Webworms on Pecan Trees

Fall webworms are responsible for the massive, gray silken tents enshrouding entire branches of more than 100 tree species. Pecan trees are among fall webworms' favorite host plants. These North American and Mexican natives have made their way -- with human help -- to Europe, where they first appeared in 1940s Yugoslavia. They have also invaded Japan, North Korea and China.

Pecan Webworm Description

Fall webworm moths (Hyphantria cunea) have completely white or black-spotted white wings and yellow or orange forelegs. The caterpillar responsible for pecan tree damage is yellow to light green, with a black or red head. A wide, dark stripe runs down its back. A yellow stripe runs along each of its sides. A pair of tiny, black or orange bumps tops each of its segments. The pest's most distinctive characteristics, however, may be the long tufts of gray hair emerging from those bumps. A mature webworm is up to 1 inch long.

Life Cycle

Adult moths emerge from their cocoons in early to mid summer to mate. The females deposit masses of 400 to 1,000 green eggs on a pecan's lower leaf surfaces. The webworms hatch in seven to 14 days and spin a communal tent around the leaves. They enlarge it over six weeks as they feed and grow. The young webworms consume the tissue between the leaves' veins and midribs before graduating to entire leaves. Mature webworms leave the web to pupate in the soil or in the tree's bark crevices. In cold winter climates, they overwinter as pupae and emerge between March and August to lay eggs. In mild summer climates, the moths may produce four generations of webworms each year.

Symptoms and Damage

Large webworm colonies envelop pecan tree branches in webs measuring several feet around. Some webs cover small trees. Their feeding habits skeletonize the leaves so that only veins and midribs remain. The heaviest damage to pecan trees occurs in late summer to early fall, when webworm populations are at their highest.

Manual Control

Webworm damage, from webs to destroyed leaves, is largely cosmetic. The worms feed most heavily when the trees are about to shed their foliage naturally in fall. Webworm control is seldom necessary. Removing webs with a pole or high-pressure water spray, or pruning their supporting twigs, disposes of the worms while improving the pecan's appearance.

Chemical Control

Their protective nests make webworm control with pesticides difficult. Sprays are seldom powerful enough to penetrate the webs. The best approach is to spray the pesticides on the foliage outside the existing nest to prevent the worms from enlarging their feeding area. Spraying pecans with Bt (Bacillus thuringiensis) pesticide at the first sign of infestation is also effective. This pesticide contains digestive tract-paralyzing bacteria. Webworms that ingest it eventually starve.

References

• The Bugwood Network; Fall Webworm; G. Keith Douce; March 2003
• Texas A&M AgriLife Extension; Fall Webworms; B. M. Drees, et al.; 1999
• Utah State University Cooperative Extension Utah Pest Fact Sheet; Fall Webworms; Ryan S. Davis, et al.; November 2010
• UC IPM Online Pests in Gardens and Landscapes; Walnut Fall Webworm; C. Pickel, et al.; December 2007
• University of Florida IFAS Extension; Fall Webworm, Hyphantria Cunea (Drury) (Insecta: Lepidoptera: Arctiidae: Arctiinae); Andrei Sourakov, et al.; December 2010

Resources

• UM Extension; Pecan Pest Management: Insects and Diseases; George S. Smith, et al.; June 1995