Everyone hates mosquitoes. But the little monsters are highly diverse, and with 3,500 species out there, some are more despicable than others.
Many mosquitoes don't bite humans, for example, or just lump us in with lots of hosts. A few "anthropophilic" species, however, actively seek human blood, often living near — or even inside — homes. They can be especially scary, and not just because they excel at stealing our blood. Some also spread deadly diseases, from malaria carried by certain Anopheles species to a wide variety of viruses.
One of the most notorious people-seekers is Aedes aegypti, an African species that has followed humanity to tropical, subtropical and temperate regions worldwide. Its ancestral form still inhabits some forests of sub-Saharan Africa, where it mostly bites wildlife, but the "domestic form" now specializes in humans. It's the main vector of several viruses, including dengue, yellow fever, chikungunya — and Zika.
This is a transmission electron micrograph (TEM) of Zika virus particles, shown in red. (Photo: Cynthia Goldsmith/CDC)
Scientists have known about Zika virus since 1947, when it was discovered in Uganda, but until lately it seemed mild and localized. It began surging through Brazil in 2015, and soon leapt to more than 50 countries in the Americas and elsewhere. This outbreak has revealed a far worse Zika than anyone knew, with symptoms ranging from microcephaly in infants to Guillain-Barré syndrome in adults. It can be spread between people by sex, but the main mode of infection still seems to be mosquitoes.
"Zika is unprecedented," Centers for Disease Control and Prevention (CDC) director Tom Frieden said at a press conference in Florida, where at least 15 people have been infected with Zika by local mosquitoes. "We've never before had a mosquito-borne disease that can cause birth defects."
While Aedes aegypti is the primary vector for Zika, it's not alone. The virus can also be transmitted by Aedes albopictus, a species that originated in Asia and, like aegypti, now ranges from tropical to temperate areas around the world.
Until there's a Zika vaccine, our best hope may be to fight or flee these mosquitoes. Even if we can't wipe them out, we can at least limit them near people — and not just with well-known steps like emptying birdbaths, or with broad-spectrum pesticides that can pose new problems. It helps to be well-versed in your enemy's strengths and weaknesses, so here are a few key facts about Zika mosquitoes:
1. These maps show the potential range of Zika mosquitoes.
The predicted global distribution of Aedes aegypti mosquitoes. Colors indicate the species' probability of occurrence, from zero to 1, in a given section of land measuring 5 km by 5 km. (Image: Kraemer, et. al./eLIFE)
The predicted global distribution of Aedes albopictus mosquitoes. (Image: Kraemer, et. al./eLIFE)
Aegypti probably learned to like humans by necessity, a 2013 study suggested, as ancient people displaced other animals in its habitat. And by stalking world travelers like us, it became one, too. "[O]f all species, humans occupy the widest range of habitats on Earth," the authors wrote," [so] once a species evolves the ability to co-exist with humans it will likely be spread by the great mobility of humans."
Aegypti apparently used trade routes to reach Europe, then showed up in the Americas centuries ago, likely thanks to European explorers and colonists. It's now widespread in areas with suitable climates. (For a closer look at the distribution of aegypti mosquitoes in the U.S., see these maps from NASA.)
Albopictus is a global invader, too, but rose to power more recently. Native to Southeast Asia, it was first reported in the Americas in the mid-1980s, followed soon after by Europe and Africa. It can tolerate lower temperatures than aegypti can, giving it an edge in some parts of the world. It has also invaded more of the U.S. than aegypti has, and even outcompetes that species in much of the country.
2. Aedes aegypti and albopictus look similar, but one is a bigger threat to people.
Aedes aegypti (left) and Aedes albopictus (right) are difficult to tell apart by sight, aside from slight differences in dorsal patterns, but each species has its own preferences for habitat, hosts and hunting style. (Photos: CDC)
Albopictus is known as the Asian tiger mosquito due to white bands on its black legs, although aegypti has similar bands. One of the few visible ways to distinguish them is a pattern on the upper back: Aegypti has a "bright silvery lyre-shaped dorsal pattern," the CDC explains, while albopictus has "a single, longitudinal silvery dorsal stripe."
