The Periodicity of Cicadas

During a return journey from Pennsylvania this past weekend I witnessed something I had never noticed before. The leaves at the ends of the branches of many of the trees along the highway in every state were brown and appeared dead. It didn't seem to matter what kind of tree it was; every species of tree seemed affected. Upon investigation I learned something new – that it was a periodic occurrence due to cicadas, called flagging, with minimal harm done to the trees.

It seems that female cicadas, nearing the end of their lives, make tiny slits in the ends of tree branches, deposit their rice-shaped eggs there and then die. The eggs begin to feed on the tree branch tips, which in turn die and drop like a flag. Eventually, the newly hatched eggs drop to the ground if the branch tip falls.

The tiny cicadas burrow into the ground and feed on the shallow roots of things like grass, and as they grow they dig deeper into the ground where they remain in a juvenile stage for years until they emerge again en masse in the next hatching season.

I have always wondered about cicadas. It is said that they only hatch once every 17 years, or some say 13 years, yet every year I hear their droning buzz just like the year before. Apparently there are different breeds of cicadas – 15 different varieties in all – 12 broods follow a 17-year cycle, while 3 broods are on a 13-year cycle. Each of these broods emerge in different years, so many areas of the country rarely spend a summer without them. On top of that, many other species of cicada aren't periodical but annual, meaning they emerge every year. That explains that.