University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

August 24, 2007

Webworms Damaging Alfalfa And Soybeans

We have received a couple reports of webworms damaging alfalfa, one in southeastern and one in northeastern Nebraska. Webworms also have been reported damaging soybeans in central Nebraska, near Curtis. Several species of webworms can damage alfalfa and soybeans, including garden webworms. Last week Missouri entomologists reported garden webworms damaging soybeans in Missouri. We have not yet received a sample to identify and don't know which species is damaging crops in Nebraska.

Several generations of garden webworms develop each season, the last of which may overwinter in the soil in the pupal stage. Larvae feed on a wide variety of plants, including alfalfa, clover, beans, soybeans, sugarbeet, and corn. Feeding causes leaf surfaces to take on a brown, netted appearance as green matter is stripped, leaving only the network of leaf veins to dry and curl inside the webbing.

Garden webworms, which are not common pests in Nebraska field crops, are pale green or yellow-green caterpillars with rows of strong, somewhat elongate, black spots on the upper half of the body, and rows of lighter (gray) spots on the lower half of the body (below the spiracles). The head is whitish-tan with brown speckles and has a distinctive black spot on each side near the thorax.

The best control method in alfalfa is to harvest the crop, if possible. KSU entomologists suggest that treatment may be needed in alfalfa if significant defoliation involves 25% to 30% of terminals. If defoliation exceeds 20% in soybeans in reproductive stages and webworms are still present, an insecticide may be warranted. A variety of foliar insecticides should provide control of webworms. See the UNL Department of Entomology Web site at http://entomology.unl.edu/fldcrops/pestipm.shtml for a list of insecticides labeled for use in alfalfa and soybeans.

Bob Wright
Extension Entomologist, Lincoln

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Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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