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 Across the Biscayne Bay in Miami Beach, wind and high-rise buildings
make aerial spraying challenging. So, the effort in the popular
tourist destination has focused on ground-sprayed pyrethroids -
pesticides that are safer but don't always work.
The arrival in Florida of Zika, a virus that can cause a crippling
birth defect known as microcephaly, has drawn into focus the
limitations of the U.S. mosquito control arsenal.
Larvicides reduce future populations relatively safely. But for use
against the mature mosquitoes that spread disease, only two classes
of pesticides are approved. Each has drawbacks.
Organophosphates, such as naled, are effective. But there are strict
controls to limit risk. Pyrethroids are safer but have been used so
much that mosquitoes, in many places, are immune.
"That's really the weak link in much of the United States," said
Michael Doyle, director of the Florida Keys Mosquito Control
District. "We're kind of caught off guard."
DENGUE PREVIEW
Doyle led a 2009 effort against a dengue outbreak in South Florida,
the first in the United States in nearly a century. Authorities
threw everything they had at the Aedes aegypti, the same mosquito
that carries Zika: backpack fogging, door-to-door yard inspections
looking for watery breeding sites and larvicide spraying.
Still, 88 people were infected before the virus was brought under
control more than two years later, and there continue to be sporadic
cases in Florida.
The outbreak highlighted gaps in the mosquito control arsenal that
remain, according to pesticide makers, abatement officials and
entomologists. Few companies make pesticides for use in public
health outbreaks, a niche market that is expensive to get into, has
a limited upside and varies season to season.
Safety testing a new pesticide can cost up to $250 million and take
10 years, said Karen Larson, vice president of regulatory affairs at
privately held Clarke Mosquito.
As long as a product remains on the market, companies must continue
testing for unforeseen side effects, an expense that some makers
have blamed for decisions to abandon products.
"There's not a lot of profit," Larson said.
Sales of the Dibrome brand of naled have been estimated at $12
million a year. By comparison, total crop pesticide sales for some
companies can exceed $500 million in a single quarter.
Bayer, Dow Chemical, BASF and other agricultural pesticide makers
"are not interested in going after a $20 million or $30 million a
year market," said William A. Kuser, investor relations director at
Dibrome maker American Vanguard Corp.
The U.S. Environmental Protection Agency has approved several new
pesticides in recent years. But it has received few requests for
using them against mosquitoes, said Jim Jones, Assistant
Administrator for the agency's Office of Chemical Safety and
Pollution Prevention.
"Although it's of critical importance, the amount one can sell is
small and it's variable, which makes it difficult for business
planning," Jones said. "You can go many years without having much of
a market at all, then suddenly, whether it's because of a nuisance
outbreak of mosquitoes or something like West Nile or Zika, the
market grows significantly."
Abatement authorities have pressed for help with the cost of
developing mosquito control pesticides. The 1996 U.S. Food Quality
Protection Act includes a provision for subsidies to defray the
expense of safety testing, but Congress has never funded it.
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RISK AND RESISTANCE
At least 49 cases of locally transmitted Zika infections have been
reported in Florida, most in Wynwood and Miami Beach. Most people
have no symptoms or mild illness.
Because of the microcephaly link, efforts are focused on preventing
infection among pregnant women.
In Wynwood, the campaign began with pyrethroids, synthetic versions
of a chemical derived from chrysanthemums. Amid signs of resistance,
authorities switched to naled.
Developed as nerve agents, organophosphates, at high doses, can
cause nausea, convulsions and death. They can be toxic to wildlife,
including bees. The EPA considers naled safe at permitted ultra-low
concentrations, and it is sprayed annually over 16 million acres in
the United States.
But it is banned in Europe, where the risk is seen as unacceptable.
In the U.S. territory of Puerto Rico, where Zika is widespread, the
governor prohibited naled amid protests over safety concerns.
Although naled killed more than 90 percent of mosquitoes in traps
set in Wynwood, the Aedes aegypti's resilience remains a concern.
"This is truly the cockroach of mosquitoes," said Tom Frieden,
director of the U.S. Centers for Disease Control and Prevention.
DROPPING PESTICIDES
CDC entomologist Janet McAllister said pyrethroid resistance
typically is limited by the mosquito's small range. When resistance
to one pyrethroid develops, another often works.
Still, she said, "we would love to see additional classes of
insecticides available because, even in places that may have an
effective tool today, that doesn't mean it is going to last down the
road."
The EPA can fast-track its evaluation of new pesticides and expand
the use of old ones. In response to Zika, it expedited new uses for
pesticide-treated bed nets and mosquito traps.
Still, development of pesticides is painstaking. Even if the EPA
speeds up its evaluation, required safety data can take years to
collect. And the expense of ongoing safety testing has prompted
companies to drop products.
Bayer CropScience, for example, told distributors it dropped the
pyrethroid resmethrin in 2012, rather than do additional testing.
Clarke Mosquito gave up temephos, a larvicide, six years ago,
because of costs, Larson said.
That decision led to stockpiling in southwest Florida, said Wayne
Gale, director of the Lee County Mosquito Control District.
"We purchased just about every bit," he said.
(Reporting by Julie Steenhuysen; Editing by Michele Gershberg and
Lisa Girion)
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