No. 6 Article 5/May 5, 2006

Alfalfa Weevils Still Noted in Several Areas

Cowpea and pea aphids have captured a lot of attention in alfalfa this year, primarily because these insect pests have been observed infrequently in large numbers in Illinois alfalfa fields. However, don't let the aphids divert your attention from our old friend the alfalfa weevil, which has caused noticeable injury in some fields in southern and central Illinois. Fortunately, alfalfa weevil infestations in most areas have not been very heavy, and spraying insecticides for their control has not been widespread. A lot of alfalfa growers may be able to cut before alfalfa weevils reach economic thresholds. (Refer to the Bulletin issue no. 4, April 15, 2005, for a table of economic thresholds.) Insecticides should be considered only if densities of alfalfa weevils exceed economic thresholds and the alfalfa is not quite mature enough to cut.

Very soon, alfalfa weevils will make their presence known in northern Illinois. Once again, I refer you to the Degree-Day Calculator at As of May 2, 2006, 400 degree-days (base temperature 48°F) had accumulated from January 1 across most of central Illinois. In northern counties, 300 degree-days had accumulated from around the Quad Cities in the west to Joliet in the east. You should be able to find first-instar alfalfa weevils when approximately 300 degree-days have accumulated. So we anticipate reports of alfalfa weevils in northern counties very soon.--Kevin Steffey

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