How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Loxostege spp.
(Reviewed 1/17, updated 1/17)
In this Guideline:
Description of the Pest
Webworms are greenish black caterpillars 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. They are often found within webbed leaves that may be connected to the ground by a silken tunnel through which it will retreat rapidly when disturbed. They vary in size, reaching up to nearly 1.5 inches in length.
Webworms overwinter as larvae in the ground adjacent to their fall food host. Moths emerge in early spring and lay eggs on leaves of host plants. Larvae will feed for 3 to 5 weeks.
The larval stage feeds inside of webbed leaves on the upper parts of the plant in summer and fall. If numbers are abundant, this webbing will be clearly visible and will cover extensive areas of foliage.
Management of webworms in alfalfa can be achieved by early cutting of the hay if close enough to harvest, because the larvae cannot survive on dried alfalfa. Monitor alfalfa stubble and the new regrowth, especially under the windrows, for 2 to 3 days after cutting. If the webworms are defoliating the stubble after the hay is harvested, an insecticide may be warranted, although a pesticide application is rarely justified in California.
If found in combination with other worms:
UC IPM Pest Management Guidelines: Alfalfa
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
Acknowledgment for contributions to Insects and Mites:D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center