Cicada is more of an enigma than a pest

Consider cicadas the bug-a-doon of insects. They lie dormant underground for 17 years and then emerge -- like the mythical Scottish town of Brigadoon -- in a riot of wings and singing. In a few weeks, they're gone.

This year's crop, Brood X, is one of the largest. Trillions of these insects will emerge over nearly one-third of the southeastern and eastern part of the country. The most heavily infested areas could see more than a million-and-a-half bugs per acre.

New England is immune; Brood X prefers Washington, Philadelphia, and Cincinnati to these cooler climes. In those regions, cicadas have begun blanketing backyards over the last few weeks. President Bush was chased by one last week as he headed toward Air Force One.

But they pose little threat to most trees or humans. They don't bite and they don't have the same strip-mining effect as, say, the gypsy moth.

"We call them more of a nuisance pest than anything else," said Michael Schauff, an entomologist with the US Agricultural Research Service.

To scientists, cicadas remain more enigma than pest. Researchers speculate that their quirky 17-year cycle -- which in some broods has been truncated to 13 years -- acts to throw off would-be predators. Although predators can devour some of the swarm, they can never get them all, and because cicadas appear so infrequently, they cannot be the staple of another creature's diet.

Beyond that, however, cicadas keep naturalists guessing. "The more we know, the more we realize we don't know," said Gene Kritsky, a professor of biology at the College of Mount St. Joseph in Cincinnati, who recently published a book titled "Periodical Cicadas: The Plague and the Puzzle."

Seventeen years ago Brood X hatched as nymphs, smaller than a grain of rice. They burrowed deep in the soil, feeding on the root systems of trees as they slowly matured. A few weeks ago as the soil warmed -- Kritsky believes the critical temperature is 64 degrees -- these nymphs climbed out of their subterranean suites. Emerging at dusk, they scrambled to a tree and clung to its trunk, where overnight, they metamorphose into adults.

Within about three weeks of reaching maturity, the females start laying eggs -- between 400 and 600 each. Their mating job done, the adults die. Six weeks later the next generation of Brood X will hatch, head underground, and in another 17 years the cycle will begin anew.

But researchers now wonder whether every member of a 17-year brood does a full term underground. Perhaps, they speculate, the 13-year broods derived from 17-year denizens that departed the dirt early.

Lending fodder to that theory, Kritsky found a number of Brood X denizens among the cicadas of 2000. This year, he said, he's seen some representatives of Brood XIV -- which is due in four years -- jumping the gun, perhaps to avoid a fungal disease.

Theoretically, leaving early could make these renegades more vulnerable to predators. But if they do so in conjunction with another 17-year brood on a different cycle, they'll enjoy the same protection as if they popped up with their original group.

Recently, Chris Simon, a professor of ecology and evolutionary biology at the University of Connecticut, traveled to North Georgia, to capture a moment when both 13-year and 17-year cicadas are hatching together. There are two types of 13-year cicadas, Simon has shown, those that have an identical song, markings, and mitochondrial DNA as their 17-year cousins, and those with distinct features. Now, she wants to know what happens when all these cicadas meet.

"We want to know if these populations are exchanging genes or if one is converting to another," said Simon. "We're documenting the origin of a new species."

Other researchers are more interested in what happens after the cicadas head underground. Keith Clay, a professor of biology at Indiana University, disputes their innocuous reputation. Cicadas, he suspects, have an extensive impact on native forests, draining resources from the tree roots they call home. He's embarking on a study in which he'll prevent cicadas from colonizing the root system of some trees to see whether these fare better than their unprotected neighbors.

It's also possible, he concedes, that the cicadas help trees. When large broods like this year's die, their bodies decompose into natural fertilizer, which may provide a nutrient jolt that will give trees a growth spurt.

"The question is, if all the cicadas disappear tomorrow, would the forest be the same?" he said.

Cicadas are not likely to disappear tomorrow -- or any time soon. In four years, we can expect another large brood. And this one lives in Southeastern Massachusetts.