Cover story In the past few decades, the Asian tiger mosquito has travelled from its natural home in Southeast Asia to the ends of the earth, becoming one of the world's most invasive species.
Bites of passage
In the past few decades, the Asian tiger mosquito has travelled from its natural home in Southeast Asia to the ends of the earth, becoming one of the world's most invasive species. Tom Scocca reports from the frontlines of the battle to halt the winged invasion. The picnic cooler, resting up against the back side of a row house in Trenton, New Jersey, was an artefact designed solely for the use of human beings. It had been invented to meet a particular and exclusive set of needs, unique to 21st-century Homo sapiens: to allow the transport of cold beverages to locales without refrigeration - and, once there, to allow those humans, while drinking, to free up their opposable thumbs by setting their beverages down in the wilderness without a spill. For this, humans needed more than a picnic cooler; they needed one whose lid was inset with four cup holders. No one was picnicking along this particular alley, on a sweltering afternoon in a poor part of the city. But Ary Farajollahi, a government entomologist for Mercer County, New Jersey, had spotted the cooler through a fence, and his hunch was that the cup holders were in use - though not by humans. "All right, check this out," he said. Holding one end of a long-handled dipper, he leaned into a gap in the sagging chain-link fence. He hooked the edge of the cooler and dragged it toward him, so that the murky rainwater in the cup holders came into view. "Oh, yeah," Farajollahi said. "There's larvae in there." The larvae, if left alone, would emerge from their plastic cradle in the cup holders as adult Asian tiger mosquitoes, Aedes albopictus, a low-flying, aggressive biter. The Asian tiger mosquito was first classified in 1895 by an Australian entomologist, FAA Skuse, who described a black and silvery-white "banded mosquito of Bengal", which, he noted, was already "a great nuisance in Calcutta". The modern city of Kolkata sits toward the western end of what could, until recently, have been called the natural range of the Asian tiger mosquito: a swath of land stretching from Pakistan to North Korea. Its ancestral habitat is in the forests of Southeast Asia, where it laid eggs in water-filled tree holes or the hollow insides of bamboo stalks, rarely travelling more than a few hundred metres from the cavity where it was born. Over the course of the past two or three decades, however, the Asian tiger mosquito has become considerably, prodigiously more mobile. It has circled the planet, emerging in Trinidad in 1983; Mexico in 1988; France in 1999; Cameroon in 2000; Nicaragua, Greece, Israel and Switzerland in 2003. It has colonised North and South America, Africa, and Europe, until it has become a great nuisance in Brazil, Albania, Nigeria, Mexico and Italy - "the most invasive mosquito in the world", a team of researchers wrote in 2007, in the journal Vector-Borne Zoonotic Disease. The mosquito from the Southeast Asian jungle is also now a great nuisance in Trenton, New Jersey - the greatest present-day mosquito nuisance in a wetland state whose history is written in waves of mosquito infestation. "There was no Jersey Shore before mosquito control," Farajollahi, the county mosquito control superintendent, said - the coastal areas now filled with tourists originally belonged to clouds of hostile mosquitoes, which ranged far inland as well. At the beginning of the 20th century, the state began claiming the land for humans, cutting ditches through the marshes to drain the pests' breeding areas. Farajollahi's division is attached to the county transportation department, an arrangement that draws on the traditions of human-mosquito conflict: big digging machines reshaping the earth and its waterways, transforming mosquito-friendly habitat into land fit for human beings. Nowadays he fights a house-by-house battle in the heart of the city. A transplanted Californian with close-cropped hair and a neat, glossy goatee, Farajollahi wore a tan polo shirt with the Mosquito Control Division logo embroidered on it, and jeans and work boots in the heat. He had been up in the dark before 3am, supervising a truck-mounted insecticide-spraying operation against the adult mosquitoes, but he was poking enthusiastically through backyards in search of larval habitat, clambering through knee-deep thickets of poison ivy. And the dipper kept coming up with larvae. New Jersey is merely the northern fringe of the Asian tiger mosquito's US territory, which now includes the entire southeast, from the Gulf of Mexico across the south and more than halfway up the Atlantic coast. Mercer County and neighbouring Monmouth County, which together form a belt across the middle of the state, are in the second year of a five-year, $3.8 million project, backed by the United States Department of Agriculture, to try to bring the invading swarms under control. The agriculture