A new study of the local habitats of the Asian tiger mosquito (Aedes albopictus) in five neighborhoods of Baltimore, Maryland, shows precipitation, number of abandoned buildings, and vegetation were the three most influential factors affecting the mosquito’s nearby abundance. (Photo credit: Ary Farajollahi, Bugwood.org)

By Meredith Swett Walker

A neighborhood watch generally keeps an eye out for potential burglars and vandals, reporting them to local law enforcement when spotted. But if you want to improve safety and health in your neighborhood, it may be wise to keep an eye out for small pockets of water that collect in objects like discarded food containers, recycling bins, and garden planters. These water sources attract unwelcome visitors that can make your neighborhood an itchier, less healthy place.

Meredith Swett Walker

In research published this week in the Journal of Medical Entomology, a team led by Eliza Little, Ph.D., of Columbia University, examines how socio-economic, ecological, and climatic factors interact to drive populations of the Asian tiger mosquito (Aedes albopictus) in an urban environment. Their project is part of the Baltimore Ecosystem Study at the Cary Institute and is supported by the Dynamics of Coupled Natural and Human Systems Program of the National Institutes of Health.

Ae. albopictus thrives in temperate climates and is widely distributed in the eastern and central United States as well as parts of the southwest. The species is adept at taking advantage of human-altered environments, and humans are its preferred targets.

The Aedes albopictus mosquito is widespread in the eastern, central, and southwest United States, as of 2016. (Image credit: Centers for Disease Control and Prevention)

Ae. albopictus is a particularly maddening mosquito because it is active and bites during the day when people generally spend the most time outdoors. More importantly, Ae. albopictus may be a vector for viruses including dengue, chikungunya, Zika, and West Nile.

This species is also very difficult to control. Because it is active during the day, night-time insecticide treatments that are used to control other mosquito species are ineffective in controlling Ae. albopictus. Likewise, reduction or treatment of standing water is also less effective on this species because it is a container breeding mosquito. Ae. albopictus can breed in tiny pockets of water that may collect in natural cavities as well as manmade containers like rain gutters and discarded food packaging. Eliminating these manmade pockets of water is extremely difficult.

Aedes albopictus mosquito larvae can often be found in water in functional manmade containers and objects such as these. (Portion of photo by Bradley Gordon/Flickr.)

Scientists have a pretty good idea of where to find mosquitoes on larger spatial scales such as counties or towns. But habitat suitability for Ae. albopictus varies at a much finer scale than that, and control efforts need to happen at this same fine scale. So, Little and colleagues looked at field-collected data on containers (potential mosquito breeding sites), infrastructure, vegetation, precipitation, and abundance of larval and adult Ae. albopictus across five neighborhoods of varying socio-economic status in West Baltimore. Their aim was to determine what factors best pinpoint city blocks with large populations of Ae. albopictus.

Overview maps: (A) Location of Baltimore, Maryland; (B) Location of West Baltimore neighborhoods and GHCN (Global Historical Climatology Network) weather station within the city limits of Baltimore; and (C) Close-up view of the neighborhoods showing the spatial distribution of block clusters within neighborhoods, street segments surveyed for neighborhood attributes, parcels sampled for juvenile mosquitoes, and location of BG-Sentinel traps for adult surveillance. (Originally published in “Socio-Ecological Mechanisms Supporting High Densities of Aedes albopictus (Diptera: Culicidae) in Baltimore, MD,” by Little, et al.)

The researchers found that the three most important variables determining where Ae. albopictus hotspots would occur were precipitation, number of abandoned buildings, and vegetation. The more abandoned buildings on a block, the more likely it was to also have semi-permanent dumping grounds and an abundance of litter. These blocks were also less likely to have regular garbage collection. Trash provides an abundance of containers that can collect water and serve as Ae. albopictus breeding sites, so these blocks had high populations of Ae. albopictus when conditions were wet. If abundant vegetation was also present, these areas could become hot spots for the shade-loving Ae. albopictus.

Trash provides an abundance of containers that can collect water and serve as Aedes albopictus breeding sites. (Photo credit: Eliza Little, Ph.D.)

But, when precipitation was low, blocks with fewer abandoned buildings and higher socio-economic status had higher populations of Ae. albopictus than did blocks with more abandoned buildings. The researchers believe this is because these areas receive artificial precipitation from sprinkler systems and other sources, so they are less dependent on actual rain to create pockets of water. While these areas had less trash, other types of “functional” or “structural” containers were present. Functional containers included items like watering cans and recycling bins that are stored outdoors and can collect water. Structural containers are permanent features like bird baths and planters.

As far as controlling Ae. albopictus, Little says reducing breeding sites is critical, but how these are reduced may vary according to the socio-economic status of a given neighborhood.  For instance, Little notes that “cleaning up trash in lower socio-economic status neighborhoods may provide the greatest reduction of mosquitoes.” But in higher socio-economic status neighborhoods, the focus needs to be on structural and functional containers. In these areas, “informing residents as to the potential consequences of supplemental watering without reducing water-holding containers could be an important component,” says Little.

This research also has important implications for urban greening initiatives. Increasing shade and greenery makes urban neighborhoods more pleasant for their human residents but also for mosquitoes. Little warns that “increasing vegetation without decreasing immature development habitat (containers) may lead to a local increase in mosquitoes.”

Understanding how mosquito habitat varies on a fine-scale will help mosquito control professionals achieve more effective control and reduce the spread of mosquito-borne diseases. It will also help residents make their neighborhoods healthy places to enjoy time outdoors.

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 Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.