Status of Alfalfa Weevil Development

May 21, 1999
We're at the time of year when a lot of additional information about alfalfa weevils offers nothing new. In several previous issues of the Bulletin, we have discussed alfalfa weevil identification, biology, status, and management, including early cutting and application of insecticides. We have provided information about the parasitic wasps Bathyplectes curculionis and B. anurus and the fungus Zoophthora phytonomi. However, few people have reported any significant effects attributed to these natural ene-mies. On Monday, May 17, I observed a pupa of Bathyplectes curculionis within the netlike cocoon of an alfalfa weevil in a field in Champaign County. However, I also observed a significant level of injury; obviously, the impact of natural enemies in that field was not great enough to keep weevils below economic levels.

Heat-unit accumulations (base 48°F) from January 1 through May 17, 1999, are shown in Figure 3. The second peak of third-instar alfalfa weevils (575 heat units) should have occurred throughout the southern two-thirds of the state. Development of alfalfa weevils is progressing rapidly in northern Illinois. By May 31, 1999, accumulated heat units throughout the state will be well beyond the need to measure (Figure 4).

A lot of alfalfa has been cut, and as I mentioned in last week's Bulletin (issue no. 8, May 14, 1999), early cutting can be as effective as an insecticide for managing alfalfa weevils. I reiterate to all alfalfa growers who have cut their hay: Watch the regrowth carefully to make certain that weevils don't cause any more trouble. Look for mature larvae and new adults in the stubble for several days after cutting. If 50 percent of the stubble is defoliated for 3 to 5 days, an insecticide may be warranted. Also keep in mind that as soon as we finish with this first crop of alfalfa and get our second crop under way, the next nemesis--potato leafhopper--moves to the top of our list of concerns. Stay tuned.--Kevin Steffey

Author: Kevin Steffey