In many urban areas, low-income neighborhoods have more mosquitoes, making residents more vulnerable to mosquito-borne diseases. A new study co-authored by scientists at Cary Institute of Ecosystem Studies found that invasive, nonnative plants, which tend to be more common in low-income areas , boosted mosquito populations in Baltimore, MD, and Washington, DC.
Lead author Sarah Rothman, a former postdoctoral researcher at the University of Maryland, explains, "Leaves that decay in water-holding containers provide the resources that mosquito larvae eat. Our study showed that the leaves from canopy trees more commonly found in low-income neighborhoods helped boost mosquito growth in ways that were measurable and significant."
The research team found these mosquitoes were more likely to survive and reach adulthood sooner. One of the mosquito species also had larger wings as adults compared to mosquitoes reared with leaves more common to higher-income neighborhoods. Wing length indicates body size in mosquitoes, and larger-bodied mosquitoes are better at transmitting disease.
The results, published in the Journal of Medical Entomology , suggest that cities and their residents could reduce mosquito infestations by planting native tree species.
Mosquito mystery
The new paper builds on research that Cary Institute disease ecologist and co-author Shannon LaDeau started in 2011 in Baltimore as part of the Baltimore Ecosystem Study . Over the years, LaDeau and collaborators have investigated how the social and ecological environment shape people's experiences with invasive mosquitoes in urban settings, documenting interactions between socioeconomics, vegetation, and mosquito traits.
Mosquito larvae eat bacteria that grow on decomposing matter. In urban areas, this often means plant litter that falls into water-filled containers such as tires or trash cans. But a few years ago, LaDeau and colleagues noticed a discrepancy: In Baltimore, higher-income areas had more abundant and more diverse tree canopies , whereas low-income areas had more abundant and larger mosquitoes that are better at transmitting disease.
"We wanted to understand if differences in tree canopy might help explain why we found more abundant and larger mosquitoes in some neighborhoods," explained LaDeau, whoco-advised Rothman's PhD research. "Our earlier studies demonstrated that you're more likely to find nonnative plants in neighborhoods with lower housing values and more abandoned lots. These fast-growing species tend to take over when land is vacant, whereas wealthier neighborhoods have the resources to establish and maintain slower-growing native trees. Decades of municipal and state disinvestment in low-income neighborhoods plays an important role in this disparity."
Testing tree leaf teas
To investigate how different tree species affect mosquito growth and survival, the researchers tested mixtures made from the leaves of the most common trees in low- and high-income areas in Baltimore and Capitol Heights, MD, and Washington, DC.
One mixture contained leaves from the two most prevalent trees in low-income areas: tree-of-heaven and princess tree, both introduced species. The second was made from leaves of American elm and red maple, both native species that were among the most widespread trees in wealthier neighborhoods. And the third contained leaves of two species found in both high- and low-income areas: white mulberry (nonnative) and black walnut (native).
After soaking the leaves in water for five days, the researchers added larvae of two species of invasive mosquito: Asian tiger mosquito (Aedes albopictus, which spreads diseases including dengue, Zika, yellow fever, and other diseases) and the common house mosquito (Culex pipiens, a vector of West Nile and other harmful viruses). Some of the cups contained just one of the mosquito species, and others contained both to measure how the mixtures influenced mosquito competition. Over time, the mosquito larvae ate the bacteria that had been breaking down the wet leaves, and the researchers measured how well the young mosquitoes survived and grew.
"The hardest part of doing this type of lab experiment," said Rothman, "was getting hundreds of mosquito eggs to hatch in synchrony so we could start many containers at the same time for comparison."
Novel strategies for managing mosquitoes
"Mosquito growth and survival were significantly enhanced when reared on leaf mixtures that contained at least one nonnative tree species," said LaDeau. "We also found that the nonnative vegetation may reduce competition between the two mosquito species, allowing them to coexist in low-income areas."
The researchers conclude that the disproportionate amount of introduced plants in low-income areas may help support high mosquito populations, potentially putting local residents at greater risk for mosquito-borne diseases.
"Scientists have found multiple reasons that explain why mosquito infestations are worse in low-income areas, often having to do with quantity — for example, more containers to breed in or more rats to feed on," said Rothman. "We're showing that quality matters, too. Municipal governments and residents can use this information when deciding which trees to plant along streets or in yards."
Introduced plants may have boosted mosquito populations because of their high nitrogen content, which not only makes them easier to digest, but also helps the leaves decay faster, making nutrients available to microbes sooner compared to leaves from native plants. Another possibility is that, because the invasive plants and invasive mosquito species in this study shared native habitats in China, the mosquitoes may be adapted to thrive among these plants.
As a next step, the researchers hope to repeat their tests with other tree species, to see if the findings hold up, said Rothman.
Interestingly, cups with native and nonnative leaves supported similar microbial abundance, but this experiment did not examine which types of microbes were present in the different treatments. "In future studies, we aim to dive deeper into the microbial communities to see whether certain types of microbes make better or worse food sources for mosquitoes," said coauthor Jane Lucas , a community ecologist at Cary Institute.
LaDeau added that future studies should test whether the effects found in this study translate into greater mosquito survivorship and fitness in real-world neighborhoods, and whether leaf types influence mosquitoes' ability to spread disease.
"Our findings suggest that mosquito control agencies could better predict where infestations may occur by surveying the local vegetation," said LaDeau. "They also provide hope that governments and city residents can protect public health in low-income communities by replacing nonnative plants with native ones."
Authors
Sarah Rothman - University of Maryland
Shannon LaDeau - Cary Institute of Ecosystem Studies
Jane Lucas - Cary Institute of Ecosystem Studies
Paul Leisnham - University of Maryland
Funding
The research was supported in part by the National Science Foundation (grant DEB-1824807), the Cary Institute of Ecosystem Studies through the Lang Assael Family Science Innovation Fund and the Bentley Holden Scholarship, and the Washington Biologists' Field Club.
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Cary Institute of Ecosystem Studies is an independent nonprofit center for environmental research. Since 1983, our scientists have been investigating the complex interactions that govern the natural world and the impacts of climate change on these systems. Our findings lead to more effective resource management, policy actions, and environmental literacy. Staff are global