Mosquitoes, it turns out, might be their own worst enemy.
In a six-month field trial launched in South Miami last year, Miami-Dade County teamed up with a Kentucky-based pest control company to release male mosquitoes infected with Wolbachia. The bacteria is common in other insects, but when introduced in male A edes aegypti,it makes them sterile.
After releasing more than 6.8 million treated mosquitoes, the study found the wild population was reduced by 75 percent in the half-square mile treatment area near Brewer Park in the Twin Lakes neighborhood.
"That's a very good result," said Bill Petrie, who was hired to run the mosquito control division in 2017 after the Zika outbreak and expanded it from a staff of about 17 to more than 50. "I think it's very promising."
The $4.1 million test is the first in the nation to try to control the mosquitoes in an urban area, a more difficult population to control, Petrie said. The method has been tried in the Keys and California, but neither area is as densely populated area, he said. South Miami was chosen after Miami and Miami Beach declined to participate in the trial.
Miami-Dade County began looking for other methods to control aedis aegypti mosquitoes, which breed year round in as little water as a bottle cap, after the county became ground zero for the Zika virus in 2016. Wynwood became the first active transmission zone in the contintental U.S., followed by a second neighborhood in Miami Beach.
Aerial spraying angered residents and environmentalists because Naled, the pesticide used to kill adult mosquitoes, has been linked to developmental problems in babies. It also kills butterflies, bees and other pollinators. Europe banned its use in 2012.
The Centers for Disease Control continues to recommend its use in active disease-transmission zones, along with spreading organic larvicdies to kill mosquito eggs. But Miami-Dade County has focused most of its efforts on spraying an organic larvicide, BTI, from the back of a truck called a Buffalo turbine to kill mosquito eggs.
"It's preventative and pre-emptive," Petrie said. "You're trying to kill mosquitoes before they become adults."
The county has also upped its community outreach and beefed up efforts to educate residents about inspecting their yards for possible breeding sites.
That's not to say the county would not use Naled if active disease zones appeared, he said.
"Whether you like it or not, you've got to kill adults," he said. "Remember, the larvae aren't carrying the disease. The adults carry the disease."
How the Wolbachia method could be used for the entire county remains unclear, Petrie said. For example, the CDC covered the cost of shipping male mosquitoes by Fed Ex from the MosquitoMate lab in Kentucky to South Florida. Those mosquitoes would likely need to be bred locally to cover the 2,400-square mile county.
"You have to work out your economies of scale," Petrie said. "The lab would need to be much larger."
For now, Petrie said he wants to see how research in other parts of the country and Brazil play out.
"So while it's a slightly difficult position to be in as to how do we move forward," he said, "it's a good problem to have."
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