It Was Already the Deadliest Animal in the World. Now the Most Dangerous Species Are Coming Deeper and Deeper Into America.

Science How the Mosquito Is Conquering America The deadliest species of the world’s deadliest animal used to find it hard to survive in the United States. Now they’re here—and they’re thriving. On an overcast Wednesday morning in Salt Lake City this summer, four bicyclists gathered in an industrial area not far from the city’s airport before fanning out across town. They were on a mission to kill. Almost anyone in America would have supported them. The riders work for the Salt Lake City Mosquito Abatement District, the first mosquito-control program in the nation to launch a bicycle brigade, according to Ary Faraji, the district’s executive director. One rider stopped to kneel alongside the road, another at the edge of a park, to spritz stormwater catch basins with larvicide, a chemical designed to kill mosquito larvae before they develop into adults and begin feasting on people. Each of the riders covers upward of 30 miles a day to treat every catch basin in the city’s urban and suburban areas, about 75,000 total, over the course of a month. Then, they start all over again. Faraji is an entomologist who sometimes calls the insects he studies “skeeters” and who sent me a photo of his desk piled high with mosquito-related history books when I asked him for reading recommendations. He and his team take what’s known as an integrated mosquito management approach to protect Salt Lake City residents from the incessant itch that the pests bring. “Really, that means understanding the pest that you’re trying to control, understanding its biology and its ecology, and intervening with appropriate control measures that take into consideration larval control, adult control, disease surveillance, adult surveillance—it’s really a holistic approach,” he told me. The Salt Lake City Mosquito Abatement District even has its own fish hatchery, where it rears the aptly named Western mosquitofish. The tiny fish have a voracious appetite and work wonders as natural mosquito-control agents in abandoned swimming pools, small residential ponds, and other confined aquatic habitats. Residents need only call the abatement district to get a special delivery of mosquito-eating fish for their backyard at no cost. These might sound like extreme measures just to prevent a common nuisance. But it’s not only annoying pink welts that Faraji’s team is concerned about. The threat of mosquito-borne disease is increasing in the U.S. as invasive mosquito species expand their range across the country and their seasonal populations last longer and grow larger during global warming. Studies have also found that warmer temperatures push some species to bite more, develop into adults faster, and serve as better virus incubators. Already, mosquito-borne diseases kill more than 1 million people and infect about 700 million worldwide each year. Cases in the U.S. comprise only the smallest fraction of those numbers, with annual deaths usually in the low hundreds and diagnosed cases in the thousands. (Cases are much more common in Puerto Rico and other U.S. territories than they are in the 50 states and D.C.) But that could soon change as two particular mosquito species, Aedes aegypti and Aedes albopictus, make their way deeper and deeper into the country. These are the species to worry about—and they are increasingly around us. Since its arrival in the Americas centuries ago, Aedes aegypti has conquered the Southern states, swaths of the Midwest, and much of California, and has begun to advance up the East Coast. The insects were found for the first time in Oregon in 2024 and in Idaho in 2025, and they have now established themselves across parts of mountainous Colorado, where conditions have historically been far too harsh for them to survive. Aedes albopictus, a much more recent arrival, now has a range that stretches well into New England and the lower Great Lakes and has even been found to overwinter in the Midwest. Aedes mosquitoes are vectors for some of the most serious and widespread mosquito-borne diseases. Four in particular cause the most human suffering worldwide: chikungunya, Zika, dengue, and yellow fever. Each of these viruses comes with a raft of symptoms ranging from unpleasant to, in some cases, fatal—fever, headache, muscle and joint pain, shock, organ failure. Zika can spread from a birthing parent to their child in the womb and cause congenital disabilities, which notoriously led to panic after an outbreak erupted in Brazil in 2015. Of the four diseases, dengue is the most prevalent, with more than 3.9 billion people in over 132 countries at risk of contracting it, an estimated 96 million symptomatic cases annually, and about 40,000 deaths per year as a result. As populations of Aedes mosquitoes grow and spread across the U.S., researchers expect we’ll see more mosquito-borne disease outbreaks. Warmer states where the mosquitoes are already prevalent are at the greatest risk. “Florida is on the front lines,” John-Paul Mutebi, director of Miami-Dade County’s Mosquito Control division, told me. The list of locations where mosquitoes are only a nuisance—and not a serious public health threat—is shrinking. Preventing mosquito-borne diseases from taking root here will require more than just covering ourselves in bug spray (though that’s always a good idea too). To grasp what we’re facing, we should look to the past to understand how these insects have spread so far, why they are so at home in our cities—and what it will really take to stop them. Getty Images Plus Some 400 years ago, a six-legged invader disembarked on the coast of Spanish Florida and began making its way across North