When the Zika virus struck last year, Miami-Dade County Mosquito Control immediately began fogging with three pesticides: BTI, a group of bacteria that kills mosquito larvae; naled, a controversial chemical compound banned in Europe over links to developmental disorders in children; and permethrin, the active ingredient in home bug-killers such as Raid. Permethrin was sprayed at least seven times in Wynwood and five times in Miami Beach, but by the end of August, the county realized the poison had little effect and stopped using it.
The county could have learned that lesson much earlier. Documents obtained by New Times
show that officials didn't get around to testing which pesticides kill Aedes aegypti
until October — midway through Zika season and about a month after the county spent an extra $20,000 on permethrin. Once they finally peformed tests, the results were unambiguous: Permethrin is nearly useless against Zika-carrying mosquitoes.
Three county mosquito-control employees each denied in a phone interview that Miami-Dade should have conducted tests sooner to figure out which pesticides worked. There simply wasn't time, they insisted.
"We had an emergency declaration from the state, and when we have that, we need to respond in the community quickly," Deputy Director Paul Mauriello said. "We did it when we felt it was appropriate."
But Gary Rand, a Florida International University toxicologist who worked in the pesticide industry and studies pesticides' environmental impact, says any mosquito-control district should make slow, informed decisions before spraying chemicals over a public area.
"This is not candy," he said. "It’s a toxic agent."
Though permethrin is a commonly used pesticide, concerns remain about its impact on the environment. A Penn State study last week linked chemicals such as permethrin to childhood autism
. It appears county residents and local wildlife were exposed to a toxic agent with little actual benefit.
Now county officials say Miami's Zika-carrying mosquitoes have become largely resistant to the family of pesticides permethrin belongs to, called pyrethroids. The county tells New Times
it's ceasing use of pyrethroids to fight Zika mosquitoes and is switching to an old standby in the insect-control trade: malathion, an organophosphate pesticide from the same family as naled.
But that October test also warned that Miami's Aedes aegypti
were also "developing resistance" to malathion, raising concerns about how quickly that chemical will remain useful in the fight against tropical disease.
The issue highlights an ongoing feud between mosquito-control experts, who say mass-level spraying is necessary to combat disease, and environmentalists and biologists, who accuse those experts of being too careless with toxic chemicals long known to be dangerous to humans and animals. In response to public criticism last year, Miami-Dade Mosquito Control has begun preemptively spraying BTI in high doses across Miami-Dade to kill Aedes aegypti
mosquitoes before they start to fly.
County officials says they're also conducting insecticide-efficacy tests at regular intervals to avoid a situation like last year's, which wasted time and money on permethrin.
During an interview with the Atlantic conducted in Miami last year, CDC Director Tom Frieden warned that, until a Zika vaccine is produced, cities will be forced to play whack-a-mole with various insecticides as Zika becomes "endemic" to Florida.
"That situation is what everybody across the country is confronting," county Mosquito Control Operations Manager Chalmers Vasquez said. "It’s pretty much just a handful of pesticides we can choose from."
A quick, cheap, and simple test called a bottle bioassay shows entomologists and mosquito-control experts which populations are resistant to certain chemicals. Though the results of those tests aren't definitive, a simple bottle test could have pushed county officials toward faster-acting pesticides during a public-health crisis.
These testing decisions have real-life consequences: Had more effective pesticides been used from the get-go, the public could have been exposed to fewer infected insects, as well as fewer doses of pesticides, which are also demonstrably harmful to humans. The final two "active transmission zones" for the virus, in Little River and Miami Beach, were lifted December 2 and December 9, respectively.
There are specific health concerns when it comes to permethrin. A study released last week shows that autism rates are far higher in cities where permethrin is sprayed aerially
. Pesticides are also highly imprecise tools to kill insects: Permethrin, for example, also kills bees and butterflies, which pollinate plants, and can be harmful to fish and some species of wildlife, including cats.
