Native to Southeast Asia, the Asian tiger mosquito (Aedes albopictus) showed up in Houston, Texas, in 1985. By 1986 it had reached St. Louis, Missouri, and Jacksonville, Florida. Today, the aggressive, daytime biter can be found in all of the southern states and as far north as Maine.
Ae. albopictus has an affinity for humans and is also a vector for human disease, says Kim Medley, interim director of the Tyson Research Center at Washington University in St. Louis.
“Even Darwin, in the 1800s, was interested in how species travel from point A to point B.”
The mosquito arrived in the US in a shipment of used tires from Japan. Ae. albopictus lays eggs that can survive even if any water evaporates, which makes them very easy to transport, says Medley. “It’s widely accepted that global trade and travel have led to many species introductions,” she says.
“But how introduced species spread and adapt to novel conditions after introduction is less well understood,” she says.
We’re moving them around
To reconstruct what happened, Medley and her colleagues turned to the new discipline of landscape genetics. Correlating genetic patterns with landscape patterns, they concluded that the mosquito had traveled by human-aided “jump” dispersal followed by slower regional spread.
The jumps occurred when mosquitoes hitched a ride in cars or trucks, traveling in style up major highways.
Their study, published online in Molecular Ecology, is one of only a handful of landscape genetics studies to track an invasive species and the first to detect hitchhiking.
Medley’s concern is that human-aided dispersal will accelerate the adaptation of the mosquitoes and the viruses they can carry to their new surroundings and to one another.
So far this year the state of Florida has reported 11 cases of locally transmitted chickungunya, a disease original to Africa that wasn’t seen in the Americas until 2013.
“The movement of invasive species by human-aided transport can have far reaching consequences,” Medley says.
Looking at a map of the current range of Ae. albopictus in the US, it is impossible to know how the mosquito spread from its point of introduction, although it could hardly have been by wing power alone, since an adult flies less than a kilometer in its lifetime.
“Even Darwin, in the 1800s, was interested in how species travel from point A to point B,” says Medley. “But measuring dispersal rates by hand is nearly impossible,” she says. “I’ve tried it. You can dust mosquitoes and release them and then try to recapture them, but capturing enough of the releases to reach any solid conclusion is really difficult.”
So to figure out how Ae. albopictus spread from the tire pile in Houston, the scientists used a technique that relies on high-tech tools such as genotyping and GIS. First introduced by French scientists in 2003, landscape genetics provides a way to rigorously test competing hypotheses for dispersal.
As a first step they had to establish the genetic structure of the US Ae. albopictus population. A container mosquito, Ae. albopictus lays its eggs just above the waterline in old tires, flower pot saucers, water bowls, bird baths—and cemetery flower vases.
To sample the mosquito population the scientists collected larvae from abandoned flower vases in cemeteries both on the edge and within the core of the mosquitoes’ US range in both rural and urban areas.
“The green cylinders with the spike on the bottom are the best for collecting immature stages of mosquitoes,” Medley says.
The immature mosquitoes were raised to adults in the laboratory so the species could be accurately identified (there are 174 recognized species in the US) and then the DNA was extracted from clipped legs and typed at nine different microsatellite locations.
The scientists then compared the genetic structure of the mosquito population to that predicted by 52 different models of mosquito dispersal that variously took into consideration habitat and highways.
It turns out that gene flow over long distances was correlated with highways and bodies of water. People had carried mosquitoes from the core of their range to its edge along highways, likely by semi-trailers or in cars. Wetlands and lakes were important, not because they are breeding sites, but because they tend to occur in areas where frequent rainfall refills artificial containers and supports mosquito growth.
The scientists also looked more closely at what was happening at the range edge. Because Ae. albopictus lays eggs in treeholes and is often found resting at forest edges, they expected forests at the northern edge of the mosquitoes’ range to act as natural corridors for dispersal.
It turned out forests were barriers rather than corridors, perhaps because Ae. albopictus had not been able to displace the native treehole mosquito, Ae. triseriatus.
The forest finding is an interesting example of the ability of landscape genetics to test assumptions, Medley says.
Taking mosquitoes seriously
Ae. albopictus is now well established in the US, so what bearing do the findings have on what we do now? Medley suggests that they should inform mosquito control efforts, particularly at the range edge where the population is fragmented and sub-populations blink in and out naturally and as a result of local eradication.
“If we eliminate a local population but leave dispersal routes open, we’re just going to have re-introductions,” she says.
“But,” she says, “my broader concern is the effect of human-aided dispersal on evolution, both the adaptation of the mosquito to northern climates and its co-evolution with chikungunya. By stopping human-aided dispersal of the mosquitoes, we might be able to prevent the further spread of the mosquitoes and the diseases they vector. ”
To shut down the dispersal routes, however, we’d have to take the risk more seriously than we now do.
After all, one of the containers Ae. albopictus favors are those small pots of “lucky bamboo” sold on Amazon and eBay, and at Target, Home Depot, Walmart, Walgreens, and many other retail outlets.