Start Now to Manage Alfalfa Weevils

Mike Schuldt
Friday, April 9, 2021

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.

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 South East Montana typically begins between May 10 and June 16, depending on the location and the seasonal weather. This year we will need to start earlier as the warm spring is going to bring on earlier weevil activity.

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 managing forage pests, please contact the Custer County Extension office at 406-874-3370 or stop in for a visit. We are located in the basement level of the courthouse.

(Mike Schuldt is the Custer County Extension Agent.)

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