The following article was prepared by Kevin Wanner (MSU Extension Entomologist).
Alfalfa weevil is the key insect pest of alfalfa, causing variable levels of economic damage across Montana each growing season. Montana weevil populations are a bit lower this year with some pockets of economically damaging numbers. With the cool spring weather alfalfa has been out growing the weevil. Weevil development is starting to ccelerate but first bloom and harvesting has also begun, so first harvest should take care of most of the weevil larvae this year. However, we continue to see adult weevils active.
We dissected a few and the females still have plenty of eggs. Fields with alfalfa weevil should keep an eye on the regrowth this year, these adult females may lay more eggs in the stems after first harvest.
Monitoring: Weevil populations can be estimated using sweep nets (net with a 15 inch diameter, can be purchased online) 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 3 samples are recommended for fields up to 30 acres and 5 samples for larger fields.
Thresholds: 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 harvesting is not an option then an insecticide can be used to reduce weevil populations below economically damaging levels. Additional management information including insecticide options is listed online in the High
plains IPM guide:
http://wiki.bugwood.org/HPIPM:Alfalfa_Weevil
Montana State University, U.S. Department of Agriculture and Montana Counties Cooperating. MSU Extension is an equal opportunity/affirmative action provider of educational outreach.
