It is time (or soon will be!) to start scouting for alfalfa weevil damage in established alfalfa stands. We suggest spot-checking for early signs of damage when 300 weevil degrees have accumulated. This degree day map supplied by UW Extension Ag Weather will give you near real-time degree day accumulations for your area. Click on “Thermal Models” then select the model for alfalfa weevil. While you are at the UW Extension Ag Weather site check out the other degree day models. It is a great resource!
Adult weevils overwinter in plant debris along fence rows, grassy waterways, woodlands, etc. During the first warm spring days adults become active and females start to lay eggs. At 300 weevil degree days (Base 48°F) eggs start hatching and early signs of tip feeding should start to be noticeable. Alfalfa weevils go through 4 larval instars. Maximum feeding should occur between 600 and 800 weevil degree days. Scouting at 300 degree days will give you a heads up on damage potential and allow more time to reach a control decision if needed. Although weevil populations have been spotty the past two decades, I always hear about someone getting caught off guard. If you initiate spot checks at 300 degree days you can avoid a surprise.
A treatment threshold of 40% tip feeding is suggested. This is not to advocate treating at 40% defoliation but rather when 40% of the stems have signs of weevil feeding. If you are over the suggested threshold consider harvest if the timing is correct. Timely cutting is still our best control option. For those fields with heavy first crop weevil feeding, plan to check second crop regrowth for feeding. Larvae and/or adults can survive harvest and cause significant damage to regrowth.