It’s Time to Scout for Alfalfa Weevils

Identification

Figure 1. (A) Alfalfa weevil larvae. (B) Alfalfa weevil adult. Courtesy: Adam Varenhorst

The larvae of the alfalfa weevil are small, green, and resemble caterpillars. They have a white stripe on their backs and a dark brown/black head capsule (Figure 1-A). The larvae will have 3-pairs of legs near their head.

The adults are small brown beetles with a darker brown stripe that runs down their midline. Alfalfa weevil adults also have an elongated snout that is characteristic of the weevil family. (Figure 1-B).

Scouting

The best way to determine if alfalfa weevils are present in a field is to use a sweep net and a five-gallon bucket. If you confirm alfalfa weevils are present in a field, the next step is to determine their population density. To do this, we recommend walking in a “Z” pattern and randomly sampling a total of 30 plants (i.e., sample approximately 10 plants per leg of the “Z”). Each plant that is selected needs to be pulled out gently and then hit against the inside of the five-gallon bucket to dislodge any larvae that are present. We recommend this method versus trying to count the beetles and larvae on the intact plant because adult alfalfa weevils will fall off the plant when disturbed as a defense mechanism.

Once completed, the plant should be measured to determine height and the number of larvae in the bucket should be counted. Repeat this process until a total of 30 plants have been sampled. Lastly, calculate the average number of larvae and the height of the plants. Tables 1-4 contain threshold information for alfalfa weevils based on plant height and the number of larvae present.

Economic Thresholds