Monitoring Canker Worms Now Protects Trees Later

​An infestation of fall canker worms can wreck a forest in a couple of years if something isn’t done to keep them under control. The people with the Prince William County Mosquito and Forest Pest Management Program keep an eye on the worm populations in order to keep the pests in check.

Nate Nagle, the field supervisor for the program, said monitoring the bugs in the fall helps to determine how trees and forests should be treated to control the bugs later in the year. People from the program will be out-and-about, counting female canker worms in upcoming weeks.

The native insect’s favored host tree is the chestnut oak, but they’ll also attack ash, beech, elm, other oaks, linden, maple and hickory trees, as well.

Adult female moths emerge from the ground, usually after a hard freeze in the fall. The dull-gray, wingless adult females climb up the tree trunk where they mate with winged males and lay eggs. The egg clusters, laid around small tree limbs, remain in the trees over the winter then hatch into caterpillars in late April to early May. The caterpillars then eat new spring leaves as they come out. By late June to early July, the caterpillars mature and descend to the ground on silk threads, burrow into the ground, spin silken cocoons and the cycle starts over.

Nagle said a couple of years of fall canker worm infestations can be deadly to trees. “What happens is, when the tree loses so much of a portion of its leaves, it stresses the tree out. It forces the tree to press out more leaves and it causes the tree to use a great amount of energy to do that. When you couple several years of defoliation and drought and other environmental factors, it can actually kill the tree.”

Keeping tabs on the pests involves placing sticky band straps around the trunk of a tree. “The females climb up to lay their eggs and they get caught on the band; and we come out and collect the females,” Nagle said.

Spraying from helicopters helps control the worm in the spring, Nagle said. “We control the population through aerial application of an insecticide. It’s a biological insecticide. It’s not a chemical. It’s the same thing we use for mosquito control. The helicopter will apply the insecticide to the leaves. They [the caterpillars] eat the leaves and it kills them.”

In addition to counting female moths, program field personnel watch the leaves in the spring to find out which trees are infested. “If we know there’s defoliation in one area and combine that with the female count in the following fall and winter, we know what areas to target in the spring,” Nagle said.

Historically, fall canker worm infestations have been concentrated in the Cherry Hill and Bull Run Mountains areas, and that’s where people are likely to see helicopters spraying, Nagle said. People shouldn’t be concerned. The insecticide spray coming from the helicopters is poisonous to the caterpillars, but non-toxic and harmless to humans and other mammals. The only bother to humans will be the noise from the helicopters, Nagle said. “You’ll see a helicopter flying over. It’s going to be loud.”

For more information, visit pwcgov.org/mosquito or call 703-792-6279.