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Just the Beginning for Alfalfa Weevils

April 6, 2001
The onset of alfalfa weevil activity is off to a slower start this spring than it has been the past couple of years. We do not have maps of accumulated and projected degree-days from January 1 (base 48°F), but observations from the field indicate that alfalfa weevil eggs have just begun to hatch and initial signs of injury are apparent. Omar Koester, Unit Assistant­Crop Systems in Monroe and Randolph counties, found first-instar alfalfa weevils feeding in folded terminal leaves in some alfalfa fields in Randolph County on April 2. His observation suggests that the presence of small alfalfa weevils and symptoms of their feeding (pinholes in tip leaves) are obvious in the southern three tiers of Illinois counties.

A few weeks ago, John Shaw, director of the Insect Management & Insecticide Efficacy Program, sampled an alfalfa field in Champaign County and found numerous healthy alfalfa weevil eggs, suggesting that they survived the winter reasonably well. Although one sample site is not necessarily representative of the situation throughout the state, John's observation is a "heads up" for alfalfa growers everywhere in Illinois.

Fully grown alfalfa weevil larva.

Skeletonization of alfalfa leaves by alfalfa weevil larvae.

As you begin to scout alfalfa fields for alfalfa weevils, look first in areas of fields that might warm up early, for example, south-facing slopes and areas of fields with lighter soils. After 300 degree-days have accumulated from January 1, you should be able to find small, first-instar weevils in the folded terminal leaves. These small, yellowish larvae with black heads feed on the leaves, resulting in observable pinholes. This injury is not economic because the larvae are too small to cause significant defoliation. However, by the time alfalfa weevils grow into third instars, they begin to cause more economic damage by skeletonizing the leaves. At this stage of development, alfalfa weevil larvae are bright green with a distinct white stripe along the center of the back.

As temperatures warm up this week and next, expect alfalfa weevil activity to increase rather dramatically. We will begin projecting degree-days next week so we can "predict" alfalfa weevil activity in all areas of the state.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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