WITH ITS ALGAL LAGOONS and moss-dripping trees, Florida is the buckle of the southern mosquito belt. Like much of the South, the state circa 1900 was a wasteland: a bog whose main export was yellow fever. That Miami is now a retirement capital rather than a death zone can be credited to turn-of-the-century scientists who finally made the mosquito–disease connection—and to the nationwide eradication campaigns that followed. Anti-mosquito brigades drained swamps and bulldozed wetlands. Canisters of DDT were handed out like hard candy. By 1950, the lower 48 states were mostly habitable. And there, as far as most of us know, the matter ended.

Except it didn't. The current pleasant state of things, it turns out, is about as

Mosquitoes have been known to kill cattle in Florida by sheer numbers, packing the cows' noses and mouths so densely that they suffocate. "They completely cover your face," Vlach says. "You really can't help but panic."

low-maintenance as the gardens of Versailles. And the prevailing, comparatively blithe attitude toward mosquitoes in America—as barbecue spoilers and canoe-trip pests—relies heavily on the work of an infantry of government employees who spend their days enforcing a kind of entomological Homeland Security: preemptively killing off billions of the bugs before they have a chance to become a threat.

In Boston, researchers hoist caged chickens into the trees as bait for mosquitoes that could be carrying West Nile virus, a scourge that arrived in New York seven years ago and quickly moved across the country. In Texas, roving patrols tip the standing water out of discarded tires. And in the Florida Keys, Andrea Leal and John Snell kill mosquitoes by hand.

"You know it's a bad day when you come back with blood spots all over your clothing," Leal remarks as she winches a government-issue low-draft motorboat into the shallows north of Key West. As the field crew for Florida Keys Mosquito Control District, Leal and Snell spend their days bushwhacking through dozens of unpopulated offshore islands, setting traps and killing larvae to keep infestations from spreading to the mainland. They also conduct counts, a process that consists of pushing up both sleeves and recording how many bugs land on your arms in a minute.

The job is much worse than it sounds. Despite their celebrity reputation, the majority of Florida's Keys are lousy real estate: small, mangrove-knitted islets infested with overgrown spiders and collared by sulfurous mud. Their capacity to produce mosquitoes is legendary. In previous years, crews could expect to get 200 bites a day despite wearing net suits, canvas gloves, and thigh-high rubber waders.

Annett Key, where we land, is no different. On the day I go out with Leal and Snell, the tide is at mid-ebb, forcing Snell to push the boat over sandbars with one leg, like a huge skateboard. On shore, the pair wade through the mud and force a path through the matted jungle of branches, seeking out pools of standing water. When they find one containing mosquito larvae, they open a Ziploc bag of gray pellets—corncob bits impregnated with the insecticide Bti (Bacillus thuringiensis israelensis)—and dribble a few in the water. Then they move on.

Because the salt-marsh mosquitoes here don't carry disease, the fieldwork is not especially dangerous. Still, the heat is oppressive and the footing slick, exacerbated by mangrove roots that grow upward out of the ground like pale, rubbery fingers. By the time we reach the middle of the island, I'm speckled with mosquito bites and sweating through my hat. And this, I'm repeatedly assured, is a good day.

Before Bti treatment began, six years ago, the islands were truly horrendous. Leal's predecessor, Josh Vlach, recalls hanging a trap—a plastic cylinder one foot wide and six inches high—overnight on Little Pine Key and returning the next morning to find it had attracted 464,000 mosquitoes. "When you pulled up in the boat, you would literally hear the island humming," he tells me by phone from his new job in Oregon. "I truly believe they could have killed you."

This is not an exaggeration. Mosquitoes have been known to kill cattle in southern Florida by sheer numbers, packing the cows' noses and mouths so densely that they suffocate. "They completely cover your face," Vlach says. "You really can't help but panic." He describes an instance when an assistant failed to bring his head net and, overwhelmed by the resulting swarm, fled blindly through the brush, smacked into a low-hanging tree branch, and knocked himself senseless.

To amuse himself, Vlach once calculated the number of bites a person would have to get before dying from blood loss. It is 424,242—a number close to what an unclothed, unprotected person might have experienced on a midsummer night on Little Pine six years ago.