No. 4 Article 2/April 17, 2009

Alfalfa Weevils--One of Our First Indicator Insects of the Season

Despite the cold, wet weather recently and the slow start to the growing season, enough degree-days have accumulated in the southern half of Illinois that alfalfa weevil larvae could be active very soon. As of April 14, 200 degree-days (above a base temperature of 48°F) had accumulated since January 1 along a line roughly dissecting Illinois from Pike County in the west to Edgar County in the east. We recommend that alfalfa growers in southern Illinois begin to search for signs of alfalfa weevil when approximately 150 to 200 degree-days have accumulated. Alfalfa growers elsewhere can wait to begin their search until approximately 300 degree-days have accumulated. Following is the explanation for this disparity.

Because temperatures in southern Illinois in the fall and winter are relatively warmer than elsewhere in the state, alfalfa weevil adults (which overwinter throughout the state) may become active on warm days and lay eggs. Consequently, larvae hatch from eggs deposited in the fall earlier than from eggs deposited in the spring. So two distinct peaks of alfalfa weevil larval activity usually occur in southern Illinois, one from fall-deposited eggs and the other from spring-deposited eggs. Hatching of larvae from overwintering eggs usually occurs when 200 degree-days accumulate beyond January 1. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days.

To check out the relationship between temperatures and alfalfa weevil activity, go to the "Daily Pest Degree-Day Accumulations" page at the Water and Atmospheric Resources Monitoring Program (WARM) website. You can click on "Calculator" or "Maps," depending on the type of information you seek--specific sites or regional, respectively. For more information about alfalfa weevils (description, life cycle, scouting, management), go to the "Alfalfa Weevil" fact sheet at our IPM website.

Thus far, we have received no reports of alfalfa weevil activity, but it's only a matter to time. So in the midst of the forthcoming corn-and-soybean planting efforts, don't forget about alfalfa weevils.--Kevin Steffey

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