Alfalfa weevils begin to hatch at 300 GDD base 48 and quit feeding at 900 GDD Base 48. Because south facing slopes are somewhat warmer than average, they should be scouted beginning at 200 (south of I-80) - 250 (north of I-80) GGD Base 48.
A map from the mesonet base 48 is available at http://mesonet.agron.iastate.edu/data/summary/gdd48_jan1.png.
For details on managing this insect, see the April 11, 2014 ICM News article, which has updated thresholds, April 2, 2012 ICM News article, the April 8, 2011 ICM News article, pages 87 - 89 of the April 17, 2006 Integrated Crop Management Newsletter, and/or pages 48 - 49 of the April 11, 2005 Integrated Crop Management Newsletter.
If harvesting is used to manage alfalfa weevils, check re-growth in 4 - 5 days using the strategies listed on pages 2 - 3 of the June 13, 2005 Integrated Crop Management Newsletter.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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