Alfalfa is a valuable forage crop, providing high-quality feed and high yield when managed properly. Alfalfa weevils are one of the most destructive early-season pests of alfalfa. Alfalfa weevil larvae are yellowish-green in color with a white stripe on the middle of their back and a blackhead.
Scout by collecting at least 50 stems from random locations in the field. Clip a stem from an undisturbed area and carefully put it upside down in a bucket so as not to knock the larvae off the stem onto the ground. After collecting all the stems, vigorously shake the alfalfa stems in the bucket to dislodge the larvae. Count the number of weevils collected. Weevil larvae will feed on the leaves, leaving them shredded with a skeleton-like appearance.
Observe the leaves near the top of the plants for weevils that have been infected by a fungus that can quickly eliminate the population. Infected alfalfa weevil larvae are slow moving and turn from green to yellow to brown or black in color.
One management strategy for control of alfalfa weevil is to take the first cutting early removing the weevils from the field in the harvested forage. Another option is to graze the alfalfa. Generally, alfalfa weevils are only a concern for the first cutting of hay, but continue to scout the regrowth of early harvested fields.
Chemical control is dependent upon the height of the alfalfa, the cost of the insecticide and the value of the crop. The economic threshold table can be found at:
For more information contact Valerie Tate, field specialist in agronomy for MU Extension in Linn County by email at firstname.lastname@example.org or by phone at 660-895-5123. University of Missouri Extension programs are open to all.
Valerie Tate is a field specialist in agronomy for MU Extension in Linn County.