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 region.
“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.”
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