University of Illinois

No. 3/April 11, 1997

Update on Alfalfa Weevils

Untitled Document

On April 3, Steve Roberts, research entomologist with the Illinois Natural History Survey, was not able to find any alfalfa weevil larvae in 100 sweeps of a net in a field in Clinton County. He observed a slight bit of pinhole feeding injury and found one second-stage larva when he teased the tip leaves apart. However, thus far this spring, alfalfa weevils have not been easy to find. This could be a relief for alfalfa growers in southern Illinois.

The cold weather during the week of April 7 probably slowed alfalfa weevil development considerably. Some minimum temperatures throughout the state dipped well below freezing, but that occurred primarily in areas where weevil eggs have not yet hatched. Consequently, the freezing temperatures probably caused little, if any, mortality. As indicated in last week's Bulletin, alfalfa weevil eggs begin hatching when about 200 heat units (base 48F) have accumulated since January 1. According to Figure 2, as of April 7, weevil eggs are most likely hatching across the lower one-third of the state. Heat-unit projections (Figure 3) provided by the Illinois State Water Survey indicate that by April 22, alfalfa weevil eggs could begin hatching even in the northernmost counties of Illinois. Due to the very cool weather in early April, these projections may be a bit premature; however, it's better to err on this side when scouting fields than on the other. We will continue to provide updates on heat-unit accumulations and projections throughout the spring season.

Figure 2. Actual heat-unit accumulation (base 48F) from January 1 to April 7, 1997.Figure 3. Projected heat-unit accumulation (base 48F) from January1 to April 22, 1997.

figure 2

figure 3

It's much too early to consider insecticide application for control of alfalfa weevils, so we'll wait for a week or two to provide suggestions for insecticide use. Besides, reported weevil activity has been uncommon, and we can't imagine that anyone has found economic levels yet. Nevertheless, you should scout fields, looking for signs of injury and the young larvae. A return to warmer weather could foster weevil activity in a hurry.

Kevin Steffey, Extension Entomology, (217)333-6652