ATLANTA (Reuters) - Traveling through a rural part of the U.S. state of Georgia recently, Charles Seabrook heard a high-pitched whirring so loud he thought the engine of his pickup truck was overheating.
"I was getting ready to raise my hood when I realized that I was hearing the 13-year cicadas," said Seabrook, a Georgia writer and naturalist.
Throughout the U.S. South and as far north as Illinois and Indiana, a noisy and bizarre insect ritual is playing out for the first time since 1998. After living quietly underground for 13 years, billions of red-eyed cicadas -- dubbed the "Great Southern Brood" by scientists -- are emerging to mate and quickly die.
"The most common description I've heard is that it's an alien invasion," said Nancy Hinkle, a University of Georgia professor of entomology. "It sounds like the mother ship is hovering down in the woods."
The insects are called "periodical" cicadas because they remain underground for years at a time, unlike the annual cicadas that surface each summer. There are also 17-year cicadas found largely in the Northeast and Midwest, Hinkle said.
"The periodical cicadas are about 30 percent smaller than the annual cicadas," said Hinkle. "And periodical cicadas have bright red eyes."
Commonly mistaken for locusts, they don't bite and aren't harmful to humans or crops.
The cicadas are not dormant during their long life underground. "They are actively growing," Hinkle said. "The little nymphs are down in the ground, they've got their mouth parts attached to tree roots and they're sucking the juice out of tree roots."
Mysteriously, when year 13 arrives, the nymphs burrow through the soil to the surface to become adults. They shed a layer of skin, leaving a shell behind. Then they inflate and dry their wings, allowing them to fly.
The roar begins as males attract females by furiously vibrating membranes in their abdomens, producing a loud drone.
"It is one of nature's great oddities," said Seabrook.
There are several theories behind the cicadas' strange and lengthy life cycle.
One is that it is nature's "shock and awe" approach to produce an overwhelming number of cicadas at one time so that predators can't possibly eat them all.
Many animals love to munch on cicadas, including turkeys, raccoons, skunks and coyotes.
"They make tremendous dog toys," Hinkle said. "Dogs love to play with cicadas because they buzz. People let their dogs play with them just for the entertainment value."
But the fun, the feast and the noise will soon be over. After mating, females lay eggs on tree branches and, within a week or two, most of the adults die or get eaten. Little cicadas hatch, fall to the ground and burrow into the soil.
"We won't see them again until 2024," said Hinkle.
Editing by Colleen Jenkins and Greg McCune