There are 15 U.S. broods that emerge every 13 or 17 years, so that nearly every year, some place is overrun. Last year it was a small area, mostly around the Blue Ridge Mountains of Virginia, West Virginia and Tennessee. Next year, two places get hit: Iowa into Illinois and Missouri; and Louisiana and Mississippi. And it's possible to live in these locations and actually never see them.
This year's invasion, Brood II, is one of the bigger ones. Several experts say that they really don't have a handle on how many cicadas are lurking underground but that 30 billion seems like a good estimate. At the Smithsonian Institution, researcher Gary Hevel thinks it may be more like 1 trillion.
Even if it's merely 30 billion, if they were lined up head to tail, they'd reach the moon and back.
"There will be some places where it's wall-to-wall cicadas," says University of Maryland entomologist Mike Raupp.
Strength in numbers is the key to cicada survival: There are so many of them that the birds can't possibly eat them all, and those that are left over are free to multiply, Raupp says.
But why only every 13 or 17 years? Some scientists think they come out in these odd cycles so that predators can't match the timing and be waiting for them in huge numbers. Another theory is that the unusual cycles ensure that different broods don't compete with each other much.
And there's the mystery of just how these bugs know it's been 17 years and time to come out, not 15 or 16 years.
"These guys have evolved several mathematically clever tricks," Raupp says. "These guys are geniuses with little tiny brains."
Past cicada invasions have seen as many as 1.5 million bugs per acre. Of course, most places along the East Coast won't be so swamped, and some places, especially in cities, may see zero, says Chris Simon of the University of Connecticut. For example, Staten Island gets this brood of cicadas, but the rest of New York City and Long Island don't, she says. The cicadas also live beneath the metro areas of Philadelphia, Baltimore and Washington.