Webworm - web worm Hyphantria cunea (Drury) Worms
in Pecan Trees
The webworm, Hyphantria cunea (Drury), is a common pest of
trees. It attacks more than 88 different kinds of plants, including
many fruit, nut and ornamental trees and shrubs. It does not
attack conifers (pines and other needle-bearing trees).
Webworms are known for the large, unsightly webs they produce.
Heavy infestations are rarely fatal, but if they occur repeatedly
over several years they can stress trees and make them more
susceptible to drought, disease or other insect pests which
can be fatal.
The feeding preferences of webworms vary from one place to another.
In west Texas, mulberry, poplar and willow are preferred; oak,
hickory and pecan are most often attacked in east Texas. While
Pecans are favored in the Texas Hill Country.
The webworm moth is white and has a wing span of 1 to 11/2 inches.
Sometimes there are small, dark spots on the forewings. Full-grown
larvae are approximately 1 inch long, pale green or yellow,
and covered with tufts of long, white and black hairs. There
are two distinct races of the fall webworm, which can only be
identified during the larval stage. Larvae in the orange race
have orange heads and orange tubercles, while members of the
black race have black heads and tubercles.
Webworms often cover entire branches with their webs. In extreme
infestations whole trees may be covered. Larvae feed within
the web, eating the tender parts of leaves and leave the larger
veins and midrib.
There are two to four generations of webworms each year in Texas.
Four generations occur in southern portions of the state, while
two to three generations occur in the northern areas. The first
generation occurs as early as April in south Texas and as late
as June around Lubbock and Amarillo. The last generation of
the year, which occurs in the fall, is usually the most damaging
and gives the insect its name.
Webworms overwinter as pupae on the ground or on rough tree
bark. The moths emerge from silken cocoons in the spring to
disperse and mate. Female moths deposit hair-covered egg masses
on the undersides of the leaves of their food plants. An egg
mass may be deposited in either a single or double layer and
can contain up to 600 eggs. Each female moth will deposit only
one egg mass. Egg masses of the walnut caterpillar, another
common pest on pecans, are not covered with hairs.
Soon after webworm eggs hatch the larvae begin to build a silk
web. As larvae consume leaves within the web, they expand the
web to take in more foliage. All larvae within a web are the
offspring of a single egg mass. Larvae will molt six or seven
times before leaving the webbing to pupate. The life cycle from
egg to adult requires approximately 50 days.
Control of Webworms in Pecan Trees
Webworms can often be controlled without insecticides by removing
and destroying any leaves that contain egg masses. Larvae may
be knocked out of low-hanging webs, with a stick or broom, and
into a box or garbage bag for disposal. Or, webs can be pruned
from smaller branches. Many beneficial insects attack the egg
and larval stages of fall webworm. You can help these predators
and parasites get to their fall webworm prey by tearing open
the webs. If webs are too numerous or too high in a tree to
deal with individually, insecticides can be used to prevent
damage. Hose-end sprayers or commercial high-pressure sprayers
are best for reaching upper portions of tall trees. Because
webworm larvae remain inside their webbing, insecticide sprays
must penetrate the web to be effective. For best control, apply
insecticides after eggs hatch and before larvae develop dense
webs. Insecticides containing acephate (Orthene®), Bacillus
thuringiensis (B.t.), carbaryl (Sevin®), chlorpyrifos (Dursban®,
Lorsban®), diazinon, malathion, and tebufenozide (Confirm®
2F) are effective. Insecticides containing B.T. and tebufenozide
are selective for caterpillars and do not harm beneficial insects;
however, they must be applied when caterpillars are small for
effective control. Insecticide label clearances are subject
to change and changes may have occurred since this publication
was printed. The pesticide user is always responsible for the
effects of pesticides on his or her own property, as well as
problems caused by drift to other properties. Not all insecticides
are registered for fall webworm on all sites and commodities.
It is up to the user to read the label to make sure the insecticide
is cleared for the site and commodity. Always read and follow
carefully the instructions on the pesticide label.
This pesticide application is best left to a professional.