department "wanted to go to an area that was at the edge of the distribution of the mosquito, rather than at its evil heart", said Daniel Strickman, an entomologist overseeing the project for the federal government. That Trenton lacks bamboo groves and jungle flora poses no obstacle to the Asian tiger mosquito. In the United States, Aedes albopictus makes use of vegetation for food and sometimes shelter, but its breeding and dispersal depend on other means entirely. The picnic cooler in Trenton was not a makeshift substitute for a tree hole; it was precisely what the Asian tiger mosquito was looking for. There is an indigenous American mosquito, Aedes triseriatus, that breeds in the woods and takes blood from chipmunks. As far as albopictus is concerned, the natives can keep the trees. Its life revolves around human-made containers. The American population of Asian tiger mosquitoes entered the country sometime around 1986, through the Port of Houston, in a shipment of used tyres from Japan. It spread out along the interstate highways, wherever tyres were stored or dumped, laying its eggs in those round, sheltered, rubberised cavities supplied by human beings, or in anything else that would hold water. Suddenly, the little mosquito was moving as fast as human commerce. As recently as 2003, Mercer County had not received a single complaint call about Asian tiger mosquitoes; today they are the number-one source of complaints to mosquito control - and those calls don't come from marshlands or swamps but from densely populated areas. The old distinction between mosquito territory and human territory - and the idea that the one can be reclaimed from the other - has collapsed. Now humans come first and mosquitos follow. "The albopictus are colonising human-made habitats," Strickman said. The Asian tiger mosquitoes breed in artificial containers, shelter and feed in domestic shrubbery. Given a choice of animals, they will take their blood meals from humans."Albopictus here," Strickman said, "is acting like a domesticated mosquito."
The domestication of animals has traditionally been viewed as one of mankind's great and defining accomplishments. Primitive humans bravely and ingeniously decided to tame the wolf into a loyal companion, to capture wild horses to serve as steeds, to wrestle the wild aurochs to the ground and make it submit to the yoke, laying the foundations of agriculture and civilisation alike. But domestication has not all been by human design, or even with human consent. For many species (including, in all probability, some of our cherished companion animals), humans were not lord and master, let alone friend: they were habitat. Wherever humans made their homes, they bent the prevailing natural conditions into something new and unusual: humans stored grain in large quantities all year round, kept structures warm through the winter, burnt and flattened the land, and drove off larger predators. And this array of comforts and opportunities moved wherever humans did. Living things evolved to settle into this new niche, whether humans had invited them or not: the house mouse and the house sparrow, the bedbug and cockroach, the dust mite, the housefly, the Norway rat and the pigeon. With them came their own predators, parasites and diseases. Humanity has not subdued nature so much as been infiltrated by it, co-opted to generate food and water and shelter to an entire roving ecosystem. For the Asian tiger mosquito, the benefits of human settlement include a near-limitless supply of cavities and containers of every shape and material, all of them possible breeding reservoirs: styrofoam cups, plastic bottles, metal roof gutters, ceramic saucers. In cemeteries, people not only put out vases, they are thoughtful enough to put water in them. "The containers in which it is found range from tin cans to 55 gallon drums," wrote William A Hawley in a survey of albopictus biology in the Journal of the American Mosquito Control Association in 1988, "and may be made of any material?metal, glass, stone, earthenware, plastic, wood or rubber." "Everyone's got something in their backyard that's going to hold water," Farajollahi said. Aedes albopictus will lay eggs in inflatable wading pools and the bed liners of pickup trucks - in anything but a natural puddle on the ground. The larvae have turned up swimming in the tiny space left in a crushed aluminium can. If you turn some flowerpots upside down to drain them, larvae will hatch in the shallow depression on the bottoms. Within a week, the mosquitoes will grow from millimetre-long eggs to centimetre-long larvae, feeding on whatever is available in the water: bacteria, algae, plant detritus, rotifers, the dead bodies of other larvae. They favour water that is somewhat clean, but not overly so. The Asian tiger mosquito is less fastidious - or more sensible - than its container-breeding competitor, Aedes aegypti, the yellow fever mosquito, which has been known to lay eggs in a spotless drinking glass, Strickman said, where larvae will hatch and starve.
