Even in areas where temperatures have been on the cool side in recent weeks, alfalfa growers will want to be on the lookout for signs of alfalfa weevil activity.
Alfalfa weevils have already become active in the southern reaches of Pennsylvania, says John Tooker, Penn State University Extension entomologist. Young alfalfa weevil larvae cause pin-hole-sized damage to leaves near the tips of plants, often on folded leaves, he points out. 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’s good to recognize the damage and be aware of the fields that have greater activity.”
University of Illinois Extension entomologist Mike Gray was expecting alfalfa weevil eggs to begin hatching last week across the southern third of his state. “Producers should begin to pay attention to early signs of injury, particularly along southern exposures of fields,” he says.
Alfalfa weevils complete four larval instars, Gray points out. Shortly after they hatch, larvae are yellowish-green, each with a white stripe down the middle of its back. When full grown, larvae are roughly 3/8” long and have shiny-black head capsules. Severe weevil infestations result in leaf skeletonization, causing fields to take on a frosted appearance. Consider a rescue treatment when 25-50% of leaf tips are skeletonized and three or more larvae are present per stem, he recommends.
Weevils have already, or will soon, become active in the southern half of Iowa, according to Iowa State University entomologist Erin Hodgson. North of Interstate 80, the insect will likely become active by April 20-25.
To initially detect alfalfa weevil larvae in spring, Hodgson advises sampling with a sweep net. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake it into a bucket to dislodge larvae. Average the number of larvae per stem and plant height to determine if an insecticide is warranted.
“Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days,” says Hodgson.
See weevil threshold table and/or check out this Penn State factsheet.