The Right Moment
The eastern U.S. is home to 15 broods of cicadas that appear on either 17- or 13-year cycles. Brood X is one of the 17-year variations. Why they wait 17 years to emerge, nobody is quite sure. One theory is that they have an “internal molecular clock” that allows them to sense the passing of time by queues in the xylem they rely on to sustain them underground.
Xylem helps water move around plants and trees, and the fluids in it contain water and nutrients. The theory goes that cicadas can gauge tree leaf out to count the years.
But it’s ground temperature that triggers their emergence from underground. Specifically, accumulated ground temperature. Once soil temperatures hit 64 degrees Fahrenheit (18 degrees Celsius), the cicadas come crawling out of their burrows to start looking for love.