America. Capable of flying only about the length of a football field, it relied on humans for transport—and sustenance. As the Aedes aegypti mosquito expanded its range across more of the Americas, it brought with it the delirium, jaundice, and aching joints of yellow fever. Researchers later used the archival record of yellow fever outbreaks, paired with genome analyses of modern mosquitoes, to piece together the insect’s travels. Though Aedes aegypti is native to the forests of Africa, it evolved long ago to live in human settlements. “These mosquitoes really like living with us,” Douglas Norris, a vector biologist at Johns Hopkins Bloomberg School of Public Health, told me. That’s because they don’t have a large flight range and have evolved to breed in just about anything that holds water—from a bottle cap or the plant boxes on your patio to parts of the stormwater management infrastructure in our nation’s cities. And because Aedes aegypti feeds almost exclusively on humans, the insects are great at transmitting disease from one person to the next. In fact, in a crowded field of more than 3,500 mosquito species, Aedes aegypti has been the vector for almost all major epidemics of some of the world’s worst mosquito-borne diseases. The subspecies of Aedes aegypti now established across large swaths of the U.S. began its evolutionary journey in sub-Saharan Africa. Ancestral populations can still be found in wooded areas there, depositing their eggs in tree holes and other natural pockets of water and feeding on wildlife, including a variety of mammals and occasional birds or reptiles. But when humans began living in villages and towns on the outskirts of those forests, some mosquitoes found the water stored in containers in human settlements a more attractive breeding ground. (Unlike forest nooks, stored water tended to last through seasonal droughts.) The mosquitoes that bred in settlements eventually evolved a preference for blood meals from their human neighbors, who were now the most available food source. The insect’s first trips west came soon after, as the transatlantic slave trade brought ships packed with enslaved Africans to the New World. Those ships also carried supplies of fresh water for the monthslong voyage, stored in barrels fit for mosquito breeding. The yellow fever virus, also indigenous to Africa, must have stowed away too. Tracing Aedes aegypti’s colonization of the Americas using records of yellow fever epidemics places it in Atlantic port cities in the British colonies in the 1690s and limited parts of the American South in the mid-1700s. By the 1850s, the new arrival had spread to much of the south-central U.S. Aedes albopictus, a mosquito native to Asia that can transmit the same diseases as Aedes aegypti, conquered the country even faster thanks to developments in human infrastructure. Researchers believe that it first arrived with military equipment and tires shipped back from the continent following World War II. Tires make excellent mosquito incubators. They collect heat and organic matter, which mosquitoes feed on during their larval stage, and are almost impossible to empty once water pools inside. Turn a tire upside down or on its side, and the water will flow within it to whatever section is now at the bottom. Roll or shake the tire, and you are likely to succeed only in sloshing out some of the water. Additional Aedes albopictus mosquitoes arrived via the used tire trade. The U.S. imported millions of used tires in the 1970s and ’80s, largely from countries where the mosquito is indigenous. By the end of the ’80s, the mosquito had established itself in at least 15 states. Once the invasive Aedes mosquitoes arrived on U.S. soil, they found American cities comfortable places to live. Our urban centers tend to be low in mosquito predators (like frogs, fish, and birds), offer a buffet of options for blood meals (humans, mostly), and provide plenty of habitat where mosquitoes can breed, as well as places for the insects to shelter from scorching or freezing temperatures that would otherwise kill them. Many wealthy neighborhoods feature environments that support Aedes mosquitoes, but the situation tends to be worse in lower-income areas. One study of mosquito prevalence in Baltimore and D.C., in which researchers tested for mosquitoes in water-holding containers in public alleyways and residents’ backyards, found that Aedes albopictus pupae (a stage the mosquito passes through after the larval stage but before it emerges as an adult) were more common in neighborhoods with below-median incomes. More than half of the Aedes albopictus pupae recorded in Baltimore were in a single low-income neighborhood. Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies, worked on that study. She told me that mosquito populations spiked in areas with more human-made mosquito habitat, such as abandoned buildings, unkempt vacant lots, and piles of discarded bulky items. Rain can gather in those areas and attract mosquitoes, but the spaces do not tend to attract many natural mosquito predators. “The garbage that had been dumped and collected around abandoned spaces, like toilets and tires and big Tupperware containers, created semipermanent water bodies that were in some cases just under an overhang, so they had a little bit of environmental protection, and there was also vegetation growing over the whole thing—so it was like the perfect mix of habitat for mosquitoes with no mosquito predators,” LaDeau explained. The type of vegetation in a neighborhood also affects mosquito populations. Wealthier neighborhoods tend to have better-managed green space, with the leafy parts of trees growing far above lawns or planted flowers. That leaves fewer shaded, humid spaces