Pictured: Alfalfa weevil larvae in sweep net. Photo by K. Hamilton DATCP
Department of Agriculture Trade and Consumer Protection field specialists conducted spring survey work in first-crop alfalfa this week. The survey measures levels of a variety of early-season pests, with emphasis on alfalfa weevil leaf feeding damage.
Random sampling of 56 alfalfa fields south of Highway 33 found light defoliation in 30 fields (53%) and moderate leaf feeding in 15 fields (27%). Heavy defoliation at or above the 40% threshold was observed in 11 of the fields (20%). Counts of larvae in the fields with economic defoliation ratings ranged from 2.4-7.2 per sweep and averaged 4.7 per sweep. The heaviest alfalfa weevil defoliation was found in Dane, Grant, Green, Lafayette, and Rock counties during the May 22-28 sampling period.
Across the lower one-third of the state where the harvest of first-crop alfalfa is approaching the halfway point, the alfalfa weevil threat will subside as remaining acreage is cut by early June. Alfalfa regrowth should be checked for signs of weevil injury about five days after harvest and until new growth of the second crop is established. The economic threshold for regrowth is reached if 50% of new stems or crown buds show damage and weevil larvae are still actively feeding in the field.
In contrast, the alfalfa weevil feeding period is entering the most damaging phase in central and northern Wisconsin fields. Scouting for alfalfa weevil larvae and leaf tip feeding should be prioritized for uncut alfalfa north of Highway 21 in the week ahead.
Learn more: https://cropsandsoils.extension.wisc.edu/articles/managing-alfalfa-weevil-in-wisconsin-alfalfa-fields/
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