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Weevil control: UW studies pesticide application combinations

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University of Wyoming (UW) scientists have been hard at work researching if applying both an insecticide and an herbicide at the same time early in the season controls alfalfa weevils.  

“Herbicides to control weeds are usually applied earlier than the insecticide used for weevil control, but some producers are skeptical that enough larvae have emerged at this time to offer any control,” states Randa Jabbour, plant sciences associate professor at UW.  

She notes, because of this, some companies are offering the option to apply the two pesticides in one application for increased convenience. 

“Many producers say it’s helping but as of right now there is no data to back it up,” says Plant Sciences Graduate Student Micah McClure.  

Therefore, researchers at UW set out to measure alfalfa weevil control after applying both insecticide and herbicide early in the season.  They also looked at how the practice affects beneficial insects.  

Alfalfa weevils 

According to Jabbour the alfalfa weevil is about three-sixteenths of an inch long, and here in Wyoming, they overwinter as adults in the crowns of alfalfa plants as well as debris around fields.  

“Once temperatures rise to about 48 degrees, females will lay eggs in alfalfa stems, usually as the alfalfa is breaking dormancy in April through May. They then hatch into larvae, move through four instars and pupate. During this stage, they look like they are wrapped in a little net, which then drops to the ground and they emerge as adults,” Jabbour explains.  

She further notes females can lay nearly 400 to 1,000 eggs, and once the larvae hatch, damage to alfalfa begins. 

Weevil control 

Jabbour explains producers who determine their number of weevils are close to an economic threshold generally like to spray at least one week before they cut their alfalfa, but oftentimes this decision is derailed by factors outside of their control, like the weather.  

“There is a really narrow window of time when alfalfa producers are trying to make these decisions,” she says.  

However, she notes spraying earlier could help solve the timing problem. 

In fact, according to a publication titled Alfalfa Weevil Biology and Management, research by Oklahoma State University showed six larvae per stem on 15-inch high alfalfa can cause a 0.5-ton per acre loss in the first cutting and 0.4-ton per acre loss in the second cutting. 

The study also indicated eight larvae per 15-inch stem height can cause a 0.67-ton per acre loss in the first cutting and 0.55-ton per acre loss in the second.  

UW study 

For the UW study, McClure notes he spent the spring netting insects on 15 research plots at the James C. Hageman Sustainable Agriculture Research and Extension Center near Lingle.  

His first sweep took place on May 6, and he finished netting his final batch the week of June 22. He collected more than 68 bags to take back to the College of Agriculture and Natural Resources and began counting alfalfa weevil larvae and adults, clover root curculios, bees, ladybugs, damsel bugs, spider, grasshoppers, wasps, aphids and lygus bugs.  

McClure explains his sweeps on May 6 netted no alfalfa weevil larvae and only two adults. By May 27, the total jumped to 348 larvae and nine adults. 

“Weevils are a one-generation, early spring problem,” explains McClure. “Some producers cut hay earlier to try to control the pests, but some weevils can survive and start damaging the second cutting. The pests can then cause further harm to production because an alfalfa plant is putting all of its energy into the secondary growth.”  

Jabbour notes UW’s study is part of a larger grant from the USDA’s National Institute of Food and Agriculture Alfalfa Seed and Alfalfa Forage System Program and study led by Utah State University. 

“Chemists there want to determine how much and how long a pesticide stays on leaves during application, as well as how long it stays toxic,” she says. “The entomologists in that study are measuring different levels of toxicity and if the insecticide will still kill an insect over time.” 

According to Jabbour, UW will collect and send leaf samples to Utah State University next year. She says her hypothesis is that spraying earlier in cooler weather will increase an insecticide’s effectiveness. 

“We think when growers spray earlier, and it’s cooler, the insecticide stays around longer and is able to kill the insect more effectively than later in the season when it’s warmer,” she says. 

More efficient spraying could also lessen weevil resistance to an insecticide, Jabbour notes. She explains there is more documentation of alfalfa weevils being resistant to some of these pesticides in the western U.S. 

“We have not measured this in Wyoming, so we don’t know if we have this resistance here, but we probably do, and it’s probably worse in certain places,” she says. 

McClure notes he may be able to give preliminary data on UW’s research this fall.  

Jabbour says, “The hope is to give producers good information on whether spraying early is effective, and if so, farmers can optimize when they use the product to reduce the rate of developing resistance in insects.” 

Hannah Bugas is the managing editor for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net. 

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