KEY HAVEN, Fla. — In this bite-size community near Key West, like so many other mosquito-plagued spots up and down the Florida Keys, residents long ago made peace with insecticides dropped into town by planes or rumbling by on trucks. Cans of Off are offered at outdoor parties. Patio screens are greeted with relief.

But Keys residents are far less enamored of another approach to mosquito control — a proposal to release the nation’s first genetically modified mosquitoes, hatched in a lab and pumped with synthetic DNA to try to combat two painful mosquito-borne viral diseases, dengue and chikungunya.

If the federal Food and Drug Administration gives the go-ahead for the trial, Key Haven, with 444 houses built on a tiny peninsula, would become the focal point of the first American release of several million mosquitoes genetically altered by Oxitec, a British biotechnology company.

For denizens of a chain of islands notorious for their renegade spirit — Key West once jokingly broke away from the United States as the Conch Republic — this possibility is fraught with suspicion and indignation.

“This is the first time they are releasing genetically modified mosquitoes in the country, and we have not given our consent,” said Mila de Mier, a Key West resident and real estate agent who helped spearhead a four-year campaign to block the trial until more research is conducted. “People can’t be experimented on without their consent. When the mosquitoes are released, there is no way to recall it.”

The Food and Drug Administration, which is still reviewing the Oxitec application, must approve the field release. But the proposal has set off a chain reaction of anxiety and protest that began in 2011 and has gathered steam as the agency’s decision approaches.

In 2012, the Key West City Commission passed a resolution objecting to the release of several million genetically modified mosquitoes there. But Key Haven is about a mile away in unincorporated Monroe County.

Opponents continue to push back hard in gatherings and town hall meetings, peppering scientists with questions. An online petition by Ms. de Mier to stop the release of the mosquitoes has drawn more than 149,000 signatures. To keep the campaign going, irate residents recently sent 1,600 emails to the Florida Keys Mosquito Control District, run by an independently elected commission.

“We feel it’s being jammed down our throats, and we are not getting answers,” said Beth Eliot, a Key Haven real estate agent who said no one she knew in the neighborhood supported the project. “The company that is saying that this is all safe is the company that stands to profit.”

Oxitec has made significant progress toward securing permission for the trial: Late last year, it won approval from several federal agencies to import mosquito eggs and build a lab for inspection. In the lab, scientists plan to inject the eggs with synthetic DNA, rear the mosquitoes and release them in Key Haven, once the field trial is permitted. The lab is in the Marathon, Fla., office of the mosquito control district, which is working with Oxitec on the project.