‘Not going anywhere’: Mosquitoes thrive in Las Vegas, gain pesticide immunity

Pesticides can reduce insect populations, but in areas around golf courses in Summerlin and Henderson, efforts to eradicate mosquitoes that carry West Nile virus are proving ineffective, according to Trishan Wickramasinghe with UNLV’s Parasitology & Vector Biology Laboratory.

While his latest research is not complete, he said he’s run several tests that show some species often survive pesticides, even at concentrations 100 times greater than the standard use.

A new generation can quickly evolve to develop immunity to pesticides that should kill them, said Louisa Messenger, assistant professor at UNLV’s School of Public Health. Wickramasinghe added that these pesticide-immune mosquitoes are commonly found at water hazards on golf courses or water installations in Summerlin and Henderson, where pesticide use is more frequent and stagnant water is abundant.

Mosquito season is in full swing in Clark County. The Southern Nevada Health District and Clark County Vector Control are monitoring the region’s mosquito population for disease, particularly after record-high mosquito activity early in summer 2024.

Without a coordinated community-wide abatement effort, Messenger warned that the mosquito problem will continue to grow. She said the problem is “very unlikely” to solve itself.

“If you’ve got one person with an untreated, stagnant swimming pool on the end of the road, they’re just gonna be seeding mosquitoes back into the community,” she said. “That’s why if you have a coordinated effort, like an abatement district, that would be much, much better.”

The health district had not reported any samples containing arboviruses like West Nile virus as of July 17, but senior environmental health specialist Christian De Haan said mosquitoes will continue to pose a public health threat until mosquito season ends in late October.

“Come August, September, there’s always a possibility that we might get some disease showing up, and if that does happen, you know, we would absolutely let the public know to take extra precautions,” he said.

‘They’re not going anywhere’

Two different mosquito species have generated a buzz among environmental and public health experts in the valley.

Culex quinquefasciatus, the most common mosquito in Southern Nevada, resides in damper places like the Clark County Wetlands Park, and is a vector for spreading West Nile virus. The disease was first logged in the valley in 2004; last year, the health district reported that 26 people were infected by the disease.

Aedes aegypti, the aggressive and resilient species, was first discovered in the valley in 2017. Bites from this invasive species generally cause harsher skin reactions and can spread viruses that cause dengue fever, chikungunya, yellow fever and Zika. The health district has not yet found any mosquitoes carrying these diseases, but given Las Vegas’ estimated 40 million visitors from around the world each year, Messenger said it’s only a matter of time before such cases are documented in Southern Nevada.

“It really doesn’t take very much for you to have a small little outbreak here in Vegas, because we have the vector. We just need a person to traffic the pathogen through the airport,” she said.

Aedes aegypti, in particular, has prospered in recent years. The mosquito species was originally identified in four ZIP codes in Clark County in 2017. Last year, it was found in 57 ZIP codes. Using predictive mathematical models based on the district’s data, Messenger said Aedes aegypti mosquitoes could be logged in 75 different ZIP codes by 2030.

De Haan said mosquito population sizes vary year by year, depending partly on the weather. The number of ZIP codes where Aedes aegypti was identified spiked in 2023, he said, after Hurricane Hilary brought increased rain to the valley.

“Last year, we had a lot of mosquitoes at the first half of the season. Then, when the heat hit in July and we had eight months of no rain … we did see our trap numbers just drop considerably,” De Haan said.

To prevent mosquito bites, De Haan recommended using insect repellent, wearing clothing that fully covers your arms and legs, finding and emptying still-water sites around your property and investing in screens for windows and doors.

“We have mosquitoes in town … and they’re not going anywhere,” De Haan said. “But everyone can take steps to reduce the number of mosquitoes around them, as well as take steps to prevent them from biting them.”

‘Building mosquitoes into our environments’

Inside the UNLV lab, Messenger leads a team of researchers studying how to improve and control surveillance of vector-borne diseases like those spread by mosquitoes. Cloth cages containing hundreds of bloodsucking mosquitoes sit on lab tables, and sealed plastic bins serve as grounds for larvae to grow.

Part of Messenger’s recent work has been trying to understand why the local Aedes aegypti population is thriving. She said she believes Las Vegas’ rapid urbanization is contributing to the species’ growth. Mosquitoes need just a bottle cap worth of water to reproduce, and as more land is developed into pools and small ponds, Messenger said it creates more potential areas for mosquitoes to breed.

“As Las Vegas is beginning to grow and expand and urbanize, we are building our mosquitoes into our environments,” she said.

Efforts to abate

Unlike other metro areas in the West, Clark County does not have a region-wide mosquito abatement program, leaving mosquito suppression up to each local municipality.

Clark County Vector Control targets mosquito abatement from public spaces and infrastructure such as washes and storm drains using mosquito-eating fish and insecticides, according to county spokesperson Stacey Welling. However, the vector control division does not provide homeowners with abatement services on private property, outside of educational outreach.

Messenger said her recommendations to establish an abatement district have been met with skepticism about necessity and cost from local governments. This prompted her to create and distribute a survey asking residents about their mosquito experiences and thoughts on establishing a county-wide control program. (If you’d like to fill out the survey, click here.)

“We want to take that information and then we want to take it up the chain of command and go, ‘Actually, there is a need,’” she said. “The best thing we can do is have the community tell us.”

Contact Spencer Levering at slevering@reviewjournal.com or 702-383-0253.