you typically expect. In a “normal” year, weevil feeding peaks about the same time as first crop harvest. This year the cooler temperature may change that crop/pest relationship. That is, alfalfa has a lower base development temperature than weevils. We may see peak weevil feeding later than normal when compared to crop development.