UW Extension and IPM Program
Judging by degree day accumulations, most areas of the northern Wisconsin are at the point where scouting for alfalfa weevils should begin and peak feeding is occurring in southern Wisconsin. Good news is that harvest is well underway and I haven’t had reports of extensive feeding. Our typical recommendation is to scout second crop regrowth for those fields which were heavily infested during first crop. A concern is that harvest was well ahead of peak feeding and those fields with heavy damage may have gone unnoticed because larvae were small and damage was easy to miss.
If those larvae survive, you could get significant feeding on either the crown/stem buds and or new foliage. A suggestion would be to spot check fields just to determine what, if any feeding, is occurring. With recent rains, alfalfa should bounce back and regrowth should be occurring. If not, check around the crown to make sure weevils are present. You will find the larvae under leaf litter, and hiding at the juncture between stubble and soil. If regrowth is not occurring 4-5 days after harvest, you’ve had sufficient rainfall and larvae are present your best option is to treat.
Additionally, check regrowth for signs of feeding. During second crop regrowth the economic threshold is bumped up to 50% of the stems showing signs of feeding. Before treating consider size of larvae present. If all are 4th and final instar (approx. 3/8 inch) you may not be able to prevent enough damage to pay for an insecticide and the loss of benefical insects. To confirm this suspicion, scout for the presence of pupae.
Alfalfa weevil pupae