Alfalfa weevil is the key insect pest of alfalfa,
causing variable levels of economic damage across Montana each growing season. After mating, the female weevils lay their
eggs in alfalfa stems, and newly emerged larvae crawl up to the developing
terminal buds where they chew small “pin” holes in the leaves. The larvae develop through four stages; the
larger 3rd and 4th stage 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
sweep nets 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 or 30-50 sweeps per field and the total number of weevil larvae is counted
to determine the average per sweep. An
alternative is to cut 10 stems from each of 3-5 different sites in a field or 30-50
stems per field and shake the stems into 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.
When the economic threshold has been met, 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. Additional management
information including insecticide options is listed online in the High plains
IPM guide: http://wiki.bugwood.org/HPIPM:Alfalfa_Weevil
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