Yet the resemblance belies major differences. Albopictus is more outdoorsy, laying eggs in places like tree holes or cupped leaves, and prefers forests, farms, parks or big backyards over denser urban areas. It's also more of a generalist, biting humans along with other animals, although it is highly anthropophilic in some places. A 2008 study found albopictus mostly feeds on mammals and birds, with top hosts including humans (24 percent), cats (21 percent) and dogs (14 percent). A study of aegypti showed more human hosts (76 to 79 percent), followed by dogs (18 to 21 percent).
"Aedes aegypti is quite unique among mosquitoes that transmit human disease in that these mosquitoes are specifically adapted to people living in urban environments," says Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College. "They generally do not bite other mammals or birds, like Culex or even other Aedes species do. They very much have co-evolved to live in human habitats."
3. Aegypti is sneaky, approaching from behind to bite ankles and elbows.
Only female mosquitoes drink blood, and since they lay eggs afterward, any that are fatally swatted can't pass on their genes. So by killing less stealthy biters over time, people may have helped aegypti become more adept at avoiding our attention.
If a mosquito bites a lot of birds or deer, it might not fully appreciate how dangerous human hands can be. Aegypti has instincts for dealing with us, though, and can be so sneaky it seems almost sinister. It's known to shy away from a person's field of vision, for example, instead flying in from behind to bite ankles and elbows. It also has a subtle bite, often leaving its victims oblivious until an itchy welt later appears.
4. Aegypti is also a 'sip feeder,' and that helps it spread diseases like Zika.
Most mosquitoes get a blood meal from one host in one bite, but aegypti is a "sip feeder," sampling a little blood from lots of people in each feeding session. That may be another trick to help it avoid detection, since it lets the mosquito spend less time on each host, thus reducing its odds of being busted.
But it also makes aegypti mosquitoes highly efficient at spreading human disease. Once a female bites someone who's infected with Zika, she can re-transmit it many times during a life span that lasts up to a month.
5. Albopictus is an aggressive biter, and that's not necessarily a bad thing.
Asian tiger mosquitoes don't share the sneaky nature of aegypti. They're "aggressive biters," as the CDC puts it, which could be related to their diverse diet. Many animals lack the insect-swatting dexterity of humans, so mosquitoes that bite those hosts don't necessarily need as much stealth. That doesn't mean albopictus is dumb or oblivious to danger, but it does improve our chances of noticing it.
There's also another reason why albopictus is less feared than aegypti. The fact that it bites lots of animals, and is less drawn to human dwellings, means more of its blood meals come from wildlife, exposing fewer people to any pathogens it may carry.
6. Neither species travels very far — at least not without help.
Most female aegypti and albopictus mosquitoes spend their entire lives near their birthplace, according to the World Health Organization (WHO). Aegypti adults are only thought to fly an average of 400 meters (about a quarter mile), so their invasions of new regions and continents were only possible by hitching rides with humans.
And now our transoceanic travel is helping viruses like Zika invade new areas, too. While a mosquito could export Zika by hopping on an airplane, an infected person is more likely to do that — especially by flying to a place with aegypti mosquitoes.
7. Both are 'container mosquitoes,' which makes them hard to contain.
Aedes aegypti females look for containers that periodically fill with water, then lay sticky eggs on inner walls above the water line, as pictured above. The eggs hatch when the water level rises. (Photo: CDC Public Health Image Library)
It's perhaps the most common advice for managing mosquitoes: Get rid of standing water. That's where females lay their eggs, and it's where those eggs hatch into larvae that become adult mosquitoes. But while stagnant water is key to mosquitoes, things are a little more complicated with Aedes aegypti and albopictus.