The yellow fever mosquito, which reached North America three or four centuries ago in the water casks of slave ships and which devastated colonial Philadelphia with disease, is in sharp decline in areas now occupied by the Asian tiger mosquito. There are multiple theories about why the older arrival is disappearing. Perhaps the Asian tiger mosquito larvae may out-compete the yellow fever mosquito larvae for food. Maybe the male Asian tiger mosquitoes mate with female yellow fever mosquitoes and produce sterile eggs, while the male yellow fever mosquito can't do the reverse. Possibly the Asian tiger mosquitoes carry a microorganism that is deadly to the yellow fever mosquitoes. It's "incredible how fast it happened, and how thoroughly", Strickman said. Meanwhile, in the countries where the Asian tiger mosquito was the native breed, the newly arrived yellow fever mosquito is taking over - in both cases, the more urbanised and domesticated newcomer is supplanting the older species. The Asian tiger mosquito is capable, in laboratory testing, of carrying and transmitting a variety of viruses, but so far, in the wild, it has been a less effective vector of disease than the yellow fever mosquito, with one notable and alarming exception. An outbreak of chikungunya - a virus that causes high fever and lingering joint pain - on the island of La Reunion in 2005, which infected 266,000 people and killed 248, was traced to a mutation in the virus that enabled it to be transmitted by the Asian tiger mosquitoes. Two years later, a tourist who had been in the Indian Ocean region passed through Ravenna, Italy. There, he encountered a local population of Asian tiger mosquitoes, which had arrived from the United States in a shipment of tyres. The convergence led to Europe's first outbreak of the disease - and raised the possibility not only of chikungunya continuing to follow the mosquitoes around the world, but of diseases such as dengue or yellow fever making the same mutational leap. Usually, when a domesticated species follows humans to a new place, it's the native species that suffer. Rats get onto an island, climb the trees, and start eating eggs until the indigenous birds go extinct - collateral damage of the human experience; pity for the birds. But when Asian tiger mosquitoes arrive, humans are the ones to bear the brunt. Aedes albopictus takes blood in the daytime, flying silently and close to the ground. It particularly favours biting the ankles. "We didn't have a mosquito in this part of the world that bit people during the day," Strickman said. Though human blood is less nutritious for mosquitoes than that of other mammals the Asian tiger mosquito, like the yellow fever mosquito, has adapted to digest it. A full blood feeding lasts something less than three minutes, but Asian tiger mosquitoes are skittish and evasive, quick to break off a meal and strike again. They do not hover at eye level, waiting to be seen and swatted, nor do they announce themselves with a whining sound in flight. Each bite tends to leave a welt. Whether they're bearing diseases or not, the mosquitoes are singularly vicious and obnoxious toward their human cohabitants - biting the very people who provide them shelter. Farajollahi estimates that in nine out of 10 complaint calls about Asian tiger mosquitoes, either the callers or their immediate neighbours are harbouring the mosquitoes on their property. Part of the New Jersey mosquito project involves studying the body mass and activity levels of schoolchildren, to see if the mosquitoes are driving them indoors into sedentary habits. After reaching full size, the Asian tiger mosquito larvae enter a shell-encased but active pupal form, a little brownish jumping bean, which hatches into an adult after a day or two. The adult mosquito's body takes 12 hours to fully harden, but the wings are pumped up and the cuticle is stiff enough for it to fly within 10 minutes. Researchers aren't sure whether the