Some southern Wisconsin counties have already reached the 300 weevil degree-day milestone, says Bryan Jensen, University of Wisconsin Extension IPM program manager. He recommends that growers start spot-checking alfalfa fields for potential damage.
“When I reminisce back to the ‘weevil days’ of the previous century, it was those years with a rapid spring warm-up that would have significant damage prior to harvest. During those years, larval development would outpace alfalfa growth, and we would reach the economic threshold of 40% tip feeding well before cutting,” Jensen says.
In the state’s central and northern counties, growers should spot-check sandy knolls and south- facing slopes. Don’t use sweep nets for sampling weevil populations to make treatment decisions, he says.
“Instead, examine stem tips for larvae and signs of feeding. Small larvae will be found in the folded leaflets and/or you can look for tiny feeding holes. Treatment can be suggested if 40% of the stems have feeding and if you are more than one week from harvest.”
Or cut the crop early, he adds. Also watch for diseased (tan/brown) larvae while scouting; their presence can be useful when deciding if control is necessary, especially if cool, wet weather is expected.
If you find what appear to be huge alfalfa weevil larvae, far ahead of anticipated development, these could be clover leaf weevils, he warns. They are similar to alfalfa weevil larvae in color but their heads are tan rather than black. Clover leaf weevil larvae are larger than alfalfa weevils when full grown; they overwinter as larvae while alfalfa weevils overwinter as eggs.