By Meredith Swett Walker
Is there a tiger lurking in your neighborhood? The Asian tiger mosquito, Aedes albopictus, is spreading in the U.S. and its preferred habitat is urban and suburban areas. Unfortunately Ae. albopictus is not simply a nuisance at backyard barbecues — the mosquito is a potential vector for Zika as well as other viruses which pose serious health threats. And many of the techniques that we have successfully used to control other species of mosquitoes are ineffective against Ae. albopictus. U.S. mosquito control programs need to adopt a new toolkit, and fast.
In the latest issue of the Journal of Medical Entomology, mosquito researchers Dr. Ary Faraji and Dr. Isik Unlu review current control techniques for both adult and larval Ae. albopictus. The good news is that there are many tools that may help tame this tiger and the viruses it spreads. But there is no magic silver bullet. Successful control of Ae. albopictus will require an integrated program of several control techniques as well as significant community engagement and cooperation.
A related mosquito, Aedes aegypti, is the main vector for the Zika virus in tropical regions like Brazil. But Ae. albopictus is also capable of transmitting Zika as well as other arboviruses like chikungunya and dengue (“Arbovirus” is a medical term for an ARthropod BOrne virus). Ae. albopictus is somewhat less likely than Ae. aegypti to spread viruses to humans because it also bites animals. But Ae. albopictus has a wider geographic range in the U.S. Because it can tolerate cooler temperatures, it can survive at higher latitudes and has already spread to Vermont, New Hampshire, and parts of Minnesota.
As the common name “Asian tiger mosquito” implies, Ae. albopictus is native to Southeast Asia, not North America. The species is relatively easy for laypeople to identify. Ae. albopictus is smaller than your average mosquito, with striking black and white banded legs and a white stripe down the back of the thorax. These markings are visible without magnification. Ae. albopictus is most active during the day and tends to hang out in shady areas.
This species is closely associated with humans both in its native range as well as in North America. We provide not only bloodmeals but also an abundance of convenient breeding sites. Ae. albopictus is a “container-inhabiting” species, which means it lays its eggs in small, often ephemeral pockets of water. Humans are excellent purveyors of these tiny pools in the form of artificial containers such as bird baths, pet dishes, discarded tires, gutters, and rain laden trash cans.
It is Ae. albopictus’s love of small, human-provided pockets of water that makes it so difficult to fight. Many places in the U.S. have long-standing mosquito control programs, but these programs were designed to combat saltmarsh and floodwater mosquitoes. These species, which include the mosquitoes that transmit West Nile virus, lay their eggs in larger, generally accessible bodies of water like marshes or flooded backwaters. The availability of these breeding sites can be predicted based on seasons, precipitation, and tides, making it relatively easy for mosquito-control managers to locate and treat these sites with insecticides or other control measures.
But control of container-inhabiting mosquitoes is much more difficult. Faraji and Unlu review more than 15 techniques that may be used to control Ae. albopictus larvae and adults, but the authors are very clear that no single technique will be sufficient to control Ae. albopictus. Instead, control requires an integrated mosquito management program employing multiple techniques. Thankfully, there are effective insecticides available and some of these, such as the biopesticide Bacillus thuringiensis (Bt), are very mosquito-specific and environmentally friendly. But a fundamental challenge of controlling Ae. albopictus is finding and accessing their ephemeral and abundant breeding sites, which are often inaccessible because they are on private property.
Area-wide spraying of insecticides like Bt using sprayers mounted on trucks or aircraft can be very effective because the insecticide is carried on the breeze over property lines. But area-wide spraying can also be expensive and requires repeated applications. In addition, many breeding sites may be protected from airborne sprays. For instance, corrugated gutter downspout extensions — a favorite breeding site for Ae. albopictus — are good shelters, and sprays are unlikely to reach the small pockets of water inside them.
