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Weed & Pest

Is Your Alfalfa Under Attack by Weevils?

Written by Jeremiah Vardiman

The arid climate and open spaces of our western states are the perfect combination for providing low disease pressure, which allows us the ability to produce high quality forages. 

However, this arid climate does not protect our hay crops from other pests, and one in particular is the alfalfa weevil, Hypera postica.  The alfalfa weevil is the most destructive insect pest to alfalfa hay in the intermountain plains of the West. 

According to Colorado State University, alfalfa weevil has been a problem in their alfalfa production since the early 1900s. 

Alfalfa weevil 101

So what is an alfalfa weevil? Adult alfalfa weevils are snout beetles, approximately one-quarter inches in length and brownish-grey with a dark brown band down the back. Adult alfalfa weevils over-winter in the crowns of alfalfa plants and become active in the spring once average temperatures warm above 48° Fahrenheit. 

During the spring, female weevils are busy laying eggs by chewing holes in new and dead alfalfa stems, then depositing the lemon-yellow colored eggs in clusters of five to 40 eggs inside these cavities.  Each female will lay between 400 and 1,000 eggs a season.

Prior to hatching, the eggs turn from the yellow-lemon color to a dark brown.  The young larvae emerge from the stem seven to 14 days after the eggs are laid. The weevil larvae are a grub that are yellowish-green with a shiny, black head and curved bodies that are one-twentieth of an inch. 

The newly emerged larvae feed on terminal alfalfa leaves prior to unfurling, while mature larvae feed on unfurled mature leaves. Alfalfa weevil larvae develop through four instar stages. 

By the third instar stage, a white stripe is visible down the center of the curved body.  It takes approximately three to four weeks for the larvae to develop from first instar to fourth instar, at which point the mature larvae then moves to the crown of the alfalfa plant to pupate into a loosely woven cocoon. 

  After seven to 14 days in the cocoon, depending on temperatures, the weevil emerges as an adult or snout beetle.  The adult beetles reduce their feeding on alfalfa plants for the rest of summer and focus on mating and surviving the winter. The complete life cycle is typically one generation per year in the intermountain plains of the west.

Monitoring

For the record, alfalfa weevil populations do not always reach economic thresholds that warrant control. Therefore monitoring your fields is advantageous. 

It has been found that a density of one larvae per 30 plants 12 inches tall can reduce yields by about three pounds per acre. However, alfalfa weevil impact on yield decreases with plant height so the same density of larvae on 16-inch tall plants was approximately 0.75 pounds per acre. 

Typically, alfalfa weevil larvae do the most damage to the first cutting through defoliation of the leaves, and they are typically out of the fields for second cutting.  However, if the damage to first cutting is significant enough then there could be major influences on the vigor of second cutting resulting in further yield loss. 

Since alfalfa weevil larvae grow and develop incrementally at almost a constant rate, which is directly related to the increasing spring temperatures above 48 degrees Fahrenheit, a growing degree day calculator can be utilized to determine the approximate date to monitor for growth stages and infestation or population. 

Estimating growth

A growing degree day calculator estimates the amount of accumulative days that have reached or exceeded 48 degrees Fahrenheit utilizing a high-low temperature gauge by adding the maximum temperature to the minimum temperature, dividing the total by two and subtracting 48.

The degree days are then totaled to represent cumulative degree days (DD) that can be correlated to development stage of the larvae – egg hatch at 310 DD, first instar at 350 DD, second instar at 425 DD, third instar at 500 DD, fourth instar 580 DD, adult emergence at 850 DD. 

Correlating the degree days to calendar days projects a date that can be utilized to concentrate monitoring efforts to determine economic threshold of an alfalfa field.

Once a date is determined utilize the six-stem count method, outlined in the sidebar, to monitor the different developmental stages of the alfalfa weevil larvae.  Since weevil populations can be spotty to uniform within a field, which is influenced by terrain, weather and soil conditions, field sampling should be done by predetermined patterns set by acreage and distinctive field sections. 

The fourth instar larval stage causes the most damage through defoliation than prior stages, therefore detection of earlier growth stages are suggested.

The economic threshold for controlling the alfalfa weevil infestation in your field is determined by the size of plants, the value of the hay, the cost of insecticide treatment, and the infestation levels or number of larvae per stem.  Contact your local extension office for current data on yield loss in your area and to discuss management options.

New development

The use of the cumulative growing degree day calculator has given our alfalfa growers a useful tool to help them better monitor and manage alfalfa weevils in their fields. Researchers and extension personnel have set average dates for the various larval growth stages by locations throughout their states.  These average dates are determined using the past 30-year daily temperature averages. 

Unfortunately, this method does not project accurate dates for unusually warm or cold springs, which can influence the development stages several weeks earlier or later.  To become more precise, calculations are encouraged to be done annually with current weather conditions. 

Current research in Wyoming is working on developing an electronic method to set the cumulative growing degree day calculator to local weather stations to give fast, accurate and local dates to keep monitoring efforts to a minimum while focusing the collection times to productive dates. 

Through this development, there is hope to benefit producers by making the monitoring and decision process more efficient and accurate.