LINCOLN, Neb. — Be sure to get out and check your alfalfa fields as alfalfa weevils have been recently confirmed in southeastern Nebraska. Scouting alfalfa fields now to monitor larval and adult weevil counts prior to the first cutting is important to determine if management strategies such as harvesting your field early or an insecticide application is needed. Remember alfalfa weevils cause alfalfa plants to wilt and turn brown, symptoms also seen with drought and cold injury. Weevil larvae can rapidly deteriorate hay quality as they spend nearly all their time feeding on the fresh leaf tissue of the plants.
Once alfalfa reaches eight inches tall, a sweep net can be used to see if alfalfa weevil larvae are present. If larvae are found with the sweep net, move to the hand sampling method. For hand sampling, collect 10 alfalfa stems cut at ground level from five locations across your alfalfa field. Next, shake the larvae off the cut stems into a deep-sided bucket. Then count the larvae in the bucket and calculate the average larvae per stem. Weevil larvae are small (1/16- to 3/8-inch long) pale yellow to dark green insects that curl into a C-shape when disturbed.
Next calculate your economic thresholds for determining if an insecticide application or early harvest might be beneficial. The final treatment decision is based on the economic threshold which varies by plant growth stage, treatment costs, projected forage value and average larvae found per stem. You can find the full economic threshold chart on CropWatch. For example, an insecticide treatment and/or harvesting early maybe recommended if you count two or more weevil larvae per stem at the early bud stage of developing alfalfa valued at $100 per ton.
— Melissa Bartels, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln