In the southern part of Wisconsin we have reached or exceeded 300 weevil degree days and damage reports have been on the quiet side.   This could be a result of low weevil populations or a weather-friendly cutting schedule. If it was the latter, it might be worth some time to spot check fields for signs of damage to second crop regrowth.    Alfalfa has a lower base developmental temperature (41° F) compared to alfalfa weevil (48° F). Historically, this scenario has resulted in harvesting prior to noticeable weevil damage giving the grower/advisor a false sense of security. Cool, cloudy weather or the shading effects of windrows can enhance larval survival after harvest. When surviving larvae mature they consume greater amounts of foliage which can result in second crop injury. 

            Shortly after harvest, monitor regrowth for signs of weevil activity, especially those fields which had elevated levels of first crop injury. If axillary or crown buds look healthy and show no signs of feeding injury you are probably “OK”. If few crown/axillary buds are forming it could be the result of weevil feeding or other unrelated circumstances, like stress or dry weather. To confirm weevil feeding, look on the soil surface around alfalfa crowns, under crop debris or at the juncture between the crown and soil for surviving larvae.  If new buds have not formed and larvae are present, an insecticide application is the best course of action. Another scenario that could lead to insecticidal treatment is when significant regrowth has been initiated and 50% of that regrowth has signs of weevil feeding. At approximately 800 weevil degree days larvae will start to pupate. Before deciding to spray an insecticide, it would be advisable to make sure weevil larvae are still present in the field. Although a sweep net should not be used to quantify weevil populations for treatment decisions, it is a useful to tool for relative estimate of weevil larvae and to confirm larvae are present.

 Image 1: Alfalfa Weevil Cocoons

Image 2: Developing Crown Buds