They're "container mosquitoes," a type that breeds in various water-holding vessels, both natural and man-made. After a full blood meal, females produce about 100 to 200 eggs per batch, with up to five batches in a lifetime. They lay those eggs on "damp surfaces in areas likely to temporarily flood," according to a fact sheet by Florida's Institute of Food and Agricultural Sciences (IFAS). "Most often, eggs will be placed at varying distances above the water line, and a female will not lay the entire clutch at a single site, but rather spread out the eggs over two or more sites."
Their eggs — which look like tiny grains of black rice — are sticky, the WHO notes, "virtually gluing themselves to the insides of containers." They wait there until it rains enough to flood the container, prompting them to hatch.
8. Aegypti prefers human-made containers.
Female aegypti mosquitoes are huge fans of humans. They like our blood, they like our homes, and when possible, they'd like to lay their eggs in containers made by us. Old tires are a popular option, but they lay eggs in all kinds of containers, explains Phil Lounibos, an insect ecologist and mosquito expert at the University of Florida.
"Containers suitable for these species may encompass a broad range of delimited habitats that include water storage tanks, storm drains and even (in Puerto Rico) septic tanks," Lounibos writes in an email to MNN. "The importance of different container types for producing adults of these species varies from place to place."
Aegypti often thrive in areas with open sewers, drainage ditches or unpiped water systems. Birdbaths, flower pots, pet bowls and rain gutters also make great aegypti nurseries, as do many other outdoor items — from tarps and grill covers to wheelbarrows and forgotten Frisbees — that can hold even a little rainwater. Both aegypti and albopictus can lay eggs in containers as small as a bottlecap.
9. But both mosquitoes are nefariously flexible.
Albopictus prefers natural containers like tree cavities, or water-holding plants like bromeliads. It's resourceful, though, and like aegypti, can adapt to many kinds of vessels with varying sizes and surfaces. "Generally, rougher surfaces are preferred," Lounibos says, "but when mosquito densities become high, the fine points of such preferences are overwhelmed by the biological necessities of reproduction."
Both can also lay eggs directly on the surface of standing water, Lounibos adds, an adaptation that helps them use containers in which water levels barely fluctuate.
10. Their eggs can become adult mosquitoes in 7 to 10 days.
About three days after feeding on blood, female Aedes aegypti mosquitoes lay eggs on the walls of a container that periodically floods. The eggs then hatch into larvae when flooded by rain. (Photo: Marvin Recinos/Getty Images)
Once aegypti or albopictus eggs are submerged in water, they can develop in two days if weather is warm enough. They hatch into aquatic larvae, which float near the surface to breathe oxygen through a siphon. They eat organic matter in the water, growing up to 8 millimeters (0.3 inch) long before becoming pupae in as little as five days. Pupae develop into adult mosquitoes after a couple more days, and under the right conditions, the full cycle from egg to adult can unfold in less than two weeks.
11. If needed, they can also wait months, even years, to hatch.
Again with the flexibility. Not only do these mosquitoes adjust their development speed based on temperature, but their eggs can also wait a ridiculously long time for water. Aegypti eggs, once laid inside a container, can survive there for months — even if they become desiccated, or extremely dry. Albopictus eggs are hardy, too, although they're "much less resistant to desiccation than aegypti eggs," Lounibos writes, "which is a difference that facilitates co-existence of these species in Florida."
Such tiny, durable eggs clearly make these mosquitoes difficult to control. Rather than trying to scour your yard for mosquito eggs, though, a more effective strategy is to remove or regularly check as many potential breeding sites as you can. And while you may not be able to eliminate every egg or every container, you do have control over another resource mosquitoes need: your blood.
12. Both aegypti and albopictus are day biters, especially around dawn and dusk.
While many mosquito species are nocturnal, Aedes aegypti and albopictus both bite primarily during the day. They're most active for two hours after sunrise and two hours before sunset, although they can bite any time of day.
