Nature Bulletin No. 87 October 12, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President Roberts Mann, Superintendent of Conservation
Those silken webs you see this time of year, enclosing the ends of tree
branches, were made by the Fall Webworm. The adult moth is all white or
spotted with black, with a wing spread of about an inch. It emerges from
its flimsy winter cocoon in June or July, mates, and the females lay patches
of 400 to 500 eggs on leaves of fruit or shade trees. In about 10 days
the eggs hatch into yellowish larvae, marked with brown, which appear to
be all head and hair. They spin a web over the leaf, enlarging it as the
leaf is eaten and they move to other leaves, but always staying within
the web. Sometimes a small tree will be entirely webbed-over and every
leaf eaten by the crawling mass of caterpillars. These larvae become about
an inch long, covered with long black hairs sprouting from rows of wart-like
knobs. After 4 to 6 weeks, each finds a secluded spot under bark or in
a hollow of a tree, or in rubbish on the ground, where it spins a cocoon
and transforms to a small brown pupa. In this stage it spends the winter.
The Tent Caterpillar, another destructive pest, hatches out in early spring
when the leaf buds are swelling, from egg masses laid the previous July.
The little larvae feed on buds and spin a web in the nearest crotch of
a tree, gradually enlarging it with new layers of silk, but not enclosing
any leaves. They come out to feed on the leaves and return to the web when
it is too hot, too cold or stormy. These two native leaf-eating caterpillars
have serious outbreaks every few years and often weaken a tree so that
it succumbs to other parasites or to disease, and dies. They can be controlled
artificially by spraying or otherwise destroying the webs and the egg-masses,
but actually they are held in control by natural enemies. They are attacked
by several kinds of small parasitic flies and wasps, by soldier-bugs, and
by many of our common birds. Nature has her own police force for such gangsters.
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Update: June 2012