Webworms hammer alfalfa, and also hit soybean

Serious webworm damage in alfalfa was reported from a dozen widely scattered counties south of Interstate 80 during the first 2 weeks of August. I have found larvae but no economic damage in the Ames area. Each summer, this insect is fairly common in Iowa alfalfa, but economic damage in alfalfa is extremely rare, with this year being the exception.

Alfalfa webworm.

During the summer, infested fields show light webs on the leaves, in which the greenish larvae can be found. Fields that are heavily infested have considerable webbing across the canopy of the alfalfa and the field takes on a light gray or "frosted" appearance. The leaves inside the webbings are nearly consumed, except for the tougher leaf veins. In new seedings of alfalfa the larvae may cause injury similar to cutworms and cut the plants at ground level.

When full grown, the larvae are slightly over an inch in length, greenish or occasionally brownish orange, with six black spots (tubercles) on each segment, and a stiff hair extending from each tubercle. The larvae quickly crawl backwards and may escape from the webbing when disturbed. Three to 5 weeks is required for the larvae to complete their development. When the larvae have exhausted their food supply, they often move en masse into an adjacent crop, similar to armyworms. They pupate in the soil and this stage of development takes 1 to 3 weeks. In the upper Midwest, there are two or three generations each season.

It was a big surprise, but this insect also has been found in soybean. The most impressive infestation was reported by Doug Johansen, Monsanto Company. A soybean field near Grant in Cass County was nearly completely defoliated on August 6. Mike White, extension crops specialist, also reports some soybean fields in south central Iowa with heavy defoliation along the field edge from webworms moving out of alfalfa.

Control of webworms in alfalfa can best be accomplished by cutting the hay, assuming it has reached the appropriate stage, because the larvae cannot survive on the dried alfalfa. Alfalfa stubble and the new regrowth, especially under the windrows, should be scouted closely within 2 to 3 days after cutting. If the webworms are defoliating the stubble, an insecticide treatment is warranted. In soybean, defoliated areas can be spot sprayed, especially if the pest is found along the field margin. Insecticides labeled for control of other caterpillars in alfalfa or soybean should give adequate control of webworms.

This article originally appeared on pages 165-166 of the IC-486(21) -- August 20, 2001 issue.

Updated 08/19/2001 - 1:00pm