next thing mosquitoes do in the wild is to mate or to get a meal of plant nectar, Strickman said. There are gaps, he explained, in human understanding of mosquitoes' routine: "We pay attention to the things they are doing that affect us." But in the laboratory, females start looking for blood to nourish their developing eggs within the first 48 hours. It takes another two or three days before the eggs are ready to be laid. Sometimes, the females take an extra blood meal in the meantime. When they're not hunting, they lurk in humid shade - most likely in shrubbery or spaces between buildings, another point on which the ethological record is not entirely clear. Once the eggs are ready, the female goes searching for vessels, usually breaking her clutch among multiple containers, placing them above the waterline, where the next rainfall will find them and hatch a new generation. Stopping this process requires somehow restricting the almost inexhaustible supply of containers to breed in - a daunting project. "It looks impossible," Strickman said. He has been part of two successful container-breeding mosquito eradications: eliminating yellow fever mosquitoes from a village in Thailand, and clearing away an Asian tiger mosquito infestation from Santa Clara County, California. "I remember the day we caught the last mosquito," he said. "We spent a year trapping after that, to be sure."
In New Jersey, the practical goal was not eradication, but reduction. Over the summer, Farajollahi and his Mercer County work crews were trying to make one small section of Trenton less congenial. They had isolated three groups of about 1,000 houses apiece, each site approximately half a kilometre square. One site was being left alone, as an experimental control; in another, residents were being taught about the importance of eliminating possible mosquito habitats from their property. This was not a problem that would be solved by reshaping the landscape to suit humans, but by reshaping human behaviour to make it less suitable to mosquitoes. At the third site - a neighbourhood of drooping porches, graffiti, flourishing weeds and litter - Farajollahi and his colleagues were out on patrol, trying to suppress the mosquitoes by every available method: public education, habitat elimination and chemical controls. By the end of the five-year project, their goal is to devise a successful countrywide control strategy, one that can be reproduced and exported to other locations. A large concrete swimming pool, shaped like a boat and sunk halfway into the ground, filled most of one yard. It was batted and moss-grown, with blue paint peeling away, but the inside was dry. Here was one place the mosquitoes would not be breeding. A worker set a backpack spraying rig filled with bacterial toxin on the edge of the pool and brought its chugging motor to life. Slipping it on, he crossed the alley and began applying a mist to the litter and weeds behind an abandoned house, where the ground was strewn with rubbish, scrap lumber and drainpipe - a wealth of nooks and crevices. For life in the landscape around humans, adaptability is in itself a valuable adaptation. The alleys of Trenton were a study in the mosquitoes' willingness to try anything. Farajollahi prodded a post on a chain-link fence to see if the cap at the top came off. If it did, Mosquito Control had discovered, the vertical pipe would hold water, and mosquitoes would lay eggs in it. Several crew members wore shirts that read "Water + 7 Days = Mosquito". Where standing water couldn't be dumped out, workers dosed the containers with two kinds of pellets: a charcoal-coloured one called Altosid, which contained a hormone to block larvae from maturing into adulthood, and a dark green-brown one called Agnique, which would leave a thin film on the surface that interfered with the larval breathing siphon, suffocating them. If they couldn't get into a yard, they would pitch pellets over the fence into the water.
