Fall Webworm, Hyphantria cunea (Drury)

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.

Importance.-The fall webworm defoliates more than 100 deciduous tree species across the United States and Southen Canada. Although seldom entirely defoliated, infested sycamore trees may be severely “ragged” causing premature leaf drop and reduced growth and vigor.

Identifying the Insect (fig. 3a, b).- There are two races of H. cunea-blackheaded and redheaded. Larvae of both are pale yellowish with a broad, dusky dorsal stripe and yellowish lateral stripes. Covered with gray hair, the larvae are about 25 mm long. Moths are pure white to white with small dark spots. The tiny gray eggs are in large masses partially covered with short grayish hairs.

Figure 3a. - Fall webworm egg mass on underside of leaf.

Identifying the Injury (fig. 3c).- Typically, large silken webs enclose one or more leaves and, eventually, entire branches in summer and fall. In sycamore plantations, however, webbing is present, but large tents may not be. Large groups of feeding caterpillars can cause sparse and ragged foliage over the entire crown.

Biology.-Moths emerge from overwintering pupae from March to July and oviposit on the underside of leaves in masses of 300 to 500 eggs, that hatch in 10 days. Distinct silken tents often do not occur on sycamore trees. Larvae may be present from April to October, but the greatest numbers occur in late summer and fall. Pupation occurs in thin cocoons near the soil surface. There are one to four generations per year, depending on locale and race.

Figure 3b. - Larvae feeding under webbing

Figure 3c. - Partially defoliated sycamore with little webbing.

Control.-More than 80 natural enemies are known. For young trees, it is possible to control the insect by clipping and destroying the webbed branches while the larvae are small. Insecticides may be required.