This oak may look sick, but there’s no need to worry. It’s just been used as a nursery by the 17-year cicadas.
The female insects have cut slits in pencil-width twigs on the tree and laid their eggs inside. That damage causes the twigs to turn brown and die, a phenomenon called flagging.
In most trees, that’s not a problem. In fact, it’s a form of natural pruning that can even make the tree healthier in the long run. Very young trees, however, can be severely damaged or even killed by the cicadas’ onslaught.
As Ashley Kulhanek of the Ohio State University Extension noted in a recent issue of the e-newsletter Buckeye Yard & Garden Line, more than 270 species of trees, vines and woody shrubs are known to support the eggs of periodical cicadas. But the insects seem to prefer some species over others.
I’ve noticed a lot of damage on oaks but almost none on the maple trees in my neighborhood, even though maples are supposed to be a type the cicadas like. Elms, chestnuts and ashes are some of the other trees the cicadas supposedly prefer.
By the way, those eggs in the trees will eventually hatch into antlike nymphs. The nymphs will fall off the trees and burrow into the ground, where they’ll spend the next 17 years feeding off plant roots and growing till it’s their time to emerge.