Usually, though, they would get into the yard. New Jersey's mosquito workers have the right to trespass, but there were other limits. In one yard, the body of a partly disassembled motorbike appeared to offer a mosquito-friendly space, but it was hard to be sure, because the view of the bike was blocked by a black-and-white pit bull snarling and lunging against the fence. The crew rounded the block to the front of the row of houses, found the dog's owner and returned to the rear, dosing a flowerpot on someone's porch along the way. "I'm waiting on the man to come and do my yard," the owner said. It was hard to determine when the yard man had last been to visit: the space smelled of dog faeces, and scattered around were a toppled high chair, abandoned exercise equipment, a weathered lawnmower. There was also a white plastic five-gallon bucket, filled to the brim with dark water. A dipper went into the bucket and came out teeming with larvae and pupae of every size and developmental stage. They skittered and tumbled in the greenish-brown broth. "The rain just filled it up," the man said, without conviction. Could they dump it out? "It's dog mess," the man said - the pit bull's waste, another task he was saving for the yard man. Into the bucket went more pellets. The dog had been moved to a smaller pen, where it stood skittishly on top of a plastic kennel. Farajollahi circled around behind the pen, toward the motorbike. The dog made a menacing twitch in his direction. "He's friendly, right?" Farajollahi said. "No," the man said. "She's sneaky as hell." The motorbike had a built-in drain hole, and was clear of water. The mosquitoes thrive on neglect, but they thrive equally on good intentions. They breed in uncovered above-ground swimming pools, and they breed in the water that gathers on swimming-pool covers. A crumpled tarpaulin is the promised land, with watery little breeding pockets everywhere; a flexible corrugated pipe, meant to improve drainage at the base of a downspout, is paradise. Walking back to his truck, Farajollahi pointed out an empty Mylar bag of crisps on the ground. The mosquito crews tried to be judicious about picking up litter, so their roundup of cups and bottles wouldn't devolve into full-time rubbish duty. Originally, they left the potato chip bags behind. But that proved to be a mistake, Farajollahi said: "If they're lying at the right angle..."
The Asian tiger mosquito made its first known appearance in New Jersey on a June weekend in 1995, when one was collected from a light trap near the marina in Keyport, in Monmouth County. Sean Healy, who is implementing the Asian tiger mosquito programme there, took me to the site on a balmy day in early September. The New Jersey light trap, as it is called, uses a design first deployed in 1932: it is a green can with a conical roof, with a 25-watt light bulb inside. Mosquitoes fly to the light and a fan sucks them down into the can. This particular historic trap was mounted on a wooden post, by the corner of a garage covered in tidy seafoam-green shingles. Matching shingles covered the accompanying house, a long-settled mobile prefab with a built-on addition. It was at the end of a cul-de-sac of trim lawns and modest-sized houses. Behind the houses were trees and solid thickets, leading down to an overgrown gully, concealing the marina beyond. How long it took for that single mosquito to show up in the trap is a mystery. The New Jersey light trap is not at all good at catching Asian tiger mosquitoes. As daytime biters, they are indifferent to the allure of electric lights. Nor are they interested in carbon dioxide, which other mosquito traps use to simulate the breath of prey, or in another established lure, a rank tea brewed from grass clippings. Until a few years ago, this made even the most basic question of Asian tiger mosquito control an intractable mystery: the only way to tell if the mosquitoes had been eliminated was to have people sit outside and wait to get bitten. Now there are special traps designed to catch Asian tiger mosquitoes, collapsible black-and-white fabric cylinders that sit on the ground. The mosquitoes hunt at least partly by sight, and the contrast gets their attention. Inside, the traps hold a waxy block composed of a compound that includes lactic acid, acetic acid, and ammonia - a sweaty-smelling mix designed to meet the mosquitoes' finicky tastes. ("It also attracts aegypti quite well," Strickman said.) The trap pulls in somewhere between 20 and 50 mosquitoes on a typical day in an infested area, Farajollahi said, but some have collected more than 500. The mosquitoes' habits likewise protect them from conventional insecticide-spraying campaigns. Fogging trucks go out at night, when other mosquito species are active and when the temperature inversion holds the fine spray near the ground. In