13. Aegypti does sometimes bite at night in well-lit areas.
That said, these mosquitoes are nothing if not adaptable. Aegypti females have been known to bite after dark if people accommodate them with outdoor lighting.
14. Pesticides aren't a panacea.
A dragonfly eats an Aedes albopictus mosquito in Maryland. (Photo: Eric Stavale/Flickr)
Insecticides are a tempting, and widely used, weapon against mosquitoes. They can be a life-saving buffer for public health in certain contexts, especially warm, humid areas where poverty benefits pests like Aedes aegypti. They're often used by health agencies to curb outbreaks, including recent Zika surges from Brazil to Florida.
Some homeowners also spray insecticides in their yards, and depending on the threat posed by local mosquitoes, that might be wise. But it's often not necessary, and it's almost never sufficient on its own. Many insecticide sprays are broad-spectrum, and while they may be safe for mammals and birds, can kill insects like bees, butterflies, fireflies, praying mantises and dragonflies — themselves an important predator of both mosquito larvae and adults.
In fact, Zika-related spraying in South Carolina recently killed millions of honeybees after officials in Dorchester County used a product called Trumpet, which is extremely toxic to bees. One beekeeper told CNN she lost 46 hives and more than 3 million bees. "Those that didn't die immediately were poisoned trying to drag out the dead," Juanita Stanley said. "Now, I'm going to have to destroy my hives, the honey, all my equipment. It's all contaminated."
If you do opt for insecticides, though, there are options beyond indiscriminate sprays like pyrethroids. Lounibos suggests "biorational" insecticides, a term for toxins that kill a narrower range of insects, often targeting mosquito larvae in water. He cites larvicides such as BTI (Bacillus thuringiensis israelensis) and methoprene, noting they "can be applied to the water without fear of serious adverse environmental effects."
You could also seek help from mosquitoes' natural enemies. Bats eat adult mosquitoes, for example, and may catch some aegypti or albopictus around dusk. Many people set up bat houses in their yards for that very reason. Dive-bombing birds like purple martins fill a similar ecological niche, and can also be lured with free housing. Several aquatic predators eat mosquito larvae, including the aptly named mosquitofish, but as the CDC points out, that only works in larger containers like fountains or ponds, not the tiny ones mosquitoes often choose.
15. Sometimes, the best offense is a good defense.
Aside from insecticides or insect-eating wildlife, what other options do we have?
"If folks want to be outdoors during daylight hours when adults of these species are active," Lounibos explains via email, "a better strategy is the use of repellents and protective attire (long pants and long-sleeved shirts)."
Wearing long sleeves and pants in August may not be ideal, especially in the warm climates where these mosquitoes live, but it can make a big difference in limiting bites. Tightly woven fabric is best, since it's harder for mosquitoes to bite through, and lighter colors can reduce your visibility to them. Combined with effective mosquito repellents — like DEET, picaridin or lemon eucalyptus oil — this is a widely recommended way to fend off mosquitoes, and for good reason.
Mosquitoes also hate wind, so hanging out in a breezy place offers some protection. An electric fan is another option, hindering the insects' flight while also dispersing our exhaled carbon dioxide, which is how mosquitoes locate us.
To keep mosquitoes out of your house, make sure windows and doors stay closed as much as possible — that's one reason why Aedes aegypti tends to be less of a problem in areas with lots of central air-conditioning. Otherwise, make sure your windows are covered by intact screens that can block mosquitoes.
On top of that, survey your property for places where aegypti or albopictus mosquitoes might breed. Regularly changing water in birdbaths and pet bowls is important, but you're probably surrounded by lots of other mosquito-friendly containers, too. And while you can't eliminate them all, every little bit helps.
It's worth noting that simply not being bitten is a form of mosquito control, too. Every bite you get is more than just an annoyance or risk of disease — it also provides fuel for the next generation of mosquitoes. We may never completely win our war on these pests, but we can win daily battles just by making sure no blood is spilled.
Editor's note: This story has been updated since it was originally published in August 2016.