Forest Insect Defoliators
Fall Webworm
Hyphantria cunea (Drury)

Hosts: Willow, alder, ash, chokeberry, cottonwood, madrone, and walnut

Figure 43. Figure 43. Adult and pupa of fall webworm.

Symptoms/Signs:  Larvae feed on foliage, forming large webs in the branches of trees. Webs are noticeable in the fall when larval feeding takes place. There are two races: a blackheaded or northern race and a redheaded or southern race. Larvae of the southern race are yellowish-tan with red or orange colored heads and brownish hair that arises from reddish-brown tubercles. Larvae of the northern race have a black head with a pale yellowish or greenish body that has a dark stripe on the back, and long white hairs rising from red or black tubercles. The black-headed race predominates in the West. Adults are white in color with orange markings on the body and legs. The wings have some black spots and a wing expanse of approximately 30 mm.

Figure 44. Figure 44. Larvae of fall webworm.

Biology:  Fall webworm has one generation per year. Adults appear and lay eggs in late June and early July. The eggs hatch and the small larvae feed on both leaf surfaces while larger larvae will consume the whole leaf. Larval feeding continues until mid-September. The insect overwinters in the pupal stage in a transparent cocoon in the soil, leaf litter, or on tree trunks.

Effects:  This insect causes minor defoliation in most forested situations. It can cause loss of visual quality in ornamental plantings.

Similar Insects and Diseases:  The western tent caterpillar is sometimes confused with the fall webworm, due to both having dark heads and dark stripes down their backs. However, western tent caterpillar feeds in the spring while fall webworm feeds in the fall.

References:  16, 24

Figure 45. Figure 45. Webbing and feeding damage on Arizona walnut by fall webworm.