Native to North
America, the fall webworm occurs throughout the United States and
southern Canada. Its hosts include more than 100 species of deciduous
forest, shade and fruit trees, with preferences varying from region
Eggs are small,
yellow or light green, and usually located in hair-covered masses
on the underside of leaves. Mature larvae are 25-31 mm (1 - 1 1/4
inches) long and covered with silky hairs. Their color varies from
pale yellow to green, with a black stripe on the back and a yellow
stripe on each side. Head color varies from red to black. Pupation
occurs in thin cocoons usually spun in the duff or just beneath
the surface of the soil. The adult moth has a wingspan of 25-31
mm (1 - 1 1/4 inches) and is snowy white, usually with dark spots
on the wings.
larvae and damage to leaf (R.A. Casagrande)
lay their eggs from May to July. Eggs hatch within two weeks and
the larvae immediately begin feeding and constructing webs. Larvae
feed for four to eight weeks. The number of generations per year
varies according to the region. The main generation occurs during
July and August in the Northeast.
larvae immediately begin to spin a silken web over foliage on the
terminal portions of the branches. The larvae feed on the leaves
within the webs. As the larvae grow, webs enlarge and enclose more
foliage. Large portions of tree branches are commonly enclosed by
such webs, and are most apparent from mid- to late-summer. Early
stage larvae feed on the upper surfaces of the leaves, and late
instar larvae eat entire leaves except for larger veins and midribs.
Fall webworms are seldom numerous enough to cause total defoliation
and severe damage to the tree. Even if they are present in large
numbers, the damage is still not likely to be a serious threat to
a tree because fall webworms are late season feeders and by the
time the leaves are consumed the leaves have already performed most
of their function to the plant. The insect is considered an ornamental
pest due to the unsightliness of the webs; however, it is ordinarily
of no great importance as a forest pest.
50 species of insect parasitoids and 36 species of predators of
the fall webworm are known in the U.S., yet they are not commercially
available. On small trees, nests can be cut out and destroyed. Soap
or other insecticides may be applied from mid- to late- summer.
If spraying is necessary, Bacillus thuringiensis is the least
toxic of the effective materials. Several more toxic chemicals are
registered and available for homeowners for this pest. Applications
can be made when larvae are first noticed (usually mid to late July).
from the Cornell Cooperative Extension and the Virginia Cooperative