Alfalfa Weevil Update

Bryan Jensen
UW Extension

I have been getting some questions regarding alfalfa weevil damage. First the good news. Harvest is well underway in southern Wisconsin and this is practice usually kills weevil larvae. The bad news is that it usually kills weevil larvae.

For those fields with higher than expected first crop damage be sure to inspect fields 3-4 days after harvest. Weevil larvae can feed on both crown and stem buds which delays regrowth and can reduce yield if feeding is severe. Make sure to inspect first crop closely. This year, temperatures have been cool which slows weevil development compared to alfalfa.   Early instar weevil feeding can be difficult to detect and may require unfolding of the upper leaflets to expose damage.

In second crop stubble, look for signs of new stem or crown bud formation. Dry weather can slow bud development. In the absence of regrowth, look under leaf litter, in soil cracks and the juncture between crown and the soil to confirm presence of larvae. You may find larval survival higher under the old windrow. A treatment threshold is not available for stubble regrowth. However, confirm the presence of larvae before spraying.

This year, because of the difference in alfalfa and weevil development, larvae may continue to feed on regrowth for a period of time. Although difficult to predict how long, typically, by the time second crop regrowth is 10” tall larval feeding is over for that generation. In this situation, consider an insecticidal treatment if tip feeding is greater than 50%.