Eastern Tent Caterpillars
 It's that time of year again, when you will see "tents" of
webbing spun on branches of trees. Close inspection will reveal
that these tents are filled with small caterpillars. These are "Eastern
Tent Caterpillars", which are native to Wisconsin. In fact,
records of their existence go back as far as 1646!
The caterpillars in these tents are the larva of a reddish-brown
moth. The adult moth lays its eggs on small branches of trees
in late fall, and the eggs overwinter and hatch in May here in
Western Wisconsin. Wild cherry, choke cherry, apple, plum and
ornamental crabapple are the most frequent hosts, although tents
can also be found on hawthorn, mountain ash, maples, birch, willow,
poplar and even oak trees.
The dense, silken tents are constructed by the tiny caterpillars
and are expanded as the larvae feed and get bigger. The caterpillars
will go out on the foliage and feed during warm sunny days, and
return to the tent at night for protection. By early to mid-June,
the host trees may be stripped of leaves and the caterpillars
will begin to wander about for additional food. About 6 weeks
after hatching, the larvae are full grown, getting as big as
2 to 2 1/2 inches long.
When done feeding and growing, the caterpillars will spin a
whitish cocoon on fences, tree trunks or other available surfaces.
About 3 weeks later, an adult moth emerges. The moth is reddish-brown
with two parallel whitish lines on the wings and have a wingspan
of 1 1/2 to 1 3/4 inches long. Those adult moths later lay eggs
on the trees to start the lifecycle over again.
While these caterpillars may defoliate a branch or even a whole
tree, there is no need to get too concerned about them. Trees
are able to withstand defoliation, and will simply break new
buds once the caterpillars are gone. If you have significant
defoliation year after year, however, it may weaken the tree
and make it susceptible to other diseases, so control of the
caterpillars would be warranted.
If you have recurring problems with eastern tent caterpillars,
there are some ways you can prevent major infestations. When
the trees are dormant, you can examine twigs and small branches
for egg masses. They will look dark brown, almost "varnished" and
will partially encircle the twig. They can easily be destroyed
by rubbing them off with your fingers, or you can coat the tree
with dormant oil sprays before the eggs hatch.
By May, keep an eye on your trees as they begin to leaf out
in spring. If you find small colonies of tiny caterpillars, you
can easily remove them by hand with a stick or other tool, and "squash" the
tiny caterpillars.
Do not try to burn the tents and caterpillars. This is a hazardous
procedure that seriously injures the trees!
Young caterpillars can also be controlled with a natural insecticide
called Bt (Bacillus thuringiensis var. kurstaki). This bacterium
is harmless to people, animals, and plants, but is lethal to
many types of caterpillars. It is sold under a variety of brand
names and there are several varieties of Bt, so look at the active
ingredients, and be sure to select kurstaki. Bt is mixed and
sprayed on the leaves, and acts as a stomach poison when eaten
by the caterpillars.
When you see these tents on fruit trees, it's nothing to panic
about! The easiest control is to destroy the tents and squish
the caterpillars. These caterpillars are not to be confused with "Forest
Tent Caterpillars" (which do not actually makes tents) or "Gypsy
Moth" caterpillars, both of which do far more defoliation
of trees.
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