for water to pool and mosquitoes to hide, as well as more access for mosquito predators such as birds and larger insects. But mosquitoes thrive in the denser and less well-managed vegetation often found in lower-income neighborhoods. The push to green our cities amid warming temperatures due to climate change has also created new mosquito habitats. “When we create a modern suburbia with fragmented parks and green space, we do a pretty good job at creating mosquito habitat,” Norris said. That doesn’t mean cities should avoid urban greening projects. But, Norris said, public administrators should recognize that “you’re not going to be able to beat biology, so if you create this green space, you should also have the infrastructure to keep those places safe.” The infrastructure Norris has in mind is control and surveillance programs that monitor both mosquitoes and mosquito-borne diseases and are prepared to respond when disease is detected in the mosquito population, like the program in Salt Lake City. But some experts fear we could be underprepared for what’s to come. The fact that Aedes mosquitoes keep showing up in new locales does not necessarily mean those places will go on to experience disease outbreaks. But it does mean that the risk is growing. Female mosquitoes need the proteins and iron in blood to produce viable eggs, so a blood meal is a vital part of the insect’s reproductive cycle. When a female mosquito bites, it uses specialized mouthparts to pierce through the skin of its chosen host and into the blood vessels. It also injects a bit of saliva, which contains anticoagulants to prevent the host’s blood from clotting as the mosquito feeds. It’s that saliva that triggers an immune response in humans—and makes us itch long after the mosquito has gone. When a mosquito takes a blood meal, it also swallows any viruses or parasites living in its host’s blood. Those pathogens can then be transferred to the next creature the insect feeds on when it deposits saliva into that creature’s bloodstream. The diseases that Aedes aegypti and Aedes albopictus are known to carry—chikungunya, Zika, dengue, and yellow fever—as well as most other mosquito-borne diseases, are most common in tropical and subtropical climates. Almost all cases diagnosed in the continental U.S. and Hawai‘i are travel-associated rather than locally acquired, meaning that an infected mosquito bit the person while they were out of the country and they brought the virus home. Several states are seeing sharp increases in mosquito-borne diseases among travelers, as case numbers abroad are already climbing due to climate change. But a case brought home from vacation can spark a local outbreak since the mosquitoes that transmit the diseases are well established in much of the country. Cases of dengue exploded in Little Havana and Hialeah in Miami-Dade County in 2023, an outbreak that ended after close to 200 locally acquired cases were recorded. Mutebi told me that, based on his research, he suspects that travelers returning from Cuba had carried it home. When it comes to spreading one of these viruses, Aedes aegypti is the most prolific vector because it bites human after human after human, and each meal is a chance for it to deposit a pathogen in its host. Aedes albopictus, on the other hand, is less effective because it is a promiscuous biter. It might dine on someone enjoying a backyard barbecue one time, then on the neighbor’s dog, then maybe a squirrel or a deer if you happen to live near wildlife. Some diseases can be spread from animals to humans by certain mosquito species (that’s how West Nile virus moves from birds to people). But chikungunya, Zika, dengue, and yellow fever are human diseases, and the chain of transmission can sputter out as Aedes albopictus feasts on other creatures between its human blood meals. Still, both Aedes aegypti and Aedes albopictus are aggressive biters capable of transmitting the same diseases. Plus, during the summer months, when mosquito populations across the U.S. peak, Aedes albopictus is more ubiquitous. It is also quick to adapt and capable of extending its range far beyond that of Aedes aegypti—and it is doing just that, fast. “We thought Aedes albopictus, because it came from a tropical climate in Asia, would never get very far north,” Norris said. But in the decades since it arrived in the U.S., he said, “the mosquito actually has adapted to those newer climates, so the ones we find in Texas or Florida have slightly different adaptations than the ones we find in Long Island. They figure out ways to get to where they want to be.” It was those Long Island Aedes albopictus mosquitoes that public health authorities blamed for a locally acquired case of chikungunya last year. That was the nation’s first locally acquired case ever recorded outside the South. LaDeau told me that Aedes albopictus’ capacity to adapt is one of the things that most concerns her about the future of mosquito-borne disease in the U.S. “It is adapting to these urban spaces, and as it does that, it will increasingly bite the animal that is the densest-population animal in these habitats, which is humans,” she said. “It’s likely that mosquito-borne diseases are only going to increase, both because it [the climate] is warming and because they’re adapting to the environments we build very quickly.” When it comes to mosquito management, the program in Salt Lake City is among the best in the nation. Faraji, its director, told me that besides the program’s bicycle brigade and backyard fish program, which help keep mosquito populations down, his team also sets hundreds of mosquito traps across the city. The traps are laid in the afternoon and picked up the following morning, allowing them to collect