Applegater
Fall 2011 9
Notes from a Rogue entomologist:
A report from inside an insect outbreak
BY riChard J. hiLton
If you have noticed large webs in with insecticides. This year I have
madrones, black walnuts and other observed the webworm in a number of
trees around the area, then you have orchard and ornamental trees such as
observed the fall webworm (Hyphantria apples, pears, peaches, plums, walnuts,
cunea) in action. Unprecedented filberts, almonds, birch, sweet gum,
populations of the fall webworm have ash, katsura, and even rosebushes. The
been observed in southern Oregon this worst infestations seem to be in black
year. As an adult,
walnuts—there are
the insect is a white
a number of black
he fact that the webworm was
moth, and the
walnut trees that
showing up in grapes, which
caterpillars, which are considered to be a non-preferred have been almost
f e e d t o g e t h e r host, was a clear indication that this entirely defoliated
within the webs year was going to be different.
and encased in
that they create,
webbing.
are very hairy and are in the same group
At our latitude, it seems that the
as the woolly worms, which are also fall webworm has just one generation
known as woolly bears. The scientific per year; farther to the south with
name for this family of moths, Arctiidae, a longer growing season, multiple
is derived from the Greek word for bear, generations can occur. The life cycle for
arktos. As an aside, the word “arctic” this insect consists of the adult moth,
has the same derivation and refers to which emerges in early to midsummer.
the northern constellations, the great The moth then lays a very large mass
and little bears, also known as the Big of eggs, and when the eggs hatch,
and Little Dippers.
the caterpillars create a web wherein
The fall webworm is native to they feed and skeletonize leaves. As
North America and occurs across the the caterpillars molt and grow, they
continent. Here in southern Oregon continue to feed on more foliage and
it is most commonly found in our enlarge their web until an entire shoot
madrones up in the hills, but this insect or branch will be enveloped. Finally,
can live on a wide array of deciduous the fully grown caterpillars will disperse
or broadleaf trees. In searching the from the web often for a considerable
literature, the number of hosts that distance to find a crack or crevice in
are attacked is quite large and the the bark or soil where they pupate over
references range from “over 80” to winter, emerging the following summer
“more than 200.” Clearly, this insect to repeat the cycle.
is not a specialist feeding on just one
In the 25 years that I have lived
or a few plant species, but is rather an in southern Oregon, the first time I
extreme generalist. The fall webworm can positively remember seeing the fall
is considered to be an occasional webworm in an orchard was last year. In
orchard pest, but it is easily controlled 2010, the fall webworm was observed
T
Rick Hilton examines webworm outbreak on a filbert tree.
in a number of
orchards, both
organic and
conventional,
as well as in the
OSU research
orchard on
Hanley Road.
If you were
observant
you may have
also seen an
A fall webworm, native to North America, is found across the continent.
occasional
web in some landscape trees around wasps) and diseases. The unusual spring
the Rogue Valley. At the time, the weather may have disrupted one or
prevalence of the fall webworm around more of these biological control agents
the valley was considered to be an and allowed the fall webworm to build
unusual population spike, curious and up to our current unprecedented levels.
notable, but nothing to get alarmed While this explanation may sound
about. Then this summer, the white convincing (or not), without more
adult moths appeared in June and July detailed study it is simply an educated
in rather large numbers, followed by guess. Unfortunately, population
the egg masses showing up. I had three ecology is a lot like economics in that
it is much easier
grape growers
learly, this insect is not a
to come up with
bring in moths or
specialist
feeding
on
just
one
an explanation as
egg masses during
to why something
a one-week period. or a few plant species…
happened than to
Considering how
much time is spent training the shoots accurately predict what will happen.
and manipulating the foliage, it is not That being said, can we predict whether
too surprising that vineyardists would the fall webworm population next
be among the first to find these egg year will continue to increase or will
masses. However, the fact that the it collapse? The short answer is no. To
webworm was showing up in grapes, quote Yoda: “Difficult to see, the future
which are considered to be a non- is always in motion.” However, we will
preferred host, was a clear indication monitor the emergence of the adult
that this year was going to be different. moths next summer to give an early
John Yungen, agronomist emeritus indication of overwintering survival
at the Research Center, moved here and then, probably depending on the
in 1955 and cannot recall ever seeing weather pattern, we will see how the
fall webworm in webworm turns.
the valley before
Richard J. Hilton
the past couple of
541-772-5165 ext. 227
years. So how can
Senior Research Assistant /
we account for the
Entolomologist
huge population
Oregon State University
increase of fall
Southern Oregon Research and
webworm over the
Extension Center
last two years? Well,
Photos by Lynn Ketchum
our best hypothesis
is that the unusual
cool and wet spring
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conditions that we
had in both 2010
and 2011 had some
Jackson County
effect that resulted
Sally Buttshaw
in much higher than
normal survival of
541-646-8418
fall webworm. The
population size of
Josephine County
plant-feeding insects
is often regulated by
Jill Wright
mortality caused by
541-324-0950
predators, parasites
Webworm nest as seen throughout the Rogue Valley. (especially parasitic
C