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.