If you feel yourself besieged in the coming days, with cicadas whirring overhead, crunching underfoot and shrilling from the treetops, you may have yourself to blame, at least in part.

A nice house in a subdivision that features a few trees here and there turns out to be a little slice of heaven for periodical cicadas as well as humans, according to biologists and geographers who study the insects.

Cicadas began crawling out of the earth this week throughout the Washington area and several eastern states, the onset of a once-in-17-years infestation that will last until the middle of next month.

For most people, the sudden appearance of hordes of black bugs with red eyes is a source of disgust, fear or wonder. For scientists, said Indiana University biologist Keith Clay, "it's a rare opportunity to examine the consequences of a large-scale biological phenomenon."

Scientists are unsure exactly how many of the bugs will emerge in the coming weeks, but most extrapolations are based on research showing that as many as 1.5 million cicadas can infest a single acre. At that rate, said evolutionary biologist Christine Simon of the University of Connecticut, there could be "trillions" overall in the brood of cicadas now leaving their underground homes in 15 states and the District.

Cicadas are dependent on trees, so it has been good news for the insects that there is more forest in the eastern United States now than there has been in 100 years. They particularly favor the edges of forests, where they can more easily bask in the sunshine necessary to heat their bodies.

That is why sprawl, which converts generally treeless farmland into housing developments with the occasional stand of trees, is a blessing for cicadas.

"Now we are creating a forest with a lot of edges, a lot of patches -- that makes a very favorable habitat for cicadas," said Indiana University geography professor John Odland.

To demonstrate that cicadas are a source of enlightenment as well as irritation, the National Science Foundation yesterday assembled four scientists whose cicada-related research it is funding. Simon described how her research into the genetics and behavior of two periodical cicada species led to the discovery of a third and has enhanced understanding of how species develop.

Indiana University's Clay said further research into cicadas may help scientists learn more about outbreaks of other insects, such as those that consume plants voraciously or cause disease in humans. Cicadas seem fairly benign during their emergence -- they do not sting, bite or swarm malevolently, and they do not eat much -- but much is not yet known. "Arguably, their effect is bigger" than that of other invading insects, said Clay, "but more subtle and harder to detect."

Female cicadas deposit their eggs in the thin branches of trees, and they can kill small trees or mangle limbs in the process. James H. Speer, an Indiana State University geographer, has discovered how cicadas affect trees during the 17 years they spend underground. During their long juvenile periods, they survive by sucking on watery sap from the roots of trees.

He measures the cicadas' impact by scrutinizing tree rings, which record trees' patterns of growth. The red maples he has studied seem to have a boom year when the cicadas leave the roots alone and come above ground to mate.

"They're not killing off the trees; they're causing a reduction in growth," he said. In this emergence, he plans to broaden his research to include several species of trees and to look further back in time to determine the long-term impact of cicadas.

Clay has determined that female cicadas, when they look around for a place to lay their eggs, seem to favor elm, sweet gum and autumn olive trees over other species. Persimmons, alders and cottonwoods are their least favorites. He is continuing his research into the impact of cicada emergence, in part netting over some areas to prevent the insects from mating and laying eggs.

A cicada leaves its shell in Chevy Chase. Research shows that as many as 1.5 million cicadas can infest a single acre.Cicada shells lie at the base of a tree. The infestation is a chance to study the effects "of a large-scale biological phenomenon," said biologist Keith Clay.