Alfalfa Weevil is Present

Courtesy PR Extension Office

A chronic problem for many producers is the alfalfa weevil, a key insect pest of alfalfa, causing variable levels of economic damage across Montana each growing season according to Powder River Extension Agent Mary Rumph.

“I started monitoring fields last week” reported Rumph, “and ended up collecting a few larvae. This week, the number increased to 25 larvae per ten sweeps, said Rumph. There was no evident damage to the forage.”

“So, weevil is present, and growing in numbers in the fields I swept,” said Rumph. “I encourage producers to monitor their fields for weevil populations,” added Rumph.

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 (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 three samples are recommended for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. 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.

The damage is done by the newly hatched larvae which 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. The adult female is thought to return to the fields in early spring and lay their eggs in the stems of the alfalfa plants. In Montana there is one generation per year.

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.