Alfalfa weevil can be an issue in Ontario, particularly in south-western parts of the province. While outbreaks tend to be isolated, they can be severe and dramatically reduce forage yield and quality. Typically, alfalfa weevil do not reach significant levels until we are ready to take first-cut. However, sometimes alfalfa weevil larvae development is advanced relative to the alfalfa crop, and can do significant damage before the crop is ready to cut. Control options are to cut early or spray. Insecticides are recommended only when cutting is impractical, such as when the alfalfa is in the pre-bud stage.
Description
The adult alfalfa weevil overwinters as a 4 – 5 mm (3/16 inch) brown snout beetle. There is a single generation per year. Adults overwinter in plant debris and become active in April. They chew small holes in the base of alfalfa stems and deposit their eggs. Larvae hatch from eggs and crawl to the tops of alfalfa, where they feed on developing leaf buds and leaves. The developmental stage of alfalfa weevil can be predicted with a Growing Degree Day (Base 10ºC) model.
Larvae are pale to bright green, with a black head and white stripe down the centre of the back. The 1st instar larvae are yellow-green and about 2 mm (1/16th inch) at hatch. They are hard to notice unless you unfold newly formed leaves. Larvae grow to a length of about 8 mm (5/16 inch) at the fully grown 4th instar stage. This usually takes 3 or 4 weeks. Mature larvae create a cocoon and pupate. Adult beetles emerge in late-June and early-July, but usually do relatively little damage.
Clover leaf weevil are sometimes mistaken for alfalfa weevil. They grow much larger, and have a light brown head. The white stripe has a pinkish edge. Clover leaf weevil rarely cause economic yield loss.
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