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Weevils make early appearance in alfalfa fields

Stop the spread of alfalfa weevils this spring with these tips for spraying or harvesting.

April 10, 2024

1 Min Read
A close up of alfalfa weevil larvae
EARLY RISER: The alfalfa weevil larvae are already appearing in fields across Missouri. Their identifying mark is the dark brown or black head. Frank Peairs, Colorado State University, Bugwood.org -

Warm weather at the end of March prompted alfalfa weevil larvae to start feeding early, which means growers should be scouting their crop.

Anthony Ohmes, University of Missouri Extension agronomy field specialist, says April is the time to walk alfalfa fields to look for larvae activity.

“The economic threshold for alfalfa weevils is an average of one or more larvae per stem, and 30% or more of the plant terminals show feeding damage,” Ohmes says. “If the field’s infestation exceeds this, it may be time to start spraying.”

Spraying do’s and don’ts

Always read and follow insecticide labels to ensure proper treatment. Some factors can reduce the efficacy of insecticide applications to control alfalfa weevils. These include:

  • Temperatures below 60 degrees F when using pyrethroid insecticides. There are other classes of insecticides that perform better in cool weather.

  • Using lower rates of insecticides when larval populations are very high.

  • Less than optimal coverage; 20 gallons per acre is recommended.

  • Incorrect sprayer tip selection reduces coverage.

  • Possible development of resistance to pesticide; rotate the mode of action being used. There are confirmed populations of weevil larvae that are resistant to pyrethroids.

Alternate options for control

If farmers do not want to spray, there is another weevil management option: Harvest early.

“Remember, it is best for the crop not to harvest earlier than seven to 10 days before the normal growth stage of one-tenth bloom,” Ohmes says. “This harvest could be done by hay-cutting or by grazing.”

MU Extension research has found that mechanical harvesting or grazing cattle can reduce 90% or more of the weevils.

If grazing, be cautious of bloat from wet foliage and damage to the crowns from trampling during wet conditions.

Source: University of Missouri Extension

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