More than just a nuisance: Insecticide-resistant mosquitoes are a public health concern.
Swat. Spray. Repeat.
This is the dance humans and mosquitoes have been doing for decades, ever since the rise of chemical insecticides following World War II. The pest control industry in the U.S. is worth an estimated $20 billion, and that number is projected to grow as climate change enables more bugs to thrive in more places than ever before. But efforts to rid our yards of blood-sucking pests in the short-term may have long-term unintended consequences, including making mosquitoes harder to kill.
In terms of its effectiveness on mosquitoes, insecticide is measured in years, not decades, because mosquitos adapt so quickly, according to Don Yee, a mosquito ecologist at the University of Southern Mississippi.
“When you spray a population of mosquitoes with a new insecticide, it is very likely that the vast majority of those mosquitoes will die because they are not immune to that particular chemical,” said Yee. “However, you will have a few individuals that will naturally be resistant to that insecticide. Because of the proliferation of mosquitoes, it doesn't take very long for those populations to become dominant and therefore you have essentially an insecticidal resistant population, and now you have to find another insecticide, so it’s a huge problem.”
This selective process is likely taking place closer to home than you might think.
“Evolution is arguably the most important force in nature, and it's happening all the time, whether we see it or not,” said Michael Reiskind, an associate professor in the Department of Entomology and Plant Pathology at North Carolina State University. “It's happening in our backyard.”
A necessary tool for public health
Reiskind and a team of researchers recently set out to look for a specific genetic mutation among Aedes albopictus mosquitoes that indicates they may have developed a resistance to pyrethroids, one of the most commonly used types of insecticide.
The team began collecting mosquitoes at Fort Bragg, working on the hypothesis that the presence of thousands of military personnel wearing uniforms impregnated with the insecticide permethrin might have a measurable impact on local mosquito populations. For comparison, they also sampled sites in Wake County.
“We saw the opposite as to what we predicted,” said Reiskind. “We didn't see any evidence for insecticide resistance at Fort Bragg, whereas we found a cluster of mosquitoes that had the genes for resistance […] in Wake County.”
The team detected the mutation in nearly a quarter of the Albopictus mosquitoes collected from a well-established suburban Raleigh neighborhood.
“We haven't tested this explicitly, but we suspect that what we're seeing is selection for resistance from backyard control approaches, primarily the spray-in-your-backyard kind of control,” said Reiskind.
The study, which was published in the July 2022 issue of The Journal of Medical Entomology, calls for increased surveillance of mosquito populations to monitor insecticide resistance.
Reiskind notes insecticide-resistant mosquitoes are more than just a nuisance; they’re a public health concern. Aedes albopictus, one of the most common mosquitoes in the Southeastern U.S., can transmit a number of viruses, including West Nile, La Crosse, and Eastern equine encephalitis. And though it’s less effective at spreading disease than its cousin Aedes aegypti, Albopictus mosquitoes can also carry pathogens like chikungunya, zika, and dengue.
Effective insecticides are a necessary tool for public health, Reiskind says, especially if new mosquito-borne diseases crop up.
“I am actually of the opinion that we should view insecticide susceptibility, that is to say, you know that you can kill an insect when you want to, as a public good,” Reiskind said. “That pool of insecticide susceptible insects is a public good. And when you start nibbling away at that pool by selecting for resistance, you're actually utilizing the public good, just like a water resource or, you know, any depletable resource.”
A long-running fight against mosquitos
Humans have been waging a long-running battle against mosquitos ever since they were identified as disease vectors around the turn of the last century.
During the Second World War, the U.S. military sought new ways to protect troops from malaria and other diseases abroad, ushering in the heyday of chemical control for mosquitoes and other pests. DDT was the most iconic of these chemical controls developed for military use, and in post-war America, it was a popular tool in the battle against mosquitoes.
But the white powder renowned for its effectiveness at killing small insects turned out to also have devastating effects on larger creatures up the food chain. It builds up in fatty tissues, and can lead to liver tumors in animals.
“The things about DDT that made it successful were that it lasted in the environment for a long time and so you could spray it once and you didn't have to keep putting it out there and putting it out there,” said Eleanor Spicer Rice, an entomologist and writer who is also the science advisor on WUNC’s CREEP podcast. “It was broad-spectrum, which means it killed all kinds of bugs and it also traveled really well through the atmosphere. Now we hear those things and we think, those are things that can make for an environmental nightmare. But back then they also made for an excellent insecticide.”
Even as early as the 1940s, reports of DDT-resistant mosquitoes began to pop up across the globe. But its popularity as an insecticide persisted largely unchallenged until 1962 when marine biologist and writer Rachel Carson laid out the case against DDT in her groundbreaking book “Silent Spring,” detailing its disastrous environmental impacts.
By the time the Environmental Protection Agency limited the widespread use of DDT in 1972, Americans had sprayed roughly 1.4 billion pounds of it all across the U.S. Half a century on, researchers are still uncovering the long-lasting negative impacts that pervasive chemical has on humans, wildlife, and the environment.
Who’s biting us?
Both Aedes aegypti and Aedes albopictus are non-native mosquitoes transported unwittingly to the U.S. by humans, but one has a much longer history in the American South.
Aegypti are considered a historical invasive, according to Yee, and likely arrived in the Americas and the Caribbean 400 years ago on ships from Africa as a small but significant part of the legacy of the transatlantic slave trade. Aegypti can spread yellow fever, zika, chikungunya, and dengue — once known as breakbone fever for the excruciating pain it causes.
They like to live among humans, and, when given the choice, they prefer consuming human blood.
“They are truly a domesticated mosquito, probably our only truly domesticated mosquito,” said Yee.
