The alfalfa weevil (Hypera postica) is the most problematic pest for alfalfa growers in the western United States. (Photo credit: Joseph Berger, bugwood.org)

For alfalfa growers throughout the United States—and particularly in the western states—the alfalfa weevil (Hypera postica) is a major concern, as damage from infestations lead to decreased yield and quality. A new resource published today in the open-access Journal of Integrated Pest Management offers growers a robust profile of alfalfa weevil and various options for managing it.

In “Ecology and Management of the Alfalfa Weevil (Coleoptera: Curculionidae) in Western United States,” Makenzie E. Pellissier, Zoë Nelson, and Randa Jabbour of the University of Wyoming outline the life stages of the alfalfa weevil, the damage it causes, ecological factors that affect it, and various management techniques for alfalfa growers to deploy.

“In 2015, we conducted focus groups with alfalfa producers from around the state and completed a statewide survey. The results overwhelmingly showed that alfalfa weevil is the most problematic insect pest for alfalfa producers here,” says Jabbour. “In addition, our own field research demonstrated that weevil densities were highly variable from one field to another and that producers were using different weevil management strategies.”

First discovered in the United States in 1904, the alfalfa weevil has expanded to all of the lower 48 states, though different strains exist in the eastern and western region. It lays its eggs in alfalfa stems, and larvae feed and grow for two to four weeks before pupating into adults, which feed on alfalfa for another one to two weeks. The larval stage causes the most damage as it chews on alfalfa leaves during its progression through four instar stages.

Alfalfa weevils cause the most damage to alfalfa in their larval stage. Clockwise from left: second instar, fourth instar, third instar, and first instar. (Photo credit: John L. Obermeyer, Purdue Extension Entomology)

The authors examined a breadth of existing research on alfalfa weevil for the JIPM profile, in which they document a variety of management methods:

• Economic thresholds. Recommended levels at which insecticide treatment may be warranted range from 1.5 to three weevil larvae per stem or 20 larvae per sweep. “However,” the authors note, “depending on the cost of treatment and the price of hay, actual economic thresholds may be lower or higher than these set numbers. It is good practice to calculate economic thresholds based on costs and prices for the year to best determine whether pest management is worth the cost.”
• Cultural controls. Early harvest, grazing, and intercropping are all potentially useful methods. Alfalfa is typically ready for a first cutting before alfalfa weevil larvae complete development, which can kill much of the larval population and reduce eggs laid later in the season. Grazing of alfalfa fields by livestock can also reduce weevil infestation. As for intercropping, “alfalfa–grass mixtures are commonly grown as forages and have been shown to reduce weevil densities when compared to pure alfalfa stands,” the authors write.
• Host-plant resistance. Some varieties of alfalfa can be selected that are more tolerant to alfalfa weevil than others, but other management strategies are often still necessary, especially with heavy infestations.
• Chemical control. Insecticides effective against alfalfa weevil include some organophosphates, carbamates, and pyrethroids. However, the authors caution that “many products registered for use for alfalfa weevil have high or moderate toxicities to natural enemies, and can be highly toxic to bees,” and many should not be applied when alfalfa is in bloom, when bees are actively foraging.
• Biological control. Several species of parasitoid wasps have been reared and released in the United States to try to control alfalfa weevil, the most common in the western U.S. being among the Bathyplectes genus. Conservation biological control also holds promise, involving practices that conserve natural predators and parasitoids of alfalfa weevil, such as reducing insecticide application, leaving refuge strips of alfalfa during harvest, or planting nearby flowering habitat.

Jabbour says the grazing method appears underused but could be particularly useful. “A number of the producers in our region mentioned sometimes grazing their alfalfa fields in winter, but not specifically for weevil management,” she says. “Previous work in Oklahoma and Montana have shown reductions in weevil infestations with properly timed grazing events. It’s possible that with more strategic grazing plans, producers could maximize winter and spring grazing in alfalfa fields for both forage and weevil management.”

The researchers also found an interesting set of interactions among parasitoids of alfalfa weevil and other pests. Aphids are another economic pest of alfalfa, but they also produce honeydew that parasitoid wasps like to feed on. Previous research has shown that aphid abundance in alfalfa boosts the parasitoids’ rate of attack on alfalfa weevil. “These interactions add another layer of complexity to pest management in alfalfa if we consider how pests may be indirectly affecting one another,” Jabbour says.