Late spring alfalfa weevil update

By Samantha Daniel, UNL

As we approach the first cutting for alfalfa across the state of Nebraska, it is important to continue to monitor your fields for alfalfa weevil.

Plant injury from alfalfa weevil feeding damage appears as pinholes in the terminal leaves, with leaves becoming skeletonized as feeding severity increases. Adult beetles are roughly 5mm in length with a blunt snout and a brown body with a darker brown stripe running down the center of the back. Larvae are 1.5mm to 8.5mm long with a black head, wrinkled green body, and white stripe running lengthwise along the top. They will curl into a C-shape when disturbed.

Scouting for Alfalfa Weevil up to and even during the first cutting can help to mitigate economic losses. First, use a sweep net to determine whether larvae are present. If larvae are present, randomly select 5 sites across the field and collect 10 stems at each site, cutting the stems at ground level. Beat the stems into a deep sided bucket, count the number of larvae to determine the average number per stem. While you are actively cutting your alfalfa field, take some time to inspect your equipment for larvae. Economic thresholds for Alfalfa Weevil can vary greatly from an average of 1-7 larvae per stem, depending on a variety of factors. Threshold tables can be found on the CropWatch website.

A variety of insecticides are labeled for alfalfa weevil control, including pyrethroids and products containing indoxacarb (such as Steward). Pyrethroids are more harmful to natural enemies of alfalfa weevil, while indoxacarb is more selective and may help to maintain beneficial insect populations. When alfalfa reaches 50% or more bud stage, it may be more profitable to cut the alfalfa early rather than treat it.

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