Although we have received no reports yet, heat-unit accumulations indicate that alfalfa weevil larvae are probably active in some counties in southern Illinois. Figure 2 shows accumulated heat units (above a base temperature of 48 deg F) from January 1 to April 4. Robert Scott with the Illinois State Water Survey generated the data from the Illinois Climate Network. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 heat units, so larvae could be active anywhere south of a line from Calhoun County in western Illinois to White County in eastern Illinois. As of April 4, scouting should have been under way anywhere south of a line from Adams County in the west to Lawrence County in the east. Projected warm temperatures over the next several days will accelerate the development of alfalfa weevils.
Again, we remind you to look first for small larvae feeding in the terminal leaves. The resulting pinhole-like feeding injury, although not economic, is an early indication of the presence of alfalfa weevils. An insecticide may be warranted when 25 to 50 percent of the tips are being skeletonized and there are three or more larvae per stem. If you decide to apply an insecticide, please read all precautions carefully, and follow all directions on the label. Insecticides suggested for control of alfalfa weevil larvae are presented in Table 2.
To look for alfalfa weevil larvae, scout in a U-shaped pattern and pick 30 stems, spaced systematically along your sampling path. Select each stem randomly to avoid picking only those that have signs of injury. Snap the stem off at ground level and invert it into a bucket. After you have gathered all 30 stems, you can determine the amount of injury and the numbers of larvae at your leisure. Shake the stems against the sides of the bucket to dislodge the larvae, and examine the stems for tip-feeding and defoliation. After you count the dislodged larvae, divide by 30 to obtain an average number of larvae per stem.
We suggest one other consideration before you decide to apply an insecticide. In recent years, natural enemies of alfalfa weevils have kept their populations in check in some areas of the state. Although we do not have supporting evidence, we suspect that the parasitic wasps Bathyplectes curculionis and B. anurus, as well as the fungal disease organism Zoophthora phytonomi, may be suppressing weevil populations. In some instances, insecticide applications could be very disruptive to natural enemies, especially the parasitic wasps. In the next issue of the Bulletin, we will provide more information about these important biological control agents.--Kevin Steffey