Cicada Swarm Returns Underground

John Roach
for National Geographic News

August 2, 2004

Brood X is gone from sight, plotting underground for a return in 17 years.

This spring and early summer, billions of black, shrimp-size bugs with transparent wings and beady red eyes carpeted trees from the U.S. eastern seaboard west through Indiana and south to Tennessee. By the middle of July they were gone, not to be heard or seen from again for 17 years.

"I will miss the stars of Brood X (their air acrobatics, those pretty eyes, and friendliness) and hope their little ones make it safely back into the Earth," read a goodbye message posted June 30 by Deborah, from Wayne County, Michigan, on the Cicada Mania Web site message board.

The bugs belong to the largest group, or brood, of periodical cicadas—insects that spend most of their lives as nymphs, burrowed underground and sucking sap from tree roots. They emerge once every 17 years, transform into adults, do the business of reproduction, and then die.

The cacophony of their courtship ritual disturbs suburban tranquility, and their nests can kill young tree branches. Females make slits in the branches and deposit their eggs inside. The process leaves many treetops with brown, dangling limbs flapping in the wind.

In addition to being a nuisance, the mass emergence aerates the soil, provides a feast to thousands of animals, prunes the treetops, and provides a pulse of nutrients into the environment, scientists say.

Broods

There are at least 12 broods of 17-year cicadas, plus another three broods that emerge every 13 years. "A brood is a class-year, like the graduates of 2004," said Gene Kritsky, a biologist and cicada expert at the College of Mount St. Joseph in Cincinnati, Ohio.

A brood emerges almost every year somewhere, sometimes overlapping with others. But none of the emergences matches the pure size of Brood X, which includes three cicada species: Magicicada septendecim, Magicicada cassini, and Magicicada septendecula.

Keith Clay, a biologist at Indiana University in Bloomington is engaged in a long-term study of the Brood X cicadas. He said people's reaction to the 17-year phenomenon runs from disgust to awe.

"Some people leave town and go west, where there are no cicadas. Other people plan camping trips timed in the middle of the outbreak, because they want to experience it in its full intensity," he said.

Seventeen-Year Cycle

The emergence of the cicadas marks the beginning of the last weeks of life for nature's longest-lived insects.

Six to eight weeks after a female adult cicada performs her last, dying act—excavating a nest in a young tree branch and laying her eggs—her eggs hatch and the nymphs fall to the ground.

The cicada nymphs keep heading down, first grubbing on grass roots and then tunneling about 12 inches (30 centimeters) deeper to where they feed on small tree roots for the next 17 years.

"If you dig in the right place, you can find 30 to 50 nymphs in a hole about a foot square [900 square centimeters]," said John Cooley, a cicada expert at the University of Connecticut in Storrs.

After the cicadas have counted 17 years—"we really don't know how they count the years," Kritsky said—they are ready to emerge, which usually happens in late spring when the soil reaches a temperature of about 64 Fahrenheit (18 Celsius).

When twilight of their emergence day hits, the one-inch-long (2.5-centimeter-long) nymphs crawl out of their holes and up just about anything vertical—trees, barbecues, walls, tombstones.

Firmly latched onto the surface of their choice, the nymphs begin their overnight transformation into adults: youthful skin breaks open, milky white cicada emerges, wings flush out, and the body darkens as its outer shell hardens.

This emergence also marks the beginning of a huge feast. "It's well known that pretty much everything starts chowing down on cicadas," Clay said. Dogs, cats, birds, squirrels, deer, raccoons, mice, ants, wasps, and, yes, humans make a meal of the insects.

According to Kritsky, the best time to eat a cicada is just after they break open their youthful skin. "When you eat them when they're soft and mushy, when they come out of their skin, they taste like cold, canned asparagus," he said.

Some scientists believe the mass emergence of the cicadas is part of a survival strategy. With so many of them, they collectively satiate their predators within a few days. Then the billions left uneaten are free to mate.

The business of finding a mate and reproducing is the sole purpose of the cicadas' short existence above the ground. It begins with the males flying to a sunny tree and, with thousands of their buddies, beating out a tune on their undersides.

"It's a high-energy activity, and they, much like a lizard basking in the sun, orient themselves to maximize sun exposure, which maximizes body temperature, which allows them to sing more vigorously and louder," Clay said.

When a male successfully attracts the attention of a nearby female, she will flick her wings as he finishes his song. A courtship dance ensues, with the male continuing to sing up until the physical act of copulation.

Shortly after mating, the male usually keels over and dies. The female buzzes off to excavate nests in a young twig for her 600 or so eggs. Once her egg supply is exhausted, the female dies. Six to eight weeks later, the eggs hatch and the 17-year cycle begins anew.

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