Wallaces Farmer

What should you do about alfalfa weevils?

So far, they’re the top insect that’s surfaced across Iowa in 2024.

Gil Gullickson, editor of Wallaces Farmer

May 14, 2024

4 Min Read
hand holding a defoliated alfalfa plant
WEEVIL FEEDING. Alfalfa weevils have defoliated this alfalfa plant and are still feeding on new growth. Rebecca Vittetoe

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:

  1. Stop in five field areas to obtain a representative sample across the entire field.

  2. At each location, break off six stems, and shake each plant into the bucket. Most larvae will be dislodged this way, but also check terminal leaves for smaller larvae.

  3. Count the total larvae collected from 30 stems and estimate the average plant height for the field.

  4. Use that information combined with estimated hay value and control costs for your area to determine if management is required. Use the threshold tables in this encyclopedia article.

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:

  • Consult the label to determine the preharvest interval. This may determine which products can be applied to fit your harvest schedule. Preharvest intervals ensure hay is safe for consumption after harvest.

  • Consider insect resistance. Although resistance to insecticides has not yet been reported in the Midwest, alfalfa weevil has resisted pyrethroid insecticides in the western United States.

  • Do not assume an insecticide application was effective. Visit the field three to five days after application to assess populations.

  • Even if alfalfa weevils are found after an insecticide application, it doesn’t necessarily mean they resist the product. Poor timing, improper rates and lack of coverage could impact insecticide efficacy.

  • If another insecticide application is required for alfalfa weevils or another pest during the same growing season (e.g., aphids, potato leafhoppers), use a different mode of action.

  • Consult the label to determine appropriate spray equipment and requirements, such as minimum volume, and use the full labeled rate of the product.

ISU Extension specialists contributed to this story.

About the Author(s)

Gil Gullickson

editor of Wallaces Farmer, Farm Progress

Gil Gullickson grew up on a farm that he now owns near Langford, S.D., and graduated with an agronomy degree from South Dakota State University. Earlier in his career, he spent 13 years as a Farm Progress editor, covering Minnesota and the Dakotas.

Gullickson is a widely respected and decorated ag journalist, earning the Agricultural Communicators Network writing award for Writer of the Year three times, and winning Story of the Year four times. He is a past winner of the International Federation of Agricultural Journalists’ Food and Agriculture Organization Award for Food Security. He has served as president of both ACN and the North American Agricultural Journalists.

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