The extremely mild winter of 2023-24 created a safe haven for a common pest in northern climates that is now enjoying a feeding frenzy in alfalfa fields.
The alfalfa weevil is largely a problem before first cutting, according to Rory Lewandowski of Ohio State University Extension. “Depending on the amount of larval insects that are present, alfalfa weevils can really shred plants and cut back on your yields,” he stated.
Because alfalfa weevils over-winter above ground, a higher-than-normal population survived the record-warm winter, ready to feast on new plant growth. According to an Integrated Crop Management bulletin from Iowa State University, entomologists believe alfalfa weevils have been showing up earlier in recent years because they break dormancy in the mild weather in the fall and lay eggs that hatch earlier in the spring.
“This leads to a prolonged feeding period, which might extend past the first cutting of alfalfa,” noted Iowa State Extension education specialist and entomologist Ashley Dean.
Weekly scouting of first-cutting alfalfa is advised, especially because of this season’s ideal conditions for alfalfa weevil. Lewandowski said the larvae – which cause the most plant damage – are lime green in color, with jet-black heads and a signature white stripe down their backs.
The larvae feed on the top of the plant, typically producing a pinhole pattern in alfalfa leaves. This top-down feeding pattern contrasts with the bottom-up feeding pattern of the other major early crop alfalfa pest, the clover leaf weevil. The clover leaf weevil also has a lighter, brownish-colored head.
To scout for alfalfa weevils, Lewandowski advised sampling plants from 4-5 areas every 25 acres, as demonstrated in this video. Included is a chart of action thresholds according to plant growth stage and degree of larval evidence.
Lewandowski said parasitic wasps and fungal pathogens can help control alfalfa weevils. But if scouting reveals damage and larval presence that exceeds thresholds, the situation may call for a rescue insecticide treatment or early harvest.
“Early harvest is the most economical form of control,” he advised. “But if you’re close to or over threshold levels, be sure to check 4-5 days after harvest for regrowth because they can also feed on new crown buds.”
In this video, Bryan Jensen with the Integrated Pest Management program at the University of Wisconsin discusses alfalfa weevil scouting after first cutting. Jensen said harvest often takes care of alfalfa weevil problems, but the larvae can survive if the weather turns cool and wet immediately after the first cutting is taken.
A delay in green-up may mean alfalfa weevils are still present. But regrowth also may be slowed because you had to take the first cutting early, so time may be the only “rescue treatment” necessary. “Before you spray, you want to make sure alfalfa weevils are in the field, and are the cause of that delay in green-up,” he suggested.
If you had a problem with alfalfa weevils early in the season, Jensen advised also scouting the regrowth again, just as you did the first crop. He said by the time the regrowth is about 8-10 inches tall, the larvae will pupate, produce silver-colored, silken cocoons that are easily identified on plants or on the ground among crop residue. “That’s a sign that you can use to determine that weevils are about done feeding,” stated Jensen.
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