Southeastern Wisconsin growers seeing increased alfalfa weevil defoliation during first-crop harvest may want to revisit fields four or five days after cutting, warns Bryan Jensen, University of Wisconsin Extension Integrated Pest Management (IPM) program coordinator.
If environmental conditions are favorable, Jensen says, weevil larvae can survive harvest and feed on new-forming crown buds. That would significantly delay regrowth and lower yields.
Make sure crown buds are forming, then look for larvae on the soil surface, under leaf litter and at the juncture between soil and the alfalfa crown, he advises. “During cool, cloudy weather you may find weevils feeding on new alfalfa buds during daylight hours. You may also see increased survival under windrows.”
Rather than basing control decisions on larvae numbers, Jensen advises treating the field when 50% of the stems have feeding injury.
“Before deciding to spray, use an insect sweep net to make sure weevils are still present and check for presence of pupae, a sign that feeding may be slowing down.”