Bryan Jensen
UW Extension and IPM Program
Based on May 9th degree day accumulations (map below), the southern Mississippi River valley area of Wisconsin has reached 300 Weevil Degree Days (base 48 F). Now is the time scouting for alfalfa weevils within that zone. The rest of Wisconsin will eventually follow. For near real-time degree day accumulations, please navigate to the UW Extension Ag Weather Site and click on “Thermal Models”.
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 hatch and early signs of tip feeding should start to be noticeable. 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.
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 an early 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.
Figure 1. Alfalfa Weevil D.D. from 1 Jan to 9 May 2016
Figure 2. Alfalfa weevil larvae
Figure 3. Early symptoms of alfalfa weevil feeding