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Alfalfa Weevil Update

May 1, 2003

Mixed reports continue to come in regarding alfalfa weevil activity. Reports are ranging from little or no activity to heavy feeding. Kevin Black, Growmark, received a report from Jeremy Hamson, Agripride FS, that several fields in Washington County have visible alfalfa weevil injury and have exceeded economic thresholds. The trouble lies with timing insecticide applications with rainfall. Due to the preharvest intervals that accompany many of the labeled insecticides, growers are left with the options to wait on insecticide applications or delay harvest, which is expected in 1 to 2 weeks.

Figure 1 has the actual accumulated degree-days (base 48°F), from January 1 through April 29, 2003. Two distinct peaks of larval activity occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days, which has already occurred throughout much of the state. Figure 2 shows the projected accumulation of degree-days (base 48°F), from January 1 through May 12, 2003. If temperatures between now and May 12 are equivalent to the 40-year averages, we should expect a second peak of third-stage larvae to have occurred throughout most of the southern half of the state (after an accumulation of approximately 575 degree-days).

Continue to scout for alfalfa weevil larvae to determine if economically damaging levels are present. Economic thresholds can be determined by the percentage of tip feeding or number of alfalfa weevils per stem. Table 3 shows some economic thresholds for alfalfa weevils based on percentage of tip feeding associated with accumulated degree-days. Table 4 shows some economic thresholds based on numbers of alfalfa weevil larvae per stem at different alfalfa heights and values of alfalfa hay. These thresholds are published in Pest Management of Alfalfa Insects in the Upper Midwest, published in 1999 by the Leopold Center for Sustainable Agriculture, Iowa State University, Ames.

Suggested insecticides for alfalfa weevil control are summarized from the Illinois Pest Management Handbook and listed in Table 5. Once again, please note the recommendations for Mustang Max are different from those listed for Mustang in the handbook.

Keep us posted on any findings in the field.--Kelly Cook

Author: Kelly Cook

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

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