Time to Begin Scouting for Alfalfa Weevils

Alfalfa weevils can be very damaging to your alfalfa stand, beginning with pinholes in the leaves and progressing to defoliation and crown injury in severe cases. The potential for this much damage, especially in years like this when hay is of high value, makes managing for alfalfa weevils very important.

The first step in management is proper identification. Adult alfalfa weevils are a beetle typically less than ¼ inch long, with an oblong shape and a snout. They are dark brown and have a stripe down the middle of their back. These adults overwinter in debris and areas surrounding the field and will begin to lay eggs in stems of the alfalfa plant when temperatures warm up in the spring. A small green larva will then hatch and progress through four instar stages before pupating in a cocoon and becoming an adult.

The larval stage is when the most damage occurs. Specifically, the third and fourth instar larvae will feed on the leaves with damage first appearing as small holes and eventually producing a skeletonized look on the leaves in which both stand yield and quality are lost.

Because of this damage from the larval stages, scouting should begin after egg hatch. This egg hatch occurs following 310 accumulated degree-days (ADD), which Richland County has now reached, indicating that now is the time to begin scouting for alfalfa weevils. Alfalfa fields should be scouted once a week up until the first cutting. There are two options for determining if your stand has reached the economic threshold of alfalfa weevil larvae: 1) calculating average larvae per stem, and 2) visual defoliation estimates. For the first option, sample ten stems from three to 5 representative sites throughout the field and dislodge the larvae by shaking and beating the stems into a bucket. Average the number of larvae per stem. If this number exceeds 1.5-2, a control method should be taken. Although not as accurate, you can make visual estimates of defoliation at sites throughout the field and average them. A control method is recommended if there is an average of 30% defoliation in a field.

Options of control methods include early cutting or insecticide application. Cutting early and getting the forage off in a timely manner salvages the stand yield before the larvae mature and cause most of the defoliation. However, some larvae may remain, and it is important to monitor regrowth. Insecticide options are available, however there is growing resistance to pyrethroids in alfalfa weevil control. Be sure to rotate your mode of action if you are using insecticide treatments. Stop by or call the MSU Richland County Extension Office at 406-433-1206 if you have any questions.