mosquitoes at dusk and dawn, when the insects are most active. They lure mosquitoes with carbon dioxide—all vertebrates release the gas with each exhale, so to mosquitoes, it usually signals a meal. Once a mosquito nears a trap, a small fan forces it into a baglike net, where it is held for pickup. Workers regularly collect the trapped mosquitoes and, after sticking them in a fridge for a while to knock them out, sort them by species and count them. A lab tests the mosquitoes for parasites and viruses. When infected mosquitoes are found, workers treat the area where they were collected with adulticide and conduct “source reduction,” a sophisticated way of describing what usually amounts to emptying containers of standing water. But Faraji added that mosquito management requires constant vigilance and a willingness to adapt and improve. “We make changes on an almost weekly basis,” he said. His team is always learning about new developments and technologies in the field, conducting research, and juggling resources to ensure they are allocated to treat the mosquito species and habitats that pose the greatest risk to residents. Not all cities are as well equipped. Some areas where new mosquitoes are arriving lack control and surveillance programs altogether. LaDeau, the disease ecologist, told me that part of the problem is that local governments tend to focus only on mosquito-borne disease when an outbreak erupts and, even then, usually only on that specific disease; they also relax surveillance and control programs in between. The federal government’s programs and research funding agenda take a similar approach. “Because we focus so much on the kind of retroactive or reactionary human caseload, we miss the opportunity to control the vector and to understand more about the vector,” LaDeau said. Salt Lake City’s program also has the benefit of being a mosquito abatement district, a governmental agency with its own budget funded by property taxes. Elsewhere, mosquito control efforts carried out by city and county public health or environmental services departments risk having their budgets commandeered for other needs and, often, their attention diverted to other tasks. Instead of having time to mount a bicycle brigade to treat the local stormwater infrastructure for mosquito larvae, a city’s public health department might have to juggle mosquito control alongside restaurant inspections, vaccine programs, and more. The mosquito abatement district model has its roots in the early 20th century, and Utah was the third state to pass legislation authorizing the agencies. Woodrow Wilson, who would go on to become president in 1913, signed the first such state legislation in 1912, as then governor of New Jersey. California followed in 1915, and Utah in 1923. What these early adopters had in common was that they were home to more than their fair share of nuisance mosquitoes, which bred in huge numbers in the Jersey Shore’s salt marshes, in floodwaters from California’s agricultural irrigation, and in snowmelt from Utah’s mountains. With residents already scratching and swatting, it took only a couple of charismatic entomologists to win over the right politicians to move legislation forward. That on-the-ground mosquito control work, which has been carried out in a growing number of states and municipalities across the country over the past century, is vital. But it is still just one piece of a much larger puzzle we’ll need to solve to survive as mosquitoes spread and their populations boom—and, unfortunately, some of the other pieces are in bad shape. Recent cuts at the National Institutes of Health, the Centers for Disease Control and Prevention, and the Department of Agriculture are expected to result in the loss of important institutional knowledge and to hobble research efforts vital to tracking mosquito spread and understanding mosquito-borne disease. Increasingly, too, the attention of the press and global health organizations—and, in turn, funding opportunities—is being pulled toward projects promising higher-tech solutions. Many mosquito experts seem to agree that there is room in the arsenal for something like mosquitoes that have been infected with a bacterium that leaves them sterile or genetically modified such that their female offspring cannot survive to adulthood, when they would begin to bite people. But developing those solutions should not come at the cost of more mundane but effective interventions. Mutebi, the director in Miami-Dade County, warns that we might not even see the next mosquito-borne problem on the horizon if research and surveillance programs receive too little support. Mosquitoes “are always going to find a way to outwit us; it is amazing how variable they are, and because they’re so adaptable, they are unpredictable,” he said. “To deal with them, we have to be vigilant, we have to continue doing surveillance, and we have to continue doing research.” It is not just the mosquito experts who have a role to play in preventing mosquito-borne disease outbreaks as the climate warms and the insects spread across more of the U.S. On the contrary, Faraji told me, “mosquito control is really a job for everybody.” If you have old tires sitting around, take them to a recycling center. Store your wheelbarrow inside your shed so rainwater can’t collect. Or if you happen to have a small pond or an abandoned swimming pool in your backyard, look online for a local mosquito control agency that can help prevent it from becoming a mosquito breeding ground. The insects have made our towns and cities their homes. But we can do more than sit back and get bitten. It’s in everyone’s interest to stop them. Animals Climate Change Environment Global Warming Research Urban Planning