Fall Webworm

Pest Description

• adults: wingspan is 1 1⁄2 inches long; pure white, sometimes with brown or black spots
• larvae: ~ 1 inch long; hairy, sometimes with distinct, paired black spots on back
• larvae: two color forms: redheaded race is dark with reddish hairs;
• blackheaded race is pale yellow to green with light-colored hairs
• larvae form silken tents around feeding area
• pupae: light brown, hairy masses
• eggs: small, round and laid in masses on leaf undersides

Host Plants, Diet & Damage

• over 100 host species; cottonwood, chokecherry, mountain-ash, elm and willow are preferred
• eed on leaves within unsightly, silken tents
• young larvae feed in groups, finely skeletonizing leaves
• older larvae can defoliate trees
• can stress or kill trees via multiple years of defoliation

Biology, Life Cycle & Damaging Life Stage

• overwinter as pupae near or on host trees
• adults emerge in June and July and mate
• deposited eggs hatch in 1 to 2 weeks
• larvae feed on foliage for about 6 weeks before pupating
• one generation per year in northern Utah
• larvae are the damaging stage

IPM Recommendations

• Manage trees to maintain overall health.
• Monitor in late-June to early-July for egg masses or larvae.
• Minor feeding from small populations can be tolerated.
• Apply an insecticide (Bacillus thuringiensis var. kurstaki; carbamate; pyrethroid) to foliage targeting larvae when they are small.

For more information, see our Fall Webworm fact sheet.