| The fall webworm, Hyphantria cunea, is a general
feeder on nearly all trees except conifers. While this
native North American insect attacks over 100
different tree species, it tends to prefer mulberry,
walnut, hickory, elm, sweetgum, poplar, willow, oak,
linden, ash, apple, and other fruit trees. The insect
makes webs at branch tips and is harmful mainly to
the beauty of the host. It is considered to be more of
a nuisance than a threat to the health of the tree.
Symptoms and Diagnosis
A distinctive web of silk, containing many
caterpillars, is constructed around leaves at branch
ends. Each "nest" may contain hundreds of larvae
that feed together for a while. By late summer, the
unsightly nest may measure three feet across and, in
addition to the growing larvae, contains excrement,
dried leaf fragments, and cast skins. An unusual
characteristic of fall webworm caterpillars is that if
alarmed, all the caterpillars in a nest make jerking
movements in unison. It is thought that this is a
potential defensive mechanism to startle and
deter predators.
Another tent-forming caterpillar is the eastern tent
caterpillar, Malacosoma americanum. However,
eastern tent caterpillars make webbed silk nests in a
fork of a branch or tree trunk and leave the nest to
feed. Fall webworm caterpillars have nests at branch
tips and feed inside the webbing.
Life Cycle
In late spring or early summer, adults emerge from
overwintering pupal cases and lay hair-covered
masses of several hundred eggs on the underside of
leaves. The larvae that emerge can be either
yellowish green with a black head or tan with a red
head. Both color forms have many long, gray hairs
and pairs of wart-like black spots running down their
backs. Once feeding begins, the larvae congregate in
masses and produce the silky web that surrounds the
entire colony. Larvae feed inside the web and expand
it as they grow. Larvae stay in the colony until their
last molt after which they may be found crawling
anywhere on the host plant. The larvae crawl to a
protected place to spin a flimsy cocoon and pupate.
The adults emerging from pupation have two color
forms: either all white or white with black spots.
There are two generations a year.
Integrated Pest Management Strategies
1. Live with the problem and let nature take its
course. Hosts are seldom seriously harmed
because defoliation usually occurs later in
summer rather than during a period of active
growth and not enough terminal growth is
consumed to affect tree growth. In addition, more
than 75 natural enemies parasitize and prey on
the fall webworm.
2. Prune out webs. Branches that have active webs
("nests") may be cut out and destroyed. Webs are
always on branch ends and are easier to remove
when they are small. Pole pruners are helpful for
reaching into trees.
3. Apply insecticidal sprays. If chemical control is truly necessary, treatment is recommended when webs first appear. This is because the smaller caterpillars are more susceptible to insecticides and, secondly, the webbed nests are somewhat waterproof and can be difficult to penetrate with sprays. The microbial insecticide Bt (Bacillus thuringiensis) is available as Dipel or Thuricide and can be used on the small caterpillars. Other pesticides registered for use include acephate (Orthene), carbaryl (Sevin), pyrethrins and spinosad. |