During a bottle bioassay, 12 mosquitoes are placed inside a test tube lined with a certain pesticide, and researchers wait to see how long it takes for the insects to die. According to state and local records, as well as conversations with county mosquito-control experts and one source with the CDC, the state health department ordered a bottle test only in October 2016, three months after active transmission was identified in Miami-Dade.
Nothing prevented the county from ordering any of those tests itself as travel-related cases began to swell in Miami-Dade as early as February 2016. Last year, the Miami Herald
outlined multiple, proactive steps the county could have taken as it became apparent Zika was set to hit mainland Florida
: Conducting pesticide-efficacy tests could have been one of those steps. Instead, the county waited for direction from the Department of Health and the CDC.
According to the bottle bioassay results, 100 percent of the mosquitoes tested from the Wynwood area in Miami-Dade were fully resistant to permethrin.
Further tests conducted by CDC expert Janet McAllister, who was brought in last August to help fight the outbreak, were also only conducted in October. In those tests, caged mosquitoes were sprayed with varying pesticides. But the results showed that permethrin killed only Aedes
mosquitoes at much higher spray-concentration rates than normal. And the tests also showed that Miami's Aedes
population was becoming resistant to the entire family of pyrethroids.
"However, by the time field testing occurred, Mosquito Control had already stopped performing treatments with those pesticides," Gayle Love, a spokesperson for Miami-Dade Mosquito Control, said via email. "In summary, resistance to pyrethroid insecticides seems to be widespread in Florida and many other areas."
According to documents the county provided, Miami-Dade spent $164,000 on pyrethroid pesticides in 2016 and 2017. That money isn't entirely wasted: Pyrethroids remain useful for black salt marsh mosquitoes, a nuisance insect that breeds wildly across Miami but does not carry Zika.
The county says it's still sending spraying trucks and employees with backpack foggers armed with pyrethroids, including the chemicals deltamethrin, duet, zenivex, and permethrin. The county says it's largely spraying on an as-needed basis when residents complain about mosquitoes. In response to getting caught flat-footed last year, the county is spraying the relatively safe bacterial bug-killer BTI across town preemptively.
Rand, the toxicologist, says permethrin isn't quite as outwardly dangerous to humans as organophosphates such as naled but lasts longer in the environment before breaking down.
"There's more of a potential for residual carryover," Rand says. "If you're applying it on a Monday and need to apply it again on Friday, there's a strong possibility that residual concentrations will still be there the second day you apply it." That means wildlife, fish, and humans could be easily exposed to the pesticide days after its application. Naled, in comparison, is potentially more dangerous to humans but breaks down faster in the environment.
Rand says that permethrin does not seem to pose the same sorts of health risks to humans as naled but that the recent study linking increased permethrin use to higher autism rates worried him.
Another opponent of aerial spraying, South Miami Mayor and Florida International University biologist Philip Stoddard says he's more hopeful that if Zika returns to Miami this year, the county will have a smarter and safer game plan to combat the insects. Last year, Miami-Dade Mosquito Control did not employ a credentialed scientist. This year, the county is bringing in a research entomologist.
"I expect to see things run more smoothly once they have their scientist on staff," Stoddard says. "If there’s a lesson learned from this, they've learned it: You need to have in-house, scientific expertise. A scientist is going to want to know this stuff." In South Miami, Stoddard has sponsored a test run to see whether wolbachia, a bacterium, could work to kill Aedes
In the meantime, Vasquez and the county say they'll reintroduce malathion this year to keep Aedes
levels down. But malathion is perhaps more dangerous to humans than permethrin: Repeated studies have shown that Florida farmers regularly exposed to organophosphates developed motor-skill issues and vision problems.
Many environmentalists for decades have been aligned against malathion use: The pesticide was criticized often in the seminal 1962 book Silent Spring,
which birthed the green movement and helped spur the creation the Environmental Protection Agency.
"These are nerve agents," Rand says. "If they affect the nerves of an insect, why would we think that they don't also affect the nerves of humans?"
Love, the mosquito-control spokesperson, implored residents to drain standing water and remove bromeliad plants from their homes. But it seems that, until a Zika vaccine is available, the county will be stuck playing whack-a-mole with pesticides until they run out of options.