The (mostly) mild winter of 2023-24 was nice, but it’s had a nasty drawback across Iowa alfalfa fields this month.

“Alfalfa weevils have been the hot topic, especially in my southern counties,” says Rebecca Vittetoe, an Iowa State University Extension field agronomist who covers east-central Iowa. She wrote in a May 7 blog that if alfalfa growers haven’t scouted their fields, she encourages them to do so now.

“Many fields have been at threshold to spray, and we are getting to peak larvae feeding in my southern counties looking at growing degree days,” she writes.

Virgil Schmitt, an ISU Extension field agronomist who covers southeastern Iowa, reports a great deal of spraying for alfalfa weevils over the last two weeks, especially south of Highway 30. “So far, alfalfa weevil numbers currently are generally lower north of Highway 30,” he says.

Aaron Saeguling, an ISU Extension agronomist who covers southwestern Iowa, says alfalfa insect pressure is high for both alfalfa weevil and pea aphids. “I encourage alfalfa producers to sweep [net] fields and apply insecticides when needed,” he writes.

They’ve also surfaced in northeastern Iowa, which Joshua Michel, an ISU Extension field agronomist, covers.

“I’ve seen several fields with early alfalfa weevil instars starting to become active,” he writes.

Vittetoe notes adult alfalfa weevils are brown in color, with a narrow stripe down their back and they have a blunt snout. The damage-causing larvae are yellow to green in color, with a black head and are legless. The larvae feed on the terminal leaves; under a heavy infestation, a field may have a frosted or silver appearance.

They’re back

This isn’t the first rodeo for alfalfa weevil, as it’s the second consecutive year they have been Iowa farmers’ most prominent spring pest, writes Ashley Dean, an ISU Extension field crop entomologist, and Erin Hodgson, an ISU Extension entomologist, in a May 7 blog.

Besides the warm winter that led to greater-than-usual overwintering populations, alfalfa weevils have been surfacing earlier than expected in spring, Dean and Hodgson say. Recent cool weather and cloud cover also favors this pest, they add.

Alfalfa weevil larvae seriously defoliate plants. Before they do so, though, their presence can be detected by using sweep nets to dislodge them from plants. However, Dean and Hodgson caution, a sweep net is not ideal for making management decisions.

Instead, stem counts are the most accurate way to determine whether populations warrant management. To do this, all you need is a bucket before heading out to the field. Here’s what Dean and Hodgson also advise to do:

Management options

Early harvest. Cutting alfalfa when plants reach a 16-inch height is the most cost-effective management tool for alfalfa weevil, Dean and Hodgson say. This removes the larvae’s food and shelter and exposes them to sunlight.

However, they advise considering other factors, such as feed value. If early harvest is chosen to manage alfalfa weevils, remember to scout alfalfa stubble for larvae or delayed regrowth.

Insecticides. If harvest is not an option, foliar insecticides are available to manage alfalfa weevil. Most options are in the pyrethroid class (Group 3A) of insecticides, but organophosphates (Group 1B), carbamates (Group 1A), and oxadiazines (Group 22) are available for use on alfalfa weevil.

For organic producers, spinosyns are also labeled for alfalfa weevil (Group 5). The University of Minnesota has a list of active ingredients and product names to reference for alfalfa weevil. Note that chlorpyrifos (Group 1B) is not on the list but is available to use in alfalfa for the 2024 growing season.

Insecticide application considerations

There are several measures to take before applying an insecticide:

ISU Extension specialists contributed to this story.