No. 2/April 3, 1998
Alfalfa Weevils Should Be Active in Southern Illinois
Although we have not geared up to produce heat-unit maps for predicting development and activity of alfalfa weevils, I am certain that alfalfa weevil larvae are active in some alfalfa fields in southern Illinois. As you probably are aware, we can accumulate heat units above a base temperature of development (48 degrees F) from January 1 and determine when certain events occur in the life of an alfalfa weevil and when certain activities should commence. Hatching of overwintering eggs usually occurs when 200 heat units accumulate, and we suggest that scouting should begin when 300 heat units accumulate. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 heat units; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 heat units. With the mild winter weather we experienced, I am certain that we have exceeded 200, maybe even 300, heat units in some of the counties insouthern Illinois.
As you begin to monitor alfalfa fields for alfalfa weevil activity, start looking in areas of the field that might warm up first, for example, south-facing slopes or areas of the field with lighter soils. After 300 heat units have accumulated, you should be able to find small, first-instar weevils in the folded terminal leaves. As these small, yellowish larvae with black heads feed on these leaves, you will observe some 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 strip along the center of the back.
If you are really interested in learning more about the presence of alfalfa weevils in a field, you can scratch around the crowns of the plants to look for the adults that overwintered. Alfalfa weevil adults are oval, light brown with a darker brown stripe along the middle of the back, and about 3/16-inch long. Like most other weevils, alfalfa weevil adults have an extended "snout," at the end of which are their mouthparts. You can also look for alfalfa weevil eggs in stems. The female weevils deposit clusters of about 9 or 10 eggs. The eggs are only 1/32-inch long, oval, and initially light yellow. Just before hatching, the eggs darken and you can see the developing larvae through the "shell."
We will provide updates about heat-unit accumulations beginning with next week's issue of the Bulletin. In the meantime, let us know if you find anything while you're scouting alfalfa fields.
Kevin Steffey (firstname.lastname@example.org), Extension Entomology, (217)333-6652