the daytime, Farajollahi said, the droplets would rise straight up in the air. But after dark, Aedes albopictus takes shelter and its metabolic rate drops; the insecticide has no effect. The trucks now spread a cocktail of the insecticide sumithrin and another substance called prallethrin, which agitates the mosquitoes and gets them up and flying. "They're kind of a closed little system there that's not very responsive to anything," Healy said. The Keyport Marina consisted of a series of shabby gravel lots, with small power boats parked close together. The area, relatively open and sunny, "doesn't look like traditional albopictus habitat", Healy said. Historically, it had been plagued by New Jersey's usual salt marsh mosquitoes. But to the rear, the gully was strewn with breeding receptacles: old tyres, rubbish, a plastic bucket. Healy poked a dipper into a skip, surrounded by slabs of crumbling styrofoam so dirty they looked like concrete. There were larvae in the water. Across Raritan Bay, in the distance, was the low, green shape of Staten Island, New York, and beyond that, faintly, were the towers of Manhattan. The Asian tiger mosquitoes in the United States are descendants of a Japanese strain whose eggs can survive winter in a temperate zone. When the daylight gets shorter, somewhere between 14 and 13 hours long, female mosquitoes begin laying eggs that will not hatch till the following spring. That time of year was drawing close, Healy said. But even after new mosquitoes stopped hatching, a few hardy ones would hang on. Last year, he said, workers bundled up in coats were still pulling adults out of traps through the end of October. Where did the late-season mosquitoes find nectar? It's possible, Healy said, that they use blood for energy. "There's some evidence," he said, "but it's not very well documented."
The new wilderness is everywhere and nowhere: for the Asian tiger mosquito, it is all the paved and built territory that humans keep expanding, indifferent to the ecology that was there before. "I thought that I would mostly be playing with bugs out in the middle of swamps," Farajollahi said, "and these pristine woodland pools and vernal pools, early in the spring." We had left the neighbourhood site and driven to an abandoned industrial park, through a gate with a hazardous-materials warning posted on it. Chest-high weeds had bored straight up through the asphalt of the car park. Here, the mosquito commission was keeping a study plot. When Farajollahi first encountered the place, the entire area, grounds and buildings alike, had been used as a tyre dump. Teams of state convicts had cleared away most of the tyres, but one pile had been left untouched in one corner of the lot, for research purposes. "If you're Aedes albopictus, you're going to love it back here," he said. "You've got lush vegetation, humidity's high, you've got plenty of shade over there, you've got your larval habitat in the tyres and containers and that sort of thing. And then you've got hosts - from feral dogs, cats, raccoons, humans - free-range humans." Homeless people, in other words, who slept in the buildings. "There were actually beagles that were being kept here by a free-range guy," Farajollahi said, "and he was using them to train pit bulls in the inner city to fight." He led the way back into the shade of the tyre pile, where he had set out buckets of water as experimental habitats. Garbage crunched underfoot. He stooped over a plastic spray-can cap that was sitting on the ground and poured the contents into a clear jar: two more albopictus larvae. Then he poured them back into the plastic cap, letting nature take its course. The air flickered with mosquitoes, like static: Asian tiger mosquitoes; the forest-dwelling native Aedes triseriatus; another invader, Aedes japonicus. Aedes atropalpos, the rock-pool mosquito, which in New Jersey has abandoned rock pools in favour of tyres. More mosquitoes, of the genus Culex. This space, shaped by humans, was rioting with mosquitoes. Without trying, I crushed one into a smear on my shirt. "Here we go," Farajollahi said, raising the back of his right hand and holding it level. A mosquito was perched on the bottom joint of his ring finger, proboscis buried, drinking blood. "That is albopictus. See how small it is? See the bands on its legs? I mean, it really, really is a beautiful, beautiful mosquito." With a quick, gentle brushing motion, he sent it flying off. On the ground, on the way back out of the tyre pile, was a rotting tree limb, with a natural cavity in it?an actual tree hole. Farajollahi poked at it with a plastic dropper. "I haven't got anything out of that," he said. "But, you know, that's the only thing that's left. It's only a matter of time."
Tom Scocca's first book, Beijing Welcomes You, will be published by Riverhead Books next year.