Cicadas return: How do they know when it's time to emerge?

17-year cicadas spend almost their whole lives burrowed under the ground, suckling on the roots of trees. How do they keep track of the time?

Chris T. Maier/Courtesy of Connecticut Agricultural Experiment Station/Reuters
An adult cicada lays eggs on an apple twig. This year heralds the springtime emergence of billions of so-called 17-year periodical cicadas, with their distinctive black bodies, buggy red eyes, and orange-veined wings, along a roughly 900-mile stretch from northern Georgia to upstate New York.

Try this backyard science project: Find a tree, dig a hole near it, bury yourself underground, and wait exactly 17 years before you reemerge, surviving off the juices from your tree's roots in the meantime. You are not allowed to use any kind of clock. 

Pretty difficult, wasn't it? 

As everyone who lives anywhere near US Interstate 95 is finding out, billions of cicadas are popping out of the ground as if on cue, seeing the sky for the first time since 1996.

How do they keep track of the time when they are underground? Do they have some sort of countdown timer in their little arthropod brains? 

That's what most researchers thought until 2000. But then University of California, Davis, researcher Richard Karban advanced a novel idea: Cicadas rely on trees to keep time.

To test this hypothesis, Dr. Karban devised a clever experiment. By manipulating the amount of light that peach trees in a climate-controlled room in Davis, Calif., received, Karban manipulated the trees to blossom twice a year. Then he transplanted a population of 15-year-old cicadas that normally emerge every 17 years, and attached them to the roots of the double-blossoming trees.

Fooled by the accelerated trees, the cicadas emerged a year early.

"I've dreamed about tricking cicadas into emerging early for most of my adult life," Karban told Discovery magazine. 

Blossoming is accompanied by a surge of sugars and proteins flowing through a tree's roots, and Karban suspects that the cicadas use this increase to mark time.

The 17-year cicadas' unusual behavior is the result of a hard-fought evolutionary battle. Biologists have suggested that cicadas' long lifespan – one of the longest in the insect world – is an attempt to outrun parasitic insects that cannot extend their own lives to keep up.

A related species of cicada emerges every 13 years. Both 13 and 17 are prime numbers, that is, they are numbers that can be divided only by themselves and the number 1. This mathematical trick, say biologists, could be another strategy to avoid being eaten. 

"The philosophy is that if cicadas have 12-year cycles, all the predators with two, three, four, and six-year cycles will eat them," Mario Markus, a physicist at the Max-Planck Institute for Molecular Physiology, who led a 2001 study investigating the relationship between prime numbers and cicada cycles, told Nature magazine. "If the cicadas mutate to 13-year cycles, they will survive."

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