Monitor Alfalfa for Pests

By PR Extension Office

This year there is a double threat for alfalfa growers, according to Powder River Extension Agent Mary Rumph. In addition to alfalfa weevil, which are a chronic problem for many producers, is the threat of grasshoppers.

"I monitored fields earlier this week" reported Rumph, "and swept 30 larvae per ten sweeps, six adults and a dozen small grasshoppers," said Rumph. "The was some shotgun damage to the leaves by the larvae," she added.

"I encourage producers to monitor their fields for both weevil and grasshopper populations and damage," added Rumph.

Alfalfa Weevil - 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.

Grasshoppers in Alfalfa - According to Montana State University Extension Entomologist Kevin Warner, the threshold for a threatening immature grasshopper infestation in an established alfalfa field is 50-75/sq. yard in crop borders and 30-45 in the field. The recommendation is to treat crop borders when the immature grasshoppers are small and numbers are moderate. Grasshopper control is most effective before the insects become large nymphs or adults, as these stages are more mobile and more difficult to control. The thresholds are developed for irrigated alfalfa, damage to dryland will be more severe at the same population levels. At high populations and if surrounded by grassland, multiple applications may be required to preserve first cutting, and then monitor early growth for second cutting. Newly established alfalfa will be more susceptible.