A relative newcomer by comparison, albopictus mosquitoes first gained a foothold in Texas in the mid-1980s.
“The thought is that this animal probably arrived in the port of Galveston in a shipment of used automobile tires,” Yee said. “Tires are notoriously a good system from the mosquito standpoint to move around the world because all they have to do is get into these tires and then we move them around and we move them to new places.”
Originally from Southeast Asia, albopictus are also called tiger mosquitoes after their white and black stripes. Less than a year after showing up on tires in Galveston in 1985, they were found in New Orleans and Jacksonville, Florida.
Since then, albopictus has successfully outcompeted aegypti mosquitoes throughout much of the South, significantly reducing the older historical invasive's range.
“We now have states like Alabama and Mississippi where aegypti used to be common, especially in the larger cities, and it's gone,” Yee said.” It's been gone for 30 years, and we think that's mainly due to albopictus's invasion.”
Today, albopictus mosquitoes have gone on to spread as far as Illinois, Pennsylvania and New York State. They are now found in at least 38 states and continue to expand their range farther to the north and west.
“Trying to control it now would be a herculean task,” said Yee. “I mean I don't think there's enough public health dollars to control this particular animal in the United States. It's everywhere.”
Insecticide resistance has been well-documented in aegypti, perhaps due to their long history of living with humans, but for albopictus, the picture is less clear, according to Reiskind.
“I think in large part, it hasn't been looked for that hard in the U.S. and maybe not at the right scale,” he said.
A short-term fix
There are about 60 known species of mosquito in North Carolina, but Albopictus is the one that likely prompts the most calls to pest control. It’s the most common these days, and perhaps the most annoying, thanks to its aggressive daytime biting habits.
When residents call a pest control service complaining about mosquitoes, workers usually strap on a device similar to a leaf blower connected to a tank full of liquid chemicals, then spray a mix of air and insecticides around the perimeter of the yard.
This can kill adult mosquitoes in the short term, but they inevitably come back. And the generations that come after might be harder to kill. Still, Reiskind said he doesn’t think the specter of insecticide resistance is sufficient to deter people from seeking out residential spraying, at least until the sprays stop working.
“That will definitely affect people's decision,” he said, “because if it doesn't work, it doesn't work.”
He warns increased insecticide resistance could have a significant financial impact on the pest control industry.
“That industry makes money when they don't have to return,” said Reiskind. “Almost all of them have a 21 to 28-day guarantee of no mosquitoes. And after 21 days, the mosquitoes come back after spraying. It's just how long it lasts out there, but if they get called back at seven days, or 14 days, or one day or three days, they start to lose money. So the profit model is built on the insecticides working, you know, and the minute they don't, it gets really expensive to run one of those businesses. Resistance is going to be a real difficult one for the industry to deal with, especially if it becomes not spotty like we've seen it but more widespread.”
More mosquitoes in more places
Climate change is forecast to make life less pleasant for humans, but the warmer temperatures and increased precipitation it’s likely to bring are two things mosquitoes love.
In the short term, that means longer mosquito seasons and more mosquitoes each summer.
“If it's warmer, their metabolic processes actually speed up and they develop faster and so you can squeeze more generations into a summer if the temperatures are warmer because you'll just produce them at a faster rate,” said Yee.
In the long-term, increased warming will make it easier for some aedes mosquitoes to thrive in places that are currently a bit too chilly for their liking.
“As these temperatures increase through time, that's going to push the range north, and so we'll start to get established populations in places like Chicago, Detroit, Cleveland, etc. And that's when we have more potential for concern because now we're bringing these species into contact with human populations that haven't experienced them before.”
New mosquitos in new places, biting new humans and new animals, introduces all kinds of potential for things to go wrong, according to Yee.
“That's where you get into the unpredictability, because if you have a new vector in a new location interacting with new hosts and those hosts have new pathogens, there's no way to tell what's going to be the outcome of that,” says Yee. “That's the biggest concern for those working in climate change and mosquito-borne disease is that we're going to have this situation in which we have all these unknowns coming together and there's simply just no way to predict what the outcome could be.”
Start in your own yard
When it comes to invasive mosquitoes, insecticidal resistance, and the impacts of global warming, we could one day be facing a collision of problems caused by the unintended consequences of human behavior.
If you want to do more than just wait for our hot, buggy, dystopian future to arrive, there are some simple and effective actions you can take right now in your own backyard.
“I honestly think personal repellents are a good way to go,” said Reiskind. “You know, we see very little evidence for the evolution of resistance to personal repellents, probably in large part because humans are the only things that wear personal repellents, whereas mosquitoes will happily feed on other things.”
In addition to limiting your exposure to mosquitoes through personal repellents on your skin, you can target the next generation of flying bloodsuckers by removing the water sources where females lay their eggs.
“The way that we design our cities, the way that we go about maintaining our individual habitats and our households are all related to the abundance of these animals,” Yee said. “If we build cities in a way that minimizes open water sources, if we as individuals try to reduce the amount of standing water that we have around our homes, if we're not storing a big tire pile in our backyard, or not letting the kiddie pool fill up […] we can mitigate a lot of these issues.”
Added Spicer Rice: “Mosquitoes aren't the devil. And pesticides aren't the devil here. They're both problems, but they're both problems that are made by us and they're mistakes that we keep making.”
We may slowly be learning we can’t just spray our way out of our mosquito problems. At least, not with the tools we’ve used in the past.
WUNC's Laura Pellicer contributed to this report.
Want to learn more about new animals in the South? CREEP is a podcast about creatures invading our space, and changing the world around us, presented by WUNC and the North Carolina Museum of Natural Sciences. Listen and subscribe wherever you get your podcasts.