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Alfalfa Weevils Are Active Throughout Southern and Central Illinois

April 19, 2002
Although I have received little information about alfalfa weevil activity in southern Illinois, entomologists in Indiana, Kentucky, and Missouri are reporting that some fields in their states are heavily infested. According to Wayne Bailey, Extension entomologist at the University of Missouri, some fields in southwestern Missouri were treated with insecticides during the week of April. Overwintering eggs in those fields were continuing to hatch.

Figure 2 shows actual degree-day accumulations (base 48 deg F), from January 1 through April 15, 2002. (Welcome back to Bob Scott, Illinois State Water Survey, who developed the maps.) Assuming alfalfa weevil larvae become noticeable after 200 degree-days have accumulated, people should be able to find larvae in central Illinois. Indeed, Matt Montgomery, Sangamon/Menard Extension unit educator in crop systems, has been finding small (~1/8-inch) larvae in the tips of alfalfa plants.

Figure 3 shows projected degree-day accumulations (base 48 deg F), from January 1 through April 29, 2002. Based on these projections, alfalfa weevils will be active in northern Illinois by the end of the month. Alfalfa growers in parts of southern Illinois will be well on their way to getting through "alfalfa weevil season." Keep in mind that the record warm temperatures we have experienced will accelerate alfalfa weevil development. If you use projected degree-day accumulations to estimate when scouting for alfalfa weevils should commence, you may want to schedule your scouting trips a bit early.

In last week's Bulletin (issue no. 3, April 12, 2002), I provided a table (Table 4) of economic thresholds, based on the numbers of alfalfa weevil larvae per stem, mediated by plant height and value of the hay. Table 1 offers economic thresholds, based on percentage tip-feeding damage and accumulated degree-day information. These thresholds appear in Pest Management of Alfalfa Insects in the Upper Midwest, published in 1999 by the Leopold Center for Sustainable Agriculture, Iowa State University, Ames. If you are keeping track of accumulated degree-days (base 48F) from January 1, you should be able to anticipate when the percentage of tip-feeding damage might reach an economic level.

Refer to Table 5 in issue no. 3 (April 12, 2002) of the Bulletin for a list of insecticides suggested for control of alfalfa weevil larvae.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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