July 16, 2010

Alfalfa caterpillars and webworms are damaging alfalfa fields in southeastern Nebraska. A wide variety of caterpillar species may be found in alfalfa. These are normally kept at tolerable levels by the diversity of predatory and parasitic insects in alfalfa, but occasionally these species may reach damaging levels.

Alfalfa Caterpillars

Alfalfa caterpillars are velvety green with a narrow white stripe running down the length of the body on each side. They reach 1.25 inches long at maturity. The adult is the orange sulphur butterfly, which is often seen in alfalfa fields. The males are orange-yellow, and females are yellow to white.

KSU entomologists recommend for alfalfa caterpillars that “Control measures are justified when there are 10 alfalfa caterpillars per sweep of the net, and cutting will not occur during the next few days.”

Webworms

Several webworm species, including the garden and alfalfa webworms, may attack alfalfa. They appear very similar in the field and need to be examined under magnification for species identification. Webworms are green to yellowish green and have three dark spots on the side of each segment. Some species may have stripes down the length of the body. They reach 1 inch long at maturity. They web together leaf tissue, hence their common name.

KSU recommendations for webworms suggest that “if the crop is more than two weeks from cutting, and 25 to 30 percent of the terminals are becoming webbed, sprays in sufficient gallonage and pressure to thoroughly cover and penetrate the foliage may be justified.”

If alfalfa is within a week of harvest, early harvest can be an effective non-chemical control measure for caterpillars. Regrowth should be inspected after harvest to be sure that any surviving caterpillars are not damaging the crop.

Treatment of Caterpillars

Insecticides suggested for control of caterpillars in alfalfa may be found on the UNL Department of Entomology website.

Potato Leafhoppers

Potato leafhoppers have built up in eastern Nebraska, and damage is evident in some fields. See the June 23 CropWatch story, Scouting for and Treating Potato Leafhoppers in Alfalfa, for a review of sampling methods, economic thresholds and control recommendations.

Bob Wright
Extension Entomologist, Lincoln