How to Identify Weevils in Alfalfa

When scouting alfalfa, there are two species of weevils that may be observed. These two species are the alfalfa weevil and the clover leaf weevil. The alfalfa weevil is known to cause serious defoliation and has the greater potential to cause yield losses. However, clover leaf weevils can also become very damaging if present in large populations. Although they are similar in size and coloration, there are some distinguishing characteristics that can be used to identify these weevil species.

Alfalfa Weevil Identification

The larvae of alfalfa weevils have three pairs of legs and range in size from 1/16” when they first hatch to 5/16” when they are fully grown. During development, the larvae also vary in color depending on their age. Shortly after hatching, the larvae are light yellow-green (Figure 1) but will turn a darker green after feeding on plant material (Figure 2). The distinguishing characteristics of the alfalfa weevil are its dark brown-black head capsule and prominent white stripe that is present on its back and runs the length of the body. The adult alfalfa weevils are approximately ¼” long. They are light brown in color with some mottling and have a darker brown stripe that runs down their midline. The adults have an elongated snout that extends and curves down from the head. Their antennae originate on the elongated snout.

Fig. 1. Alfalfa weevil larva. Notice the black head and white stripe in the middle of the back. Credit: A. Varenhorst	Fig. 2. Alfalfa weevil adult. Notice the dark brown stripe that runs down the center of the body. Credit: A. Varenhorst.

Clover Leaf Weevil Identification

The clover leaf weevil is a pest of both clover and alfalfa. The clover leaf weevil larvae are approximately ½” long when fully mature. They are similar in appearance as the alfalfa weevil larvae, but have a brown head capsule instead of a dark brown/black head capsule (Figure 3). These larvae also feed on the lower leaves of the plant, whereas the alfalfa weevil larvae feed on the upper leaves of the plant. During the later larval instars, there will also be a pink hue associated with the white line the runs down the middle of the body. The adult clover leaf weevils are brown and somewhat resemble the alfalfa weevil adults. However, they tend to be much larger. In addition, the clover leaf weevils have numerous black lines that run down their bodies that are divided by lighter lines (Figure 4).

Fig. 3. Clover leaf weevil larva. Notice the brown head and white line running down the center of the body. Courtesy: M. E. Rice	Fig. 4. Clover leaf weevil adult. Courtesy: W. Cranshaw, CSU, Bugwood.org