Miami-Dade County’s air war on the Zika-carrying mosquito is working, federal officials report — but only in a small section getting a double-barrel dose of chemicals.
In a two-square-mile zone that covers the Wynwood arts district at the center of the outbreak, the population of mosquitoes has declined under an aerial spraying program that uses two different pesticides — one that takes out adults, the other that kills mosquito eggs and larvae. But in the remaining 80 percent of the 10-mile-square zone targeted for Zika erradication where naled alone is sprayed to kill adults, the Aedes aegypti has held its ground.
“In areas without the larvicide, the adult populations are rebounding much quicker and much higher than in the area with both,” Janet McAllister, the Centers for Disease Control and Prevention entomologist advising the county, told the Miami Herald.
It’s not yet clear whether the finding, which comes after 10 days of spraying, will alter the county’s plan of attack. Deputy Mayor Alina Hudak said in a text Monday that the county is in talks with the agency about potentially expanding the dual-pesticide program. She said the CDC’s advice had been to use both chemicals in the one-square-mile area defined as “the area of active transmission.” The county has been covering an area twice that size.
“Their recommendation at all times has been to use the dual treatment in the area of active transmission,” Hudak said.
At the beginning of August, county officials launched the aerial attack to combat mosquitoes after health officials confirmed active transmissions in a square-mile zone near the trendy neighborhood and the CDC issued its first travel warning in the U.S. for pregnant women. The news made global headlines, increasing public anxiety and drawing complaints from business owners worried about losing customers during slow summer months. Word of aerial spraying also raised concerns about the use of naled, an insecticide widely used in the U.S. but banned in Europe because it harms bees and other wildlife.
In areas without the larvicide, the adult populations are rebounding much quicker and much higher than in the area with both.
CDC entomologist Janet McAllister
A county action plan called for mosquitoes to initially be controlled by ground spraying and an educational campaign urging the public to dump stagnant water where the mosquitoes breed. But between Aug. 4 and 12, with the number of South Florida cases rising, the county added the aerial assault. The total number of locally transmitted cases now stands at 27 in Miami-Dade, along with another 131 infections picked up by people traveling in foreign countries. Statewide, there have been 529 travel cases.
As of Monday, Naled has been misted over the 10-square mile area three times. A plane carrying a bacterial pesticide that kills eggs, called Bti, sprayed the smaller, two-square mile area on Aug. 6 and Aug. 10, based on a recommendation from the CDC, said county spokesman Frank Calderon.
Decisions on spraying insecticide are driven by what’s being found in mosquito traps, McAllister said. In the past, the county set 16 traps all around the county to monitor the kinds and numbers of mosquitoes. After the outbreak, the traps were moved to the transmission zone. A week ago, McAllister said the CDC installed an additional 34 traps.
The Zika virus is carried in South Florida primarily by the Aedes aegypti mosquito, an urban mosquito that feeds on humans and is active during the day, making it difficult to kill with aerial spraying. Because it is nearly impossible to catch the short-lived mosquitoes that are actively infected with the virus, officials base decisions how many adult, female breeding mosquitoes they catch. Males mosquitoes don’t bite and don’t transmit the virus.
“We’re looking for a trend. Is that trend going up or doing down relative to spray events,” McAllister said. “We know for adults the effect is transient so when the numbers start to increase again is when we discuss whether another treatment might or might not be warranted.”
You have daily rains so you have daily adults emerging and over time, the population as a whole will start to build up.
CDC entomologist Janet McAllister
The mosquitoes lay their eggs in stagnant water — water caught in tarps, the lip of a flower pot or the center of a bromeliad. Eggs take about five days to hatch and grow to adults, but not all at once, creating waves of mosquitoes that can be tougher to control without also targeting eggs, McAllister said.
“You have daily rains so you have daily adults emerging and over time, the population as a whole will start to build up,” she said. “So we also want to reduce what’s in the larval habitat as well.”
“By the times these cases get confirmed, we’ve already been there,” she said. “You call, we’re going. We are together with our community in this war against this virus.”