By Bryan Jensen
“Better late than never” may not apply to this situation. Accumulated degree days suggests that now is the time to start scouting for alfalfa weevil damage in the southern areas of Wisconsin. Although hard to predict, central and northern Wisconsin are 1-2 weeks away from scouting.
Adult weevils overwinter in plant debris along fence rows, grassy waterways, woodlands, etc. During the first warm spring days (yes we have had a few!) 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 and is the perfect time to initiate scouting.
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 allowing more time to consider a control decision if needed. I no longer consider alfalfa weevil a key alfalfa pest for several reasons. However, each year there are heavy local populations that require treatment, fields that have heavy damage that are overlook and fields that are treated unnecessarily.
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 a timely harvest especially if you are not putting additional stress on the stand. Timely cutting is still our best control option.
If an early harvest is not practical, consider treating fields with severe damage and rescouting remaining fields at a later time. For those fields with heavy first crop weevil feeding (which are not treated) plan to check second crop regrowth for feeding. Larvae and/or adults can survive harvest and cause significant damage to regrowth.