If good things come in small packages, so do some that are very bad. Consider mosquitoes. The so-called yellow fever mosquito, Aedes aegypti, weighs less than three milligrams, yet carries not only its namesake disease but a horrifying array of more than 50 viral pathogens, including dengue, Zika and chikungunya.
Once nearly eradicated in the United States, the yellow fever mosquito is expanding its range beyond the Deep South. Its unwelcome comeback now threatens three-quarters of the country.
SHAPED BY HUMAN HISTORY. Originally native to sub-Saharan forests, where it fed on monkeys and other animals as well as humans, Ae. aegypti arrived in the New World in the 1600s by stowing away on ships, particularly slaving vessels. A study published in September 2025 in Science found that the mosquito changed so drastically on this side of the Atlantic that it split from its African ancestor, Aedes aegypti formosus, into a distinct subspecies. The new subspecies, Aedes aegypti aegypti, is considered domesticated because it almost exclusively feeds on humans and lives near them.
This subspecies adapted to cities and suburbs more than forests and is as comfortable indoors as outdoors. Containers such as flower pots, clogged gutters, patio furniture, buckets, old tires and anything holding even a bottle-cap’s worth of water substitute for breeding sites in tree holes and natural cavities.
The human-associated subspecies not only spread throughout the Americas but eventually returned to the Old World, then expanded into tropical, subtropical and temperate regions worldwide. It continues to advance in North America, spreading northward from refuges along a broad southern tier of the country, coast to coast. The Centers for Disease Control and Prevention (CDC) predicted as early as 1917 that its range could eventually cover most of the country, at least for part of the year. During the study, 28 researchers sequenced genomes of 1,206 individuals from five continents to trace how the mosquito developed its relationship with humans. The shift appears to have occurred because mosquitoes transplanted to the Americas could not compete with native forest species, so they switched to humans as a readily available alternative.
MODERN CONDITIONS HELP.Aedes aegypti has benefited from a convergence of factors, including warming temperatures, urbanization, globalization and increased travel. The ideal temperature for hatching eggs is about 77 degrees Fahrenheit, but some have hatched at temperatures well below freezing.
In the Northeast, the species has been collected as far north as Connecticut and New Hampshire, though it has not yet established there. It also has been spotted as far north as Cook County, Ill.
An alarming development is that the mosquito has been setting roots in places once considered off-limits, such as dry Las Vegas and the high desert of Colorado, above 4,500 feet in elevation.
This forces mosquito control professionals to rethink assumptions about where the species can survive. Pest control professionals in regions vulnerable to invasion must understand its lifestyle and prepare for control measures.
“We were told the aegypti were very unlikely to establish, but they have and they have made it through our winters now too,” said Tim Moore, district manager of the Grand River Mosquito Control District in Colorado, where annual rainfall averages only nine to 11 inches. “We expect them to stay.” Normal winter lows in the area reach about 15 degrees Fahrenheit.
Ae. aegypti has been found in three locations around Grand Junction in recent years but is considered established in only one. There, however, it has a firm foothold.
“We have surrendered the word ‘elimination’ and turned instead toward ‘containment,’” Moore said. “We hope to control their area of expansion, as they typically do not fly very far. A few hundred feet is the usual consensus.”
BREAKING THE RULES. Aedes aegypti survives many environmental conditions and breaks the rules of typical mosquito behavior. During drought, it rewires egg development so eggs can survive longer inside ovaries and suspend hatching until conditions improve. It thrives indoors, feeds repeatedly and bites by day, especially in early morning and late afternoon.
When necessary, it can even alter its own habits. Moore says that when it is so hot people stay indoors until evening, the mosquito follows suit and shifts to night biting.
“They seem to adapt well in the summers to people’s schedules,” Moore said. “They have been touted as daytime biters, but our summers can get very hot, often in the 90s and several weeks in the 100s, so people seem to be out and about more in the evenings rather than the middle of the day and they adjust to that.”
Searching for larvae is much more difficult and time-consuming, Moore added, because breeding sites are often hidden. The mosquito also uses skip oviposition, laying one to a few eggs at a time across multiple sites. The eggs are tiny and hard to see.
Despite the dry climate, there is enough water around Grand Junction because the Colorado and Gunnison rivers converge there and the area is heavily irrigated for agriculture, commercial use and residences. “Although the climate is dry and hot,” Moore said, “lots of yards have water and spots for the mosquitoes to get out of the direct sun. This is exactly what aegypti like and they seem very good at finding and staying in these microclimates.”
Their ability to exploit microclimates could help them invade areas that normally are too cold by sheltering in urban heat islands. This may explain why infestations in cities are increasing.
UNEXPECTED PLACES. Las Vegas is one major city with scorching, dry summers now hosting yellow fever mosquitoes. Research published in the Journal of Medical Entomology outlines how rapidly the species is expanding there. Five years of trapping showed a 53% increase in the number of surveyed areas.
“Our findings show that Aedes aegypti mosquitoes are now established across much of the Las Vegas metro area,” said Saul Lozano, a CDC medical entomologist and lead author of the study.
In 2022 and 2023, researchers noted “a marked increase in mosquito abundance, particularly from late August to October,” with 2023’s peak far higher than in previous years.
The mosquitoes likely spread into Nevada from California, where they were first detected in 2013. They apparently moved north from Mexico on two separate occasions, several years before detection.
As of last December, the mosquito had been found in 27 of California’s 58 counties, firmly established in Southern California and northward through the Central Valley. In recent years, it has pushed into the San Francisco Bay Area, particularly Santa Clara County.
Two mosquitoes were detected there in 2022, none in 2023, but last year they were found in several neighborhoods, especially around Santa Clara, Gilroy and San Jose. The Santa Clara County Mosquito and Vector Control District responded with pesticide control. It is not surprising to find these warm-loving mosquitoes in the Bay Area’s Mediterranean climate (long, hot summers and mild winters). But the establishment of Ae. aegypti at significant altitude shows colder, drier regions may not be safe.
That conclusion was reinforced last September when Idaho’s Canyon County Mosquito Abatement District announced that a damaged mosquito specimen collected in August was the first known detection of Ae. aegypti in the state.
CONTROL CHALLENGES. The species’ expanding range presents major challenges because it is a domestic mosquito that thrives in human environments. Its breeding sites are small, hidden and dispersed, making surveillance and larval control difficult. Its daytime biting reduces the effectiveness of bed nets, a mainstay of malaria programs, and its preference for indoor feeding means people can be bitten even while indoors.
In addition, its ability to survive cold snaps and dry periods means control efforts must be sustained and flexible. Many programs must shift from elimination to containment, as Moore described in Colorado.
Public education is critical. Residents can reduce breeding sites by emptying containers, cleaning gutters, covering water storage and eliminating standing water. Because the mosquito can thrive in microclimates, even small changes in household water management can make a difference.
The yellow fever mosquito’s steady expansion suggests more regions will soon face its arrival. The question for mosquito control professionals is no longer whether Ae. aegypti will appear, but how quickly it can be detected and how effectively it can be contained.
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