Webworms, the Tiger Month

This is the time of year is when we start seeing Webworm around the Lowcountry.

Webworms invade trees in the fall (late summer in our area) and build large brown, webby nests in the branches of trees. The webworm is the larvae form of a tiger moth and although they usually don't harm trees, they are unsightly.

The fall webworm, Hyphantria cunea, is a moth in the family Arctiidae known principally for its larval stage, which creates the characteristic webbed nests on the tree limbs of a wide variety of hardwoods in the late summer and fall. It is mainly an aesthetic pest.   The caterpillars  will feed on almost all shade, fruit and ornamental trees but true damage to the tree is minimal. Eggs are laid on the undersides of leaves in early to midsummer and hatch in about a week. The caterpillars feed for six weeks before dropping to the ground to pupate. There may be as many as four generations in the south.

One generation per year emerges in the northern part of North America, with larvae appearing in late summer through early fall. South of an approximate latitude of 40°N there are two or more generations annually, with webs appearing progressively earlier further south. The parallel 40° north forms the boundary between the states of Kansas and Nebraska. The parallel 40° north passes through the cities of Philadelphia, Pennsylvania and Columbus, Ohio; as well as northern suburbs of Indianapolis, Indiana and Denver, Colorado. 

The caterpillars are highly variable in coloration, ranging from a pale yellow, to dark grey, with yellow spots and long and short bristles. There are two cream stripes along the sides. The two races, one more common in the north, the other in the south, differ in head capsule coloration.The maximum length is 35 mm. Webs are progressively enlarged, and much messier looking than those of tent caterpillars (which occur only in spring and have shorter hairs and very little yellow on their bodies) also (webs from the fall webworm are concentrated to the tips of the branches, where as the tent caterpillar webs are largely found in the unions). Larvae feed inside the tents until the late instars. Very young larvae feed only on the upper surfaces of leaves. Later, they consume whole leaves. The larval stage lasts about four to six weeks.

The pupa stage overwinters in the bark and leaf litter at the base of the trees. It is dark brown and about 10 mm long. The thin brown cocoon is made of silk with bits of detritus ( non-living particulate organic material ) interwoven. This stage overwinters.

Tiger Moth

The adult is mostly white in the north, but in the south, it may be marked with black or brown spots on the forewings. It is quite 'hairy', and the front legs have bright yellow or orange patches. The underwings will have less marking than the forewings, and the abdomen often has a sprinkling of brown hairs. It has a wingspan of 35–42 mm.

Management techniques: Tents may be removed by hand, or an insecticide with residual activity may be applied to foliage and twigs. Chemical treatments work best on the youngest larvae and will not penetrate the nest. Both species have natural enemies such as birds, stink bugs, wasps, and flies. Use of Bt (Bacillus thuringiensis) is also effective against young larvae. 

Systemic insecticides may be used on large trees, but use this insecticide with caution. remember  they usually don't harm trees,  but they are unsightly.