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Start scouting for alfalfa weevil now

By Assefa Gebre-Amlak
CSU Pest Management Specialist


The alfalfa weevil is the most destructive insect of alfalfa hay in the intermountain western region of the United States. Both larvae and adults feed on alfalfa; the larval stage is the damaging stage, lowering yield and quality.

First and second instars feed in the tightly folded leaves of stem buds. When half to full grown, the larvae tend to move onto open leaves near the terminals.

Alfalfa weevil larvae chew and skeletonize leaves and large larval populations may defoliate entire plants, giving the field grayish color. Damage normally occurs to the first harvest but both larvae and adults may damage regrowth when populations are high, resulting in both yield and stands.

Larval development is completed in about three to four weeks, with the peak damaging larval populations often coinciding with the first cutting of the crop. Fully-grown larvae move into the plant crowns and soil debris to pupate. The larvae spin loosely woven, net-like cocoons, in which they pupate.

Alfalfa weevil presence can be estimated using degree accumulation in your areas to help determine timing sampling for weevil larvae. Scouting for the pest should begin when 148 degree days have accumulated.

Accumulated Degree Days (ADD) can be calculated as follows:

1. Begin accumulating degree days on March 1 when temperature first exceeds 48 F. (the developmental threshold)

2. Daily Degree-days = (Maximum temp. + Minimum temp./2) - 48 F

3. Add each day's accumulation to the previous total (Accumulated Degree Days).

4. Compare the running total to the degree-days that correspond with recommendations for initiation of sampling.

According to the current temperature data, the indicated 148 degree days have already been accumulated in Front Range and northeastern Colorado, so check your alfalfa fields now.

Monitoring and sampling techniques for alfalfa weevil to determine economic threshold

1. Sweep sampling using a standard sized 38 cm diameter net is the most efficient method for estimating larval populations. Sampling should begin when 148 degree days have been accumulated, when the larvae are expected to be primarily second instars and when alfalfa hay has reached at least 10 inches in height. Ten, 180 degree sweeps are taken while the sampler is walking through the field. Count the number of larvae per sweep and repeat this sampling procedure, taking a minimum of three samples for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. The economic threshold for a sweep sample is 20 larvae per sweep.

2. Bucket method or stem count method is used to determine the number of weevil larvae per stem. Take three six-stem samples in fields one to 19 acres, four samples in fields 20 to 29 acres, and five samples in fields 30 acres and bigger. The tools and supplies needed for this method includes a three or five gallon light-colored bucket, a white cloth, a hand lens, paper and pencil. The economic threshold for the stem sampling method is 1 1/2 to 2 larvae per stem.

Insecticide applications and early harvesting are the most common growing season management strategies. For types of effective insecticide products, timing and other pesticide information, please our High Plains IPM Guide: http://highplainsipm.org.

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