Alfalfa Weevil Development

Bryan Jensen, IPM Program

There has not been any widespread insect problems reported lately, but there have been some isolated reports of a few different problems. Alfalfa weevil is one that should be mentioned. First crop harvest has been delayed in many areas because of rainy weather and alfalfa weevil development has been slowed because of cooler than normal weather. Although some defoliation may be apparent in first crop, the best decision of course is to cut as soon as possible. Larvae which survive and feed on second crop is more difficult to assess. The established threshold for second crop damage is to treat when 50% of the stems have feeding symptoms. However, you should take this recommendation to the next level and assess the age structure of the weevil population. Ask yourself how much damage can I prevent? Southern Wisconsin weevil degree days suggest that weevils have started to pupate. Alfalfa weevil larvae go through four instar stages, As fourth instars, weevil larvae are approximately 3/8 inch long and only require  91 degree days before pupating. At daily high and low temperatures of 75 and 60 degrees respectively, this takes approximately 5 days. So watch weevil degree day accumulations at http://www.soils.wisc.edu/wimnext/crops.html for your area and assess the age structure of your fields to see potentially how much damage you can prevent. 

Image 2- Fourth Instar Alfalfa Weevil