2016 Scouting Recommendations For Alfalfa Stem Weevils

By Adam J. Varenhorst

SDSU Extension Field Crop Entomologist

Last week, we received the first report of alfalfa stem weevil larvae causing serious defoliation in the crop. Lower temperatures at the end of the week likely slowed the alfalfa stem weevil larvae development and alfalfa growth, which makes it important to scout for this pest to prevent economic injury to the first cutting. Alfalfa stem weevils can be a pest of alfalfa throughout the growing season so scouting should continue after the first cutting. The alfalfa stem weevil is an introduced pest, and as a result it has very few natural enemies in South Dakota. This allows for populations of the weevil to build to levels that can cause severe feeding injury to the crop. Scouting for alfalfa weevils may take a little time, but can prevent serious yield losses.

Description

Figure 1. (Above) Alfalfa stem weevil larvae are green with a dark brown-black head. They have a white stripe down their back. Photo by Amanda Bachmann.

Alfalfa stem weevil larvae have three pairs of legs and range in size from 1/16 of an inch when they first hatch to 5/16 of an inch when fully grown. They also vary in color during larval development and are light yellow-green after hatching, and will turn a darker green after feeding.

The larvae are identified by the dark brown-black head capsule and white stripe on the back running the length of the body (Figure 1).

The adult alfalfa stem weevils are approximately 1/4 of an inch long. They are light brown with some mottling and have a darker brown stripe that runs down their midline.

The adults have an elongate snout that extends and curves down from the head. Their antennae originate on the snout (Figure 2).

Figure 2. (Above) Alfalfa stem weevil adults are small light brown beetles with a broad dark brown stripe down their midline. These beetles have an elongate snout. Photo by Amanda Bachmann.

Scouting

Although both the adult and larvae of the alfalfa stem weevil are capable of causing defoliation injury to the plant, the larvae generally cause the majority of the damage. Large populations can reduce the tonnage and quality of alfalfa. Extensively defoliated fields will have a “frosted” appearance.

To scout for the alfalfa stem weevil, you will need a sweep net and a five-gallon bucket. Start scouting an alfalfa field by using a sweep net to sample multiple areas of the field to determine if the pest is present or not. If alfalfa stem weevil larvae are observed in the sweep net samples the next step is to walk in a “W” or “Z” pattern and randomly select 30 plants along the path. Measure the height of each selected plant, and then gently break or cut the stem at the soil surface. Any unnecessary force may dislodge larvae that are present. Next, shake the plant against the inside of the five-gallon bucket, and count the number of alfalfa stem weevil larvae that are present on the bottom and/or sides of the bucket and record the total. Repeat this process for all thirty plants and determine the total number of larvae observed in the field. Economic thresholds for alfalfa are based on plant height; 0 to 12 inches tall (Table 1), 12 to 18 inches (Table 2), 18 to 24 inches (Table 3), and 24 to 30 inches (Table 4).

Table 1. (Below) Alfalfa 0 to 12 inches tall. Economic thresholds for alfalfa stem weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample.

Table 2. (Below) Alfalfa 12 to 18 inches tall. Economic thresholds for alfalfa stem weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample.
 

Table 3. (Below) Alfalfa 18 to 24 inches tall. Economic thresholds for alfalfa stem weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample. 

able 4. (Below) Alfalfa 24 to 30 inches tall. Economic thresholds for alfalfa stem weevils based on larvae per stem, and calculated from the total larvae in a 30-stem sample.