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Alfalfa Webworms in Southern Illinois

August 9, 2002
Robert Bellm, Crop Systems Extension Educator in Edwardsville, and David Feltes, IPM Extension Educator in the Quad Cities, both received reports of severe infestations of alfalfa webworms from farmers in southern Illinois. Robert visited an alfalfa field in Madison County and found 3/4-inch larvae feeding on alfalfa leaves and webbing the leaves together. Fields affected have been reported from Clark, Fayette, Madison, and Wayne counties. Obviously these pests are distributed throughout southern Illinois, from the Mississippi River to the Wabash River. Alfalfa webworms caused damage to alfalfa in Illinois relatively infrequently. So their occurrence in large numbers this year is worthy of note.

Alfalfa webworms feed on leaves within the webbing the larvae produce. As the webworms grow larger, they may venture out more and consume foliage beyond the webs. However, they still retreat to the webbing when disturbed. Although these caterpillars consume foliage, the damage is economic only if the larvae occur in large numbers. Maybe more important is the contamination of the hay with webbing and fecal material in the webbing.

Webbing of alfalfa leaves by alfalfa webworms in an alfalfa field in Madison County. (Photo courtesy of Robert Bellm.)

Alfalfa webworm larvae are distinctly marked with six black spots per body segment. Young webworm larvae are yellow-green, whereas fully grown (1-inch-long) larvae are darker green. The webs surrounding alfalfa leaves are characteristic of these pests.

Because this insect causes economic damage so rarely, economic thresholds have not been developed. Therefore, use your best judgment when trying to decide whether an insecticide application is warranted. When damage occurs near harvest, cutting the hay will accomplish as much as an insecticide. However, if the hay won't be harvested for some time, consider applying one of the following insecticides: *Ambush at 3.2 to 12.8 ounces per acre; *Baythroid 2 at 1.3 to 2.8 ounces per acre; *Mustang at 2.4 to 4.3 ounces per acre; *Pounce 3.2EC at 2 to 8 ounces per acre; Sevin XLR Plus at 1 to 1 1/2 quart per acre; or *Warrior at 1.92 to 3.2 ounces per acre. Products preceded by an asterisk are restricted for use by certified applicators. Please follow all label directions and precautions, and note the preharvest intervals.--Kevin Steffey

Author: Kevin Steffey

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

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