A grimacing Helen Geiger stands amid bikes and beach chairs in the garage of her Hawaiian home watching two men tramp through dense foliage in her backyard. Dragging a hose from a 100-gallon tank, they are seeking a tiny prey, invisible in the misty night but unmistakably present, its shrill, two-note ko-KEE mating call filling the air with earsplitting dissonance.

Unable to see the tiny frogs, no bigger than a quarter, the men follow their ears, spraying a mixture of citric acid and water over the ferns, Ohia trees and strawberry guava, killing many of the frogs on contact. "I'm so appreciative," she tells the men.

"In the daytime," she goes on, "it's beautiful and peaceful here in the rain forest." But nights are a different story. By the thousands, screeching coqui frogs have infested the trees all around her property, including those outside her upstairs bedroom window. Their nightly serenade has become so irritating that she now sleeps on the first floor with her windows shut—no way to live, she says, "if you like fresh air."

Geiger's wooded neighborhood, like scores of communities across Hawaii's Big Island, has been hit by rampaging Eleutherodactylus coqui. Biologists theorize that these tiny natives of Puerto Rico stowed away a dozen or so years ago on nursery plants en route to Hawaii. The Pacific island chain's moist, verdant environment offered the invaders an ideal habitat. With few natural predators, such as snakes and tarantulas, to keep their populations in check, the mottled yellow to grayish brown critters—almost impossible to spot on most vegetation—spread from the Big Island and Maui to Kauai and Oahu.

During the day, the coqui makes its nest in moist areas covered by leaves and leaf litter. Unlike the offspring of many other frog species, coqui froglets—about the size of a grain of rice when they hatch—emerge fully formed from eggs, skipping the tadpole stage entirely. (It's a tadpole's need for a pool of water that limits the spread of most frog species.) "We're dealing with a highly successful generalist that's capable of doing well in a variety of habitats and circumstances," says Lawrence Woolbright, a coqui researcher at New York's Siena College. "This frog is right up there with rats, cockroaches, pigeons and humans in terms of its adaptability."

With females producing an average of 28 eggs four times a year, densities of adult frogs in some Hawaiian locations have jumped to more than 6,000 an acre, some five times what they are back home in Puerto Rico, where they are eaten by spiders and snakes. The frogs begin making their fearsome racket when they are about 8 months old.

The screeching call is made by males, which contract their bodies, squeezing air out of their lungs and into their vocal sacs. Females seem to respond to both the frequency and volume of the call, which can reach from 70 to 90 decibels, comparable to a vacuum cleaner or a garbage disposal. (The federal government calls for workplace ear protection when noise reaches 85 decibels.)

"It is mind boggling if you go to an area that is infested. The noise factor is unbelievable," says Harry Kim, mayor of the Big Island. "And just imagine what the frogs are doing to the entire ecosystem." Researchers fear the coqui might severely reduce, or even wipe out, certain insects on which some rare birds rely. A thriving frog population could also mean a free lunch for other invasive species, such as rats and snakes.

Not everyone minds the little frogs: in Puerto Rico, many people find them endearing; they appear on T-shirts and license plates. In Hawaii, coqui fans include Sydney Singer, founder of the Coqui Hawaiian Integration and Reeducation Project, who sees efforts to eradicate them as cruel and unnecessary. "This place is full of aliens, and there's a big niche for them," he says. "If people can't live with the sound of nature, they should move."

But for many islanders, the coquis are beyond annoying. One nursery owner, who declined to give his name, says he nearly lost his business, despite spraying, due to frogs on his plants. Jamie Runnells, vice president of the Big Island Association of Nurserymen, says that the coqui is "the biggest single problem facing the nursery industry on the Big Island," a nearly $50 million business.

The frogs have joined termites and volcano zones as possible deal breakers in property sales. Broker Dave Lucas lost one sale when potential buyers made a sunset inspection of property on which they'd put a contract. "They sent in a cancellation the next day," says Lucas. "They said there's no way we're living by these frogs." The Hawaii Island Board of Realtors is urging its clients to spray their properties.

State authorities continue to search for eradication tools. Caffeine spray, which induces a heart attack in coquis, seemed promising until it ran up against animal rights activists and federal pesticide regulations. Even the most successful weapon, the food additive citric acid, can harm flowers. And it is far from foolproof against frogs that take cover in dense underbrush. "It's a pretty major undertaking to wipe out a population with spray," Woolbright says. "You would have to do a pretty thorough job of saturating everything from the canopy to the forest floor, and then you'd still miss the one that went to bed early that night."

The cash-strapped state has had little money to earmark for the problem, focusing instead on public education programs and inspection of nursery shipments between the islands to prevent further invasion. Earl Campbell, the Pacific region's U.S. Fish and Wildlife Service invasive species coordinator, wrote in 2002 (along with biologist Fred Kraus) that control efforts "have been hampered by limited authorities and funds, disbelief in the threat and the reluctance to act." Still, Campbell believes that a high-level Hawaii Invasive Species Council, formed this year, "can prevent the next coqui."

As for the coqui plague itself, state officials believe that aggressive spraying and clearing of underbrush can prevent their spread throughout most of the Hawaiian archipelago. But experts tend to talk in terms of containment rather than eradication on the Big Island. Which means that Helen Geiger and her neighbors may have to invest in some good earplugs.