Q. I have a
white oak tree in my front yard. Over the Fourth of July, my
grandson was leaning against it, and his shirt got all dirty. On
closer inspection, I found a number of dead grayish-brown
caterpillars. They looked very fearsome. We had the tree sprayed
earlier as a precaution against gypsy moths, since we were advised
that they might be a problem again in 2006. Were these gypsy moths?
Why were they all over the trunk of the tree?
A.
If the
caterpillars have double rows of five blue dots followed by double
rows of six red dots, they are indeed gypsy moth (Lymantria
dispar) larvae. Those distinguishing dots may be hard to see if
the insects have been killed by disease, because they shrivel up
badly.
When gypsy moth
larvae are killed by pathogens, they often remain attached to the
trunk and/or branches of their host trees. This is probably related
to the behavior of mature larvae in that they feed at night and hide
in bark crevices during the day. They probably succumbed to the
pathogen while they were down on the tree trunk.
Gypsy moths have
been a problem in the northeastern United States since their
accidental introduction here in the mid-1800s. Their common name
comes from the way they get around. Newly hatched larvae spin a long
thread of silk and are picked up and blown around by the wind.
Gypsy moth larvae
do not build nests, like eastern tent caterpillars, in the crotch of
trees or, as the fall webworms do, out on the ends of the branches
of host trees. They were introduced into Medford, Mass., and spread
south and west ever since. They are as far west as Wisconsin and
extend south along the Appalachian Mountains into Virginia.
When they first
arrive in an area, their populations skyrocket because their natural
enemies are not present or do not know yet that they like this new
food offering. They defoliate many species of trees, including oak,
apple, hawthorn, basswood, maple, elm and many others, including
conifers. Few species are safe when gypsy moth larvae populations
are high.
Deciduous trees
that are defoliated in three consecutive years often die because
they have spent all of their energy reserves by leafing out twice in
a single growing season. Conifers die after a single defoliation
because most do not have latent buds necessary to leaf out a second
time.
Once native,
naturally occurring enemies kick in -- including other insects,
birds, small mammals and pathogens -- a balance is achieved, and
they no longer seem like a biblical plague. Still, insect
populations are cyclical, depending upon favorable weather
conditions and abundant food sources.
Pathogens such as
bacteria, fungi and viruses have been critical to getting gypsy moth
populations under control in our area. Drought years can decimate
pathogen populations and allow gypsy moths to gain the upper hand.
Pennsylvania's
Department of Conservation and Natural Resources had not conducted
aerial spraying since 2002. However, forest entomologists predicted
gypsy moth outbreaks in 2006 (based on egg mass counts, populations
and damage in 2005), and conducted aerial spraying in eight
counties, mostly in the northeast and north and south central
regions of the state.
Without
laboratory analysis, it is impossible to say which pathogen killed
the caterpillars on your white oak, or if the spray application took
care of them. You are wise to protect a mature white oak, because
few of us could lose such a tree and live long enough to see its
replacement grow to similar stature.
If they aren't
too bothersome to look at, leave the dead larvae for a few more
weeks. They may continue to spread the pathogen to any surviving
gypsy moth larvae. By late July, you can safely spray them off with
a spray from the hose (NEVER use a pressure washer).
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