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Issue No. 6, Article 1/May 1, 2009

Alfalfa Weevils May Need to Be Controlled in Some Fields

Despite all of the recent rain and the occasionally cooler temperatures, degree-days associated with alfalfa weevil development (developmental threshold base temperature of 48°F) have continued to accumulate from January 1. A map of accumulated degree-days on April 28 indicated that scouting for alfalfa weevils should be underway in most counties south of a line roughly from the Quad Cities to Chicago. First and second instars could be evident as far north as Hancock County in western Illinois and Vermilion County in eastern Illinois. Serious leaf-feeding injury caused by third and fourth instars could be evident in fields south of Route 50. Kevin Black (Growmark, Bloomington, IL) has received reports of significant injury in some alfalfa fields in southern Illinois.

As a reminder, a management tactic for alfalfa weevils may be warranted when 25% to 50% of the plant tips are being skeletonized (severe defoliation) and there are 3 or more larvae per stem. The range is based on plant height; a greater percentage of skeletonization is necessary to cause economic loss when plants are tall and growing vigorously. If an insecticide is selected as a management tactic, do not apply it during bloom because of potential harm to nontarget insects such as honeybees. In lieu of an insecticide application, consider harvesting the first hay crop as early as possible, as long as yield and quality are not compromised.

Insecticides registered for control of alfalfa weevils in alfalfa are listed in Table 1. Please read and understand all label directions and precautions.--Kevin Steffey

Kevin Steffey

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