Alfalfa weevil is the key insect pest of alfalfa, causing variable levels of economic damage across Montana each growing season. The female weevils lay eggs in alfalfa stems Larvae hatch and crawl up to the developing terminal buds where they chew small “pin” holes in the leaves. The larvae develop through four instar stages. The larger 3rd and 4th instar larvae feed openly on unfurled leaves and cause the largest economic loss. Severe feeding damage will give the field a “frosted” appearance. Mature larvae develop into the next generation of adults that leave the alfalfa field to find overwintering sites. In Montana there is one generation per year. The majority of crop damage occurs prior to the first cutting as a result of feeding by larger larvae. Management decisions are based on surveying the number of weevils to determine if their population will exceed the economic threshold, the point that warrants action to be taken.
