HPIPM:Webworm Alfalfa Looper Armyworm

Authors: Sue Blodgett and Frank B. Peairs[1]

Identification (and life cycle/seasonal history)

Webworms tie up leaves with silken thread, creating a web which protects the larvae while feeding. Webbed leaves are often visible in the alfalfa terminals. Fields that are heavily infested have visible webs in the alfalfa canopy, producing a light gray or "frosted" appearance. Leaves inside the webbings are consumed leaving the tougher leaf veins. Larvae grow to 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.

Loopers are greenish caterpillars, tapered from back to front. Because of the arrangement of legs (only 3 prolegs) they walk with a typical looping motion. Usually two generations per year of this pest occur in Colorado.

Yellow striped, fall and true armyworms have been detected in the High Plains region. These pests occasionally reach damaging numbers. The caterpillars are usually black, with two prominent stripes and many narrow bright ones on each side. At maturity they reach approximately 1.5 to 2 inches long. Eggs are laid in clusters on the upper side of leaves and covered with a gray, cottony material. Eggs hatch in a few days and larvae reach full size in 2 to 3 weeks. Larvae pupate on or just under the soil surface. Adults are brown moths that primarily fly at night but may be encountered flying up as you walk through the field.

Management

Early cutting may give satisfactory control because the larvae cannot survive on dried alfalfa forage. Alfalfa stubble and the new regrowth, especially under the windrows, should be scouted within 2 to 3 days after cutting to make sure webworms are not feeding on regrowth. Insecticide applications are rarely warranted in the High Plains region.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.