Re-evaluating the Economic Injury Level for Alfalfa Weevil (Coleoptera: Curculionidae) Control in Low Desert...

Area of Science:

• Agricultural Entomology
• Crop Science
• Pest Management

Background:

• Alfalfa (Medicago sativa L.) is a major crop in the Western U.S., particularly Arizona.
• The alfalfa weevil (Hypera postica/Hypera brunneipennis) is a significant pest causing yield loss in low desert alfalfa hay production.
• The existing economic threshold for alfalfa weevil, set in 1975, is not considered suitable by regional growers.

Purpose of the Study:

• To re-evaluate the current economic threshold for alfalfa weevil in low desert-grown alfalfa.
• To provide updated guidance for pest management decisions in the region.
• To support sustainable alfalfa production practices.

Main Methods:

• Conducted four years of field trials in the western U.S. low desert.
• Monitored alfalfa weevil larval populations and correlated with alfalfa hay yield.
• Collected supporting observations from agricultural growers and professionals.

Main Results:

• The established economic threshold of 15-20 larvae per sweep is too high for current conditions.
• As few as one to three large alfalfa weevil larvae per sweep can cause significant yield reduction.
• Current hay prices and insecticide application costs support earlier intervention.

Conclusions:

• The economic threshold for alfalfa weevil in low desert alfalfa requires revision.
• Earlier weevil control is economically justified, impacting sustainable production.
• Updated pest management strategies are needed for western U.S. low desert alfalfa systems.