Reports from southwest Missouri indicate that alfalfa weevil (Hypera postica) activity is already underway, with early-season feeding observed in some fields. Recent warm periods have accelerated degree-day accumulation (Figure 1), allowing larval to hatch earlier. Similar patterns have been documented in neighboring states, reinforcing the need for Missouri growers to begin scouting alfalfa fields.

Figure 1 Accumulated growing degree days (base 48°F) in Missouri from January 1 – March 19, 2026. Map courtesy of IEM, Iowa State University

Alfalfa weevil is the most important early-season insect pest of alfalfa in Missouri. Damage is typically most severe prior to the first cutting, when larval feeding can reduce yield, forage quality, and plant stand. This species overwinters as eggs deposited in stems during the fall or adults. As temperatures exceed approximately 48°F, overwintered adults become active and begin laying eggs within alfalfa stems. Egg hatch is closely tied to accumulated heat unites and typically begins when it reaches 200-300 degree days (base 48°F).

Although adults feed on foliage, larvae cause most of the damage. Newly hatched larvae feed within folded terminal leaves, producing small "pinhole" feeding symptoms that can be difficult to detect at first. Regular scouting is essential for effective management. Field scout should start in early spring, with attention focused on terminal growth where larvae are most concentrated. Sampling multiple areas of a field and counting larvae from a set number of stems provides a reliable basis for management decisions. Early detection is important because smaller larvae are easier to control and cause less damage than later instars.

Figure 2 Alfalfa weevil feeding damage. Photo courtesy of Micah Doubledee

Treatment decisions should consider larval density, plant height, and the extent of feeding injury. Shorter alfalfa plants tolerate less feeding injury, and economic thresholds may be reached at lower larvae densities. As plants grow taller, they can tolerate greater feeding pressure; however, rapidly increasing populations can still result in significant yield loss if management is delayed. Economic thresholds for alfalfa weevil are determined based on the number of larvae per 30 stems, size of the plants, and the cost of the insecticide treatment.

Early cutting is a strategy that producers can use if alfalfa is tall enough. When early harvest is not possible, chemical treatment may be needed. There are several products registered for alfalfa weevil control. Growers should always follow insecticide label directions, rates, preharvest intervals, and restricted entry intervals. In recent years, resistance to pyrethroids has been detected in populations collected in the western United States. However, it has not yet been confirmed in Missouri. If growers suspect pyrethroid failure, they should reach out to their local field specialist for more information.