A sure sign of spring: Alfalfa weevils are becoming active in northern regions of the U.S. In Pennsylvania, instar larvae have been observed in southern counties, reports John Tooker.
Young alfalfa weevil larvae cause pinhole-sized damage near the tips of plants, often on unfolded leaves, the Penn State University (PSU) Extension entomologist says. Older larvae consume leaves that are more open, typically leaving jagged edges.
“Much of this early season feeding does not result in economic loss, but it is good to recognize the damage and be aware of which fields have greater activity. Those fields with larger populations will of course be where economically significant populations are more likely to develop.”
Economic thresholds for alfalfa weevil are determined from the size of plants, the value of the hay, the cost of insecticide application and the number of larvae on alfalfa stems representative of the field. For a sampling protocol, Tooker advises systematically selecting 30 stems from across a field and shaking them into a bucket.
“If the number of larvae exceeds the threshold, a treatment might be warranted,” he says. A table offering economic thresholds can be found in this PSU fact sheet.