Information for this article was compiled by Dr. Kevin Wanner (MSU Extension Entomologist) and Dr. Emily Glunk (Previous MSU Extension Forage Specialist).
Alfalfa weevil is the key insect pest of alfalfa, causing variable levels of economic
damage across Montana. Females lay eggs in alfalfa stems. Larvae hatch and crawl up to developing terminal buds where they chew small “pin” holes in the leaves. Larvae develop through four instar stages. 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 economic threshold is the point that warrants action to be taken.
Alfalfa weevil sampling should begin in the spring when the stand is about 8 to 10 inches
tall. Weevil populations can be estimated using a 15” diameter sweep net or by shaking alfalfa plants in a bucket. An average of 20 alfalfa weevil larvae per sweep meets the economic threshold for action. Ten sweeps are taken at each of 3-5 five sites in a field (30-50 sweeps per field) and the total number of weevil larvae counted to determine the average per sweep. An alternative is to cut 10 stems from each of 3-5 different sites in a field (30-50 stems per field) and shake the stems in a bucket to collect the larvae. An average of 1.5 – 2.0 larvae per stem meets the economic threshold for action. To get an accurate average more samples are required for larger fields. A minimum of three samples are
recommended for fields up to 20 acres, four
samples for fields up to 30 acres and five samples for larger fields. Based on historical
weather data, sampling for alfalfa weevil in Montana typically begins between May 24
and June 16, depending on the location and the seasonal weather. A typical date that alfalfa weevil monitoring begins in Lewistown Montana is June 13.
When the economic threshold has been met (more than an average of 20 larvae per sweep
or 1.5-2.0 larvae per stem) action is required to preserve yield. If stand growth is
sufficient early harvesting is the most effective and economic action. If early harvesting
is not an option then an insecticide can be used to reduce weevil populations below
economically damaging levels.
For further information about Livestock and forage pests, please contact the Chouteau County Extension office at 622-3751 or stop in for a visit. We are located in the green building next to the courthouse.
