As spring finally arrives in the Richmond region, fall cankerworms are expected to return this month with a vengeance, according to officials with the Virginia Department of Forestry.
For the past two years, people in
the area have complained about the worms hanging from silken strands in their
yards and crawling over everything. In
areas where infestations are heavy, people can even hear them eating the leaves
off their trees (the sound is actually due to the worms’ frass [bug poop] falling
from the trees).
“While cankerworms aren’t harmful
to people, they can be a great nuisance or cause distress to those who have a
particular aversion to insects,” said Dr. Chris Asaro, VDOF’s forest health
specialist. “The real problem is the
defoliation they can cause. Typically,
one year of heavy defoliation will not greatly harm an otherwise healthy tree,
but with two or three straight years of heavy defoliation, tree death becomes
much more common.”
Homeowners have just a short time
to protect vulnerable trees.
Dave Terwilliger, VDOF’s area
forester in Hanover County, said, “There’s a relatively non-toxic insecticide
called B.t. that homeowners can have sprayed on trees to control cankerworm,
but it must be applied soon after the cankerworms’ eggs hatch to be
effective. The best time to do that
spraying is when the host tree’s leaves begin emerging. If you wait until you see defoliation, it’s too
late for B.t. to be effective and the damage is already done.”
Cankerworms become moths, which
begin emerging from the ground in the fall.
Female moths are wingless and flightless, and they climb to the tops of
trees to lay their eggs. This occurs
between November and March. In December
2013, VDOF officials wrapped a band of plastic covered with a sticky substance around
the base of 70 trees between Richmond and Fredericksburg to monitor the female
moths. As the moths attempt to climb to
the top of the trees, they become caught in the sticky bands. Counting the number of female moths in the
bands serves as an indicator of potential spring defoliation levels.
Dr. Asaro said, “Typically, catching
more than 100 female moths per tree during the winter would suggest heavy
defoliation in the spring. Most of the
trees we banded had several hundred female moths with some approaching 1,000
female moths per tree.”
Large cankerworm outbreaks are
often sustained for only a year or two before their population crashes due to
natural enemies, such as birds, disease, insect predators and parasites. During the past couple of years, however,
cankerworm activity has spanned more than 2 million acres in eastern Virginia.
According to Dr. Asaro, such an
expansive infestation over several years has never been reported before in this
“On a broad scale, the current
outbreak seems to be self-sustaining, and it’s not clear when a complete
population crash will finally occur,” he said.
“Due to the limited dispersal of the adult moths, outbreaks tend to
recur in the same areas over many decades.”
Homeowners who wish to protect
the trees on their property from fall cankerworms are urged to take action now. The focus for protection should be high-value
landscape trees, particularly oaks, which are a preferred food source for
cankerworms. Contact a professional
arborist to perform the spraying operation.
Dr. Asaro said, “In ecological
terms, cankerworms and other defoliators can have a beneficial effect on the
forest by providing a food source for birds and other wildlife. In addition, all that frass falling to the
ground restores nutrients to the soil, which trees can recover through their
root systems. Most trees will re-foliate
quickly and fully recover from defoliation.
The general public should not be too concerned about environmental
impacts from this pest.”
2017 Chapter Annual Meeting
1 week ago