In the mosquito-control business, the primary measure of how things are going is called the “landing rate count.’’
It might also be called the slap test, because it entails standing in a spot for one minute and counting bites, like this: One. Slap! Two. Slap! And so on.
Judging by slap rates over the last month, South Florida could be facing another mean mosquito season. An early, wet start to the rainy season has triggered an unusually early explosion of buzzing hordes — and calls for help from bugged residents.
“This year it’s been extensive, intensive spraying since May,’’ said Chalmers Vasquez, mosquito control operations manager for Miami-Dade County. “I don’t even remember the last year we started spraying in May.’’
Sign Up and Save
Get six months of free digital access to the Miami Herald
Drier weather over the last few weeks has helped, but when the heavy rains come again mosquito control agencies across South Florida are concerned about rising public health risks from illnesses spread by some species, including Dengue fever, West Nile virus and malaria. No cases have been reported in South Florida this year — but the mosquito season doesn’t typically peak until sometime in July.
Counties combat mosquitoes with chemicals that carry their own risks, particularly to the environment. The insecticides sprayed from trucks and planes — common brand names include Bi-Mist 30+30 and Dibrom — typically contain “broad spectrum” pesticides such as naled or permethrin.
They can irritate the eyes or skin of people exposed to low concentrations in fogging treatments, but they’re potentially toxic to marine and aquatic life. Studies have show they can have ripple effects across the marine food web, killing fish and conch larvae, but also affect desirable insects such as endangered butterflies that live in coastal hammocks.
Those residual impacts make controlling mosquitoes in South Florida extremely challenging. The island chain of the Florida Keys is surrounded by mangrove islands, fragile coral reefs and seagrass beds in federally protected waters. Miami-Dade is bordered by Everglades National Park and Biscayne National, both of which ban aerial and truck spraying, as do many state parks.
“It’s a balance that we need to walk of environmental protection and protecting people from mosquitoes,’’ said Michael Doyle, director of the Florida Keys Mosquito Control District.
Because of the restrictions on chemical spraying near water, for instance, the Keys’ control program employs a lot of larvicide — bacteria that selectively target the developing offspring of the region’s most common blood-sucker, the black salt marsh mosquito.
Mosquito control operations take steps to limit human exposure as well, scheduling spraying in pre-dawn or night hours that maximize the impact on bugs and minimize it to people.
While salt marsh mosquitoes are largely an annoyance, other species can be more dangerous, passing on a host of illnesses, including dengue, a nasty, flu-like illness that sickens some 50 million people annually. In severe cases, it can be fatal and there is no known treatment.
Dengue, carried by a mosquito called √Aedes aegypti, reemerged in Key West in 2009 with 27 cases, the first recorded in the continental United States since 1945. Another 66 cases were confirmed last year. Since 2010, Miami-Dade, Broward and Palm Beach counties also have reported a handful of cases.
The threat is significant enough that the Keys district is hoping to test a genetically modified mosquito designed to breed with and kill aegypti, a controversial treatment option that has not yet received state and federal approval.
So far this year, flocks of “sentinel chickens’’ monitored by the Florida Department of Health have shown no early indicators of mosquito-borne viruses.
But the risk of an outbreak dictates that agencies stay ahead of the swarms with aerial spraying and trucks that target concentrated hot spots, said Joseph Marhefka, manager of Broward County’s mosquito control section.
“The best protection from mosquito-transmitted diseases is avoiding exposure to mosquito bites,’’ he said.
The weather is what largely shapes South Florida’s annual mosquito season - both where and when skeeters strike.
“We’re constantly watching it,’’ said Vasquez. “We’re playing with two things here, rainfall and the direction of the winds.’’
Rains hatch dormant eggs laid in moist soil and just about anything that holds water, from planters to bottle caps. One day, winds can blow hungry salt marsh mosquitoes from far-off locations, like Flamingo in Everglades National Park or from Elliott Key in Biscayne National, into suburbs. They next day, winds can shift and blow them back out. That makes border areas like the Redland hotspots in most seasons, said Vasquez.
While there have been pockets of heavy infestation, the mosquito season so far is notable mainly for its early start. Judging by landing rates tests, traps and resident complaints, the numbers are up a bit in some spots, down in others.
Miami-Dade fielded about 300 calls last week for treatment. Last July, Vasquez said, there were 6,000 calls.
In the Keys, the hot spot ran from Key Largo down to Upper Matecumbe, where the landing rate ran 7.69 mosquitoes, Doyle said. Last year, the highs ran around 20, he said.
The Keys district, with an annual operating budget of $11 million, spent more on control than any other county in the state. Miami-Dade, in contrast, spends about $1.5 million. Much of that goes into year-round larvicide treatments that dramatically knock back the adult population in inhabited areas, Doyle said.
The Keys routinely run tests at 250 “landing rate” sites from Key West to Key Largo, with employees selecting 15 to 20 sites to visit every morning. At protected areas where spraying is banned, landing rates in a minute might run into the hundreds, he said. Traps in places like Everglades National Park can produce a pound of mosquitoes overnight, hundreds of thousands or more.
“Our efforts suppress them by the billions,’’ he said. “The ones we’re seeing are the ones that got away.’’
Whatever the landing rates tests indicate, there are plenty of anecdotal reports of a heavy mosquito crop this year.
Shelby Moneysmith, a biologist at Biscayne National, said swarms have chased her while jogging near her home in Homestead’s Keys Gate community.
“It makes you run a lot faster,’’ she said.