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Alfalfa Weevil: Coming on Strong in West Central MN

by Doug Holen, Extension Educator - Crops, Fergus Falls
and Phillip Glogoza, Extension Educator - Crops, Moorhead

Just a quick note to report a significant outbreak of alfalfa weevil in WC MN. It escalated over the weekend with a lot of spraying starting on Monday. We have fields in all stages with 1st crop still standing, cut alfalfa in windrow for some time, and 16" of 2nd crop regrowth. All fields have been hit hard. All alfalfa growers in west central MN should be checking for possible infestations.

The interesting component is that all larval stages are represented, from first to third instars, indicating a staggering egg hatch over an extended time that will lengthen the run of it's "pest cycle". I've been seeing larvae in fields at some levels for 3 weeks and it looks to be at least another week.

If a farmer elects to cut infested alfalfa, be sure to scout after cutting to determine weevil survival and monitor regrowth for stubble infestation, particularly beneath windrows, and any larval feeding. After the hay has been picked up, sample the stubble and early regrowth in 20 one square foot samples, 4 chosen randomly from 5 locations. Eight or more larvae per square foot has been used in the past as a treatment threshold. The failure of a field to "green up" after 7 to 10 days with adequate moisture is a sign that stubble treatment is needed.

Insecticides labeled for alfalfa weevil include Baythroid*, carbaryl (Sevin), chlorpyrifos* (Lorsban, Warhawk and Yuma), Furadan*, Imidan, lambda-cyhalothrin* (Warrior formulations, Silencer, Kaiso and Taiga Z), Lannate*, malathion, methyl parathion*, Mustang*, permethrin* (Ambush, Artic, Pounce, and others), and Proaxis*.
* Restricted Use Pesticide
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