While most farmers are focused on getting corn and soybean in during the next few weeks, this is also the time to be aware of what alfalfa weevil larvae are doing in their forage. February and March were warm, and warmer weather on the horizon will give larvae a boost in their development. Scouting is essential to maintain a healthy alfalfa stand. Keep in mind that, while alfalfa may be resistant to potato leaf hopper, it is NOT resistant to alfalfa weevil.
As a reminder, alfalfa weevil scouting is accomplished by collecting a series of three 10-stem samples randomly selected from various locations in a field. Place the stem tip down in a bucket. After 10 stems have been collected, the stems should be vigorously shaken in the bucket and the number of larvae in the bucket counted. The shaking will dislodge the late 3rd and 4th instar larvae which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. The height of the alfalfa should also be recorded at this time.
Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. The detection of one or more large larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment. When alfalfa is between 12 and 16 inches in height, the action threshold should be increased to 2 to 4 larvae per stem depending on the vigor of alfalfa growth. When alfalfa is 16 inches in height and there are more than 4 larvae per stem, early harvest is recommended.