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Fall Applications to Combat the Alfalfa Weevil

15 October 2013

US - The alfalfa weevil, one of the most well-known and devastating pests to agriculture, can cause serious defoliation in alfalfa during the spring, if not treated in a timely manner.

Producers might have difficulty spraying insecticides timely in the spring, as spring alfalfa weevil activity can be intense for a period of three to six weeks, and spring weather is unpredictable.

Researchers at Kansas State University are evaluating an alternative treatment strategy that consists of spraying insecticides on alfalfa in the fall, followed by a spring application. Adult weevils become active in alfalfa fields in the fall where they feed, mate and start laying eggs in alfalfa stems, said Jeff Whitworth, K-State Research and Extension entomologist.

“We’re not trying to eliminate the weevils in the fall,” Whitworth said. “We’re just trying to reduce egg laying so that it will help out in the spring.”

In addition to Whitworth, the research team at K-State includes Alysha Soper, research assistant, and Holly Davis, insect diagnostician. The study began in the fall of 2012 to determine if a fall insecticide application significantly reduced spring infestations of the alfalfa weevil, and if so, what fall application timing would be most effective.

Understanding alfalfa weevil behavior is helpful in understanding the reasoning behind this study. The alfalfa weevil is a univoltine insect, Whitworth said, which means there is one generation produced each year. The weevils produced in the spring, from mid-March to mid-May, leave the alfalfa fields for the most part when temperatures get around 85 degrees Fahrenheit (85 F).

The alfalfa weevil can especially harm the first cutting, but the effects often transcend that first cutting to cause reduced quality and growth. Those weevils that aren’t destroyed in that first cutting will leave alfalfa fields and go to cooler and shadier places. A few stay in leaf litter in fields, Whitworth said, but most will come back in the fall to lay eggs around mid-October. The eggs, and some adult weevils, will over-winter inactively on the plant and in the leaf litter. Anytime the temperatures get above 48 F though, the weevils become active and continue laying eggs until temperatures cool again.

In the fall of 2011, Whitworth said chemical companies came up with registered insecticide fall application, which complied with the Kansas Department of Agriculture. It was too late in 2011 to test the fall application, so in 2012, the K-State research team put together a study to see if spraying adults in the fall would reduce spring infestation.

The researchers started the fall insecticide applications two weeks after detecting the first adult weevil. The first application last year was Oct. 9, the second on Oct. 23, the third on Nov. 6 and the fourth on Nov. 20. They evaluated alfalfa for weevils this spring on April 5 and April 12.

Results showed that the third application had less alfalfa weevils per stem compared to the other application dates.

“From a statistical standpoint, Nov. 6 (insecticide application) showed significantly reduced infestation in the spring,” Whitworth said.

Although they were statistically reduced, Whitworth said from a practical standpoint they were not reduced enough to prevent significant damage if not treated in the spring. He said producers should keep in mind that most conventional synthetic-organic insecticides provide two to three weeks residual activity, and knowing this is helpful in determining the most effective application time to delay egg laying and eggs from hatching.

The findings are preliminary, based on one year of study, but the researchers studied large plots of alfalfa from six different fields. The researchers will continue studying this fall and evaluate again in the spring of 2014.

TheCropSite News Desk

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