MARATHON, Fla. — Much has changed since 1929 when newcomers to Florida, fed up with swarms of mosquitoes, erected a bat tower in Sugarloaf Key, hoping the bats would succeed where they had failed. Badly outnumbered, the bats abandoned the tower, leaving the monumental task to the locals.
It is hard to blame them for trying. In a state largely covered by swamps and marshland, mosquitoes have long killed, sickened, bedeviled, annoyed and outwitted countless residents and visitors. They are no less potent an adversary today as mosquito fighters contend with warmer temperatures, international shipping and travel, and evolving environmental regulations, which have made it simpler for mosquitoes to hatch and travel but trickier to eliminate them.
In July, an outbreak of dengue fever, once thought to have been eradicated in Florida, occurred in Martin County, just north of Palm Beach on the east coast. Twenty people developed the disease, which causes flulike symptoms and extreme aches.
Mosquito control officials are worried that dengue has established a foothold in Florida. The last outbreak — and the first in the state in about 70 years — was in 2009 and 2010, when dengue fever hit Key West.
The problem appears to be contained for now, in large part because the area is small and Florida is well seasoned in the art of mosquito warfare.
"Florida is certainly among the leaders in the world in professional mosquito control," said Walter Tabachnick, a professor at the University of Florida's medical entomology laboratory. "Prior to that, this was not a very nice place to live in."
Imbued with Darwinian strength, the Aedes aegypti, commonly known as the Dengue mosquito because it carries the disease, is a survivalist that breeds and lives in towns and cities. It can encamp inside a bottle cap, under a house or in any container holding water, making it difficult to find and kill.
Fighting them is akin to urban warfare: armed with spray in hand-held devices, mosquito fighters go yard to yard and house to house. They warn residents to remove any containers that can fill with water or tip them over after a rainfall.
"They are sneaky devils," said Gene Lemire, director of mosquito control in Martin County, referring to the insect enemy. "They hide in people's houses, in the dark crevices and in premises where the spray doesn't get to them."
County health officials also are visiting homes to draw blood from people, with their consent, to test for dengue.
Few approach the job of fighting with the innovation of the Florida Keys, which host 45 species.
With a $9.7 million budget, money that comes from a special tax, the Mosquito Control District here relies on two planes, four helicopters and nearly 100 employees to spot mosquitoes and their hard-to-see larvae, track their movements and kill them. Most common are salt marsh mosquitoes and the Aedes aegypti.
The district is considering buying a small drone that can use infrared technology to see hidden pools of water amid tangles of mangroves and sea grass on hard-to-access islands. The drones, about 2 feet long, have never been used for mosquito control.
But a dengue outbreak is unsettling in a state where tourists expect a high degree of comfort. In Key West, where mosquito experts said local officials were slow to react initially, the 2009 appearance of dengue, which is widespread in Puerto Rico, caused some would-be visitors to stay away.
Ultimately, mosquito control employees swung into action, knocking on doors and using both adulticide (to kill adult mosquitoes) and larvicide (for larvae). Mosquito fighters still go door to door in the lower Keys.
Long gone are the days when huge amounts of the insecticide DDT were dropped over wide areas. The use of DDT was banned, with few exceptions, in 1972.
Fighting mosquitoes is particularly challenging in the Keys, and elsewhere in Florida, where large swaths of land are protected by federal and state environmental regulations. Stricter laws mean officials must formulate ecologically friendly pesticides that will not harm endangered species, while keeping pace with bugs that become resistant to certain formulas.
"The noose is tightening on our ability to control adult mosquitoes," said Michael Doyle, the Florida Keys Mosquito Control District's executive director.
So the Keys have shifted mostly to killing larvae whenever possible. Most recently, officials have used a pesticide mist that unleashes minuscule droplets that can more easily float into small containers to kill the larvae of the dengue-causing mosquitoes. New technology allows pilots, who swoop and rise in the air with stomach-churning precision, to more precisely calibrate drops of pesticide according to wind speed and drift.
Some say the best hope in fighting dengue lies in a trial that, if approved by the Food and Drug Administration, could unleash genetically modified sterile male mosquitoes whose offspring would die soon after hatching.
Trials in other countries, including Brazil, have shown sharp reductions in the Aedes aegypti population, according to Oxitec, a British insect control company that is engineering the mosquito. But vocal opposition in the Keys may block the trial, if approved.
Boots on the ground, though, remain essential. Inspectors routinely crisscross mucky mangroves on tiny, isolated islands. In high boots and long sleeves, they scoop up water and look for larvae.
"I absolutely love the swamp," said Yvonne Wielhouwer, an inspector. And then she proves it as she gleefully sloshes across Little Pine Island, keeping a lookout for mosquitoes.
Sometimes they must battle a different species: the scolding Keys resident who readily complains if just a few mosquitoes buzz close to home.
"Someone will be sitting on their porch, and a pilot flies by and kills five billion mosquitoes in the marsh next to them, and they didn't know the mosquitoes were there," Mr. Doyle said. "The next night, they see five mosquitoes on the porch and call in to say, 'What are you going to do about it?' Expectations are much higher today."
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