Two of the more innovative and promising techniques the authors discuss overcome this challenge by essentially using the mosquitoes themselves to access breeding sites and other mosquitoes. The first of these clever techniques is auto-dissemination of insecticide. There are many variations on this approach. One, called “Auto-Dissemination Augmented by Males” (ADAM), involves treating male mosquitoes with the insecticide pyriproxyfen. Male Ae. albopictus are much better at finding fertile female mosquitoes than even the best mosquito control technician. When the treated males come in contact with females or their breeding sites, they transfer a lethal dose of the insecticide to them. ADAM is currently being tested on Ae. aegypti in California’s Central Valley.
Genetic manipulation can also essentially turn mosquitoes into their own worst enemy. There are a number of different strategies which are attracting positive attention because they are very species-specific and don’t require applications of massive amounts of pesticides. Unfortunately, they are also attracting significant negative attention because, in general, the public has a great deal of anxiety and confusion about genetically modified organisms (GMOs).
One promising genetic technique is the “Release of Insects with Dominant Lethality” or RIDL. In one version of this strategy, male mosquitoes are genetically modified, and their female offspring cannot survive. Their male offspring do survive, and they spread throughout the population. Note that male mosquitoes do not feed on blood, so they do not bite humans and do not transmit viruses. RIDL has been tested on Ae. aegypti in several locations, including Brazil, where it reduced adult mosquito populations by up to 95 percent.
Another genetic strategy involves introducing a bacterium called Wolbachia into the mosquito population. Wolbachia is an insect parasite that lives in the cytoplasm of cells and is transmitted from mothers to their offspring. These bacteria can lead to infertility because sperm from infected male mosquitoes is incompatible with eggs from uninfected females or females infected with a different strain of Wolbachia. The bacteria can also shorten the lifespan of infected mosquitoes, thus reducing the likelihood that they will bite a human and transmit an arbovirus. In some cases, Wolbachia may even suppress the replication and infection rates of the arbovirus itself. Although the use of Wolbachia is very promising, large-scale field data is still needed before it can become a tool in controlling Ae. albopictus.
One of the major impediments to using genetic approaches to control Ae. albopictus is public perception.
“Regardless of the risk assessment, sound science, or environmental safety, the genetic approaches for mosquito control are not perceived positively by the public, who have a general misconception regarding genetically modified organisms and the control approach being considered,” said Faraji. Overcoming the perception problem will require significant community engagement by public health officials and scientists to address the public’s concerns and misconceptions with sound, clearly communicated science.
Regardless of the control techniques used, be they genetic or traditional insecticide applications, Faraji and Unlu argue that community outreach and engagement are an essential part of any control program. The public cannot simply be a passive participant in mosquito control. Property owners may permit technicians to inspect for, and treat, breeding sites on their property. But mosquito control programs can quickly find themselves playing a game of whack-a-mole if those property owners do not take responsibility for monitoring and reducing those breeding sites. Following treatment, owners who are not engaged in the mosquito control process may inadvertently create new breeding sites by leaving water-collecting objects in their yard. Inconspicuous items like the saucer under a potted plant or an overturned Frisbee can become breeding grounds for Ae. albopictus if property owners are not vigilant.
Faraji says that successful community engagement programs typically involve local governments, community leaders, and peers in educating the public and assisting residents in eliminating breeding sites on their properties. Fostering a community’s sense of “ownership” of the mosquito control program is important because a high rate of participation is essential for success. A single property with many untreated breeding sites can become a source of mosquitoes for an entire neighborhood. Faraji and Unlu cite one study demonstrating the effectiveness of using the community service organization AmeriCorps for active public-health education, container elimination demonstration, and services like trash can drilling.
Aedes albopictus and Aedes aegypti co-evolved with arboviruses for millions of years in their native ranges, and they already have the mechanisms to pick up a virus particle, amplify it, and re-transmit it to another host. The authors conclude that given the large, growing populations of these mosquitoes in urban and suburban areas, the outbreak of an arbovirus such as Zika is “a clear and present danger.” While panic is unnecessary and counterproductive, the public must take this threat seriously. Control of Ae. albopictus is possible, but only with extensive public participation. And control must happen soon. According to Faraji, “it is only a matter of time before [an infected] human host steps off a plane and gets bitten by a local mosquito in a new area. This will be the start of local transmission.”
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Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs http://picahudsonia.com and https://citizenbiologist.com or follow her on Twitter at @mswettwalker.