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 Fall webworm (Hyphantria cunea) ‘nests’ often start becoming
noticeable in our landscapes during the late summer and early
fall. The caterpillars will spin their communal webs on the ends
of branches on which they are feeding. They have been found on
over 400 different species of trees and shrubs. In Illinois,
they are most commonly found on crabapple, walnut, hickory,
pecan, redbud, sweetgum, maple, and oak.
The adults are bright white moths (in the southern part of its
range they may have dark spots on their wings) that will begin
to emerge in the summer (there can be two generations per year
in southern and parts of central Illinois). The females will lay
egg masses that contain several hundred eggs on the undersides
of leaves. About seven days later the larvae (caterpillars) will
hatch.
There are two races of caterpillars. The black-headed race has,
as at the name implies, a black-head. Their bodies are yellowish
with a dark stripe on the back, and long white hairs rising from
black bumps. The red-headed race has a red-head with a
yellowish-tan body and brownish hair that arises from
reddish-brown bumps (the bumps are called tubercles). The
caterpillars will spin a silk web over the leaves as they feed.
The caterpillars will remain inside the webbing as they feed, as
they exhaust their food supply they will continue to enlarge the
webbing. The webbing of mature caterpillars can be 2 to 3 feet
long. After feeding for around 4 to 6 weeks the caterpillars
will leave the webbing and spin a cocoon in leaf litter or in
bark crevices to pupate.
The webbing created by fall webworm can act as a beacon, drawing
your attention to the damage they cause. In most cases, only a
branch here or there will be attacked. Occasionally though,
entire branches, or trees in extreme cases, can be stripped of
leaves if populations get high enough. As alarming as it may be,
it’s unlikely to do any long-term damage to the tree. The damage
they cause happens later in the year and the leaves they are
feeding on have done most of the work that they need to for the
year. They also only feed on the leaves, they leave the buds of
next year’s leaves alone (the trees should leaf out just fine
next year). Add this all up and management of fall webworm isn’t
usually warranted.
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The primary problem with fall webworm is aesthetics.
If you just can’t stand the sight of them there are several things
you can do. One common way of managing them is to prune the affected
branches out of trees. Keep in mind that the webbing is only
temporary, any pruning you do will be permanent and can potentially
hurt the aesthetics of the plant. The water-resistant webbing fall
webworms create makes it difficult to control them with pesticides.
If pesticides are used make sure to use enough pressure to penetrate
the webbing, or tear it open before spraying. Keep in mind any
chemical used may have an impact on any predators and parasitoids
that may attack fall webworm. At least 50 species of parasitoids and
36 species of predators have been found to attack them, which is why
we don’t have nearly as many issues with them, compared to Europe
and Asia where they have been accidentally introduced (we’re not the
only ones who have invasive species, we send pest insects to other
countries too).
Instead of worrying about their presence, think of fall webworms as
a sign of what’s to come, cooler temperatures, changing leaf colors,
and pumpkin spice everywhere.
Good Growing Fact of the Week: Fall webworm and eastern tent
caterpillar are often confused with one another. Eastern tent
caterpillar is out earlier in the year (spring), create their
webbing in the forks and crotches of trees, and leave their webs to
feed.
[Ken Johnson, Horticulture Educator,
University of Illinois Extension] |
