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Issue No. 2, Article 1/April 8, 2011

Start Scouting Alfalfa for Weevils in Southern Illinois

It's not too early for producers in southern Illinois to begin scouting alfalfa for signs of alfalfa weevil activity. Even though the number of acres devoted to alfalfa production across Illinois continues to shrink (340,000 harvested acres in 2009, according to the USDA-NASS-Illinois Field Office 2010 Annual Bulletin), this crop can be profitable with proper management. Alfalfa weevil eggs will soon begin to hatch across the southern third of Illinois, and producers should begin to pay attention to early signs of injury, particularly along southern exposures of fields.

By using January 1 as a biofix date and 48°F as the base temperature threshold, we can begin to predict the level of weevil development and potential injury to alfalfa. As of April 5, there were accumulations of 205, 260, 260, and 233 degree-days (since January 1, base 48°F) for Belleville, Carbondale, Dixon Springs, and Rend Lake, respectively. These totals are very close to or slightly above the 11-year averages for these locations. Alfalfa weevil larvae begin to hatch from eggs when 300 degree-days (base 48°F) have accumulated since January 1. The warmer weather anticipated for April 7 to 10 across much of the state will likely result in the escalation of hatch in southern Illinois.

Alfalfa weevils complete four larval instars. Soon after hatch, the larvae are yellowish-green with a white stripe down the middle of their "back." When full grown, larvae are roughly 3/8-inch long and have shiny-black head capsules. Larval injury appears as pinholes in leaf terminals early in the season. Severe infestations of weevils result in leaf skeletonization, causing fields to take on a "frosted" appearance. Consider a rescue treatment when 25% to 50% of leaf tips are skeletonized and three or more larvae are present per stem. For complete life cycle details, scouting tips, and management information, please consult ipm.illinois.edu/fieldcrops/insects/alfalfa_weevil.

Development information for a wide variety of insect pests throughout the growing season is published at the excellent website maintained by the Illinois State Water Survey. Degree-day accumulations are updated regularly and enable you to track insects and their projected development. Equipped with this information, producers can more carefully time their scouting efforts within fields and make more effective insect management decisions.--Mike Gray

Mike Gray

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