Parts of the southern United States are currently experiencing the emergence, after 13 years underground, of the nation's largest brood of periodical cicadas. These at times annoying but generally harmless insects are emerging in a patchwork across the South, and eventually will reach as far North as Illinois.
Periodical cicadas prefer areas where there is open, vegetated ground with young trees scattered around -- in effect suburbia, says Keith Clay, a professor of biology at Indiana University-Bloomington who studies cicadas, especially Brood X.
"Probably the best habitats are these young, early successional forests where the trees are starting to fill in," he says. In mature forests scientists have measured between eight to 20 cicadas emerging up from the ground per square meter but in a suburb or younger forest it can be 100 or 200, he says.
And about that somewhat confusing nomenclature...
Charles Lester Marlatt of the U.S. Dept. of Agriculture's Bureau of Entomology came up with the system in the late 1890s. There had been much confusion between annual cicadas, 13-year cicadas and 17-year cicadas prior to that. People knew cicadas showed up pretty often, but not how one year's brood was or was not related to previous years.
In a classic paper published in 1907, Marlatt described the three types of cicadas and proposed a grouping of periodical cicadas. He gave each a Roman numeral. The 17-year cicadas were named Broods I though XVII (1-17) and the 13-year cicadas were named Broods XVII - XXX (17-30.)
Later research showed that there weren't actually 30 broods, but only 15 - 12 17-year broods and three 13-year broods.
This means that most years there's a brood hatching somewhere. Greg Hoover an entomologist at Pennsylvania State University, says there were none in 2009 and 2010, which means the arrival of this year's Brood XIX "could kind of come as a surprise to people."
All in all, periodical cicadas are " kind of refreshing," says Indiana's Clay. "We live in this highly civilized world but when something like that happens, you realize we don't control everything."
For those who'd like to see more about cicadas, the web site Cicada Mania keeps a list of sighting, information and even a wedding planner if you want to avoid (or encounter) cicadas during your outdoor wedding.