By Scout Nelson
Alfalfa weevils are significant pests. Since insect development depends on temperature, experts use growing degree day (GDD) calculations to predict when larvae become active. In Nebraska, the recommended base temperature is 48 degrees Fahrenheit, beginning on January 1. When GDD values surpass 200, it signals that weevil eggs may be hatching, and larvae could be feeding on new leaves.
Weevil damage usually appears near the tips of alfalfa stems. The larvae create small holes in young leaflets and may feed between veins. These worm-like insects have black heads, pale green bodies, and a white stripe along their backs. They often curl into a C-shape when touched. Because they remain on plants most of the time, it is vital to check folded leaves carefully.
To check for weevils, use a sweep net or inspect random sections of a field. Collect about six stems at each site and tap them into a bucket to dislodge larvae. Record how many larvae appear per stem and measure the average stem height. Compare this data with economic thresholds, which account for crop height, treatment costs, and potential hay value. These thresholds guide decisions on whether insecticidal control is profitable.
If larvae exceed these thresholds, controlling them may be necessary to preserve forage quality. For stands under 10 inches, a long-residual product can be effective. Taller fields may require different thresholds. Early cutting can help if harvest is near. Many insecticides can manage alfalfa weevils, and users should always follow label instructions.
Pyrethroid insecticides have worked well in the past, but resistance issues have arisen in western states. Although resistance is not confirmed in Nebraska, control failures with lambda-cyhalothrin or beta-cyfluthrin have been reported. Researchers plan to test for potential resistance this spring.
Alternatively, indoxacarb and chlorpyrifos have demonstrated effectiveness. However, chlorpyrifos is only permitted until June 30, 2025, under current guidelines.
By monitoring GDD, scouting fields, and applying proper treatments, alfalfa producers can protect their crops and sustain healthy stands. Responsible insecticide selection and accurate timing safeguard yields and reduce resistance risks. Maintaining vigilant scouting practices each season supports long-term alfalfa productivity. This fosters sustainable production.
Photo Credits:gettyimages-0shut0
Categories: Nebraska, Crops, Alfalfa, Weather