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As West Nile Cases Rise In South Florida, Mosquitoes Are Getting Harder To Kill

A female southern house mosquito can lay more than 100 eggs at a time.
Sean McCann
University of Florida
A female southern house mosquito can lay more than 100 eggs at a time.

It’s getting harder to kill mosquitoes spreading West Nile in South Florida.

The plentiful southern house mosquitoes can breed in drainage ponds, clogged storm drains, flower pots and more places than the better known Aedes aegypti mosquitoes that carry the Zika virus. That water is more polluted with chemicals, making the mosquitoes more resistant to insecticides, said University of Florida entomologist Eva Buckner.

“There's just so many inputs that the mosquitoes are exposed to when they're developing in this water,” she said.

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Many of those chemicals have the same structure as the agents in insecticides, she said, and that’s making the mosquitoes better at fending off efforts to kill them.

“It all influences this increasing resistance in the mosquitoes,” she said. “And it's not just in Miami-Dade County. This is across the state of Florida.”

Growing resistance to insecticides is not new, but the rising number of West Nile cases in South Florida this year is increasing the urgency to identify which insecticides no longer effectively control the mosquitoes. The southern house mosquitoes, along with two other species of mosquitoes, pick up the virus from birds, which they feed on.

Aedes aegypti are also becoming more resistant, but it appears the southern house mosquito inhabiting urban South Florida has been more efficient at resisting chemicals, Buckner said.

So far, 35 people have been infected with West Nile, with 33 of those in Miami-Dade County, according to the Florida Department of Health. Sentinel chickens also tested positive in Lee and Manatee counties.

That’s close to the total number of cases statewide in 2018, but well above 2019, when the state confirmed just three cases. Just six cases were reported in 2016 statewide and seven in 2017.

Buckner is examining mosquitoes from eight different locations in the county and testing eight different adulticides that kill mature mosquitoes. She expects to have results by the end of the week, she said, so she can identify which adulticides to stop using.

West Nile entered the U.S. in 1999 and is now considered endemic, unlike Zika and dengue, which are transmitted by Aedes aegypti and must be imported every year. The U.S. is likely to see cases every year and the disease is transmitted by birds.

It tends to spread more easily in urban areas because the southern house mosquito, one of the Culex species that spreads it, is not a picky eater.

If it can't find a bird and there happens to be a human or a horse nearby, then it's going to be opportunistic,” Buckner said. “It's going to take a blood meal from that horse or that human.”

Other animals also can get infected, she said. But symptoms are most apparent in horses and humans, which is why those cases get reported.

Jenny Staletovich is WLRN's Environment Editor. She has been a journalist working in Florida for nearly 20 years. Contact Jenny at jstaletovich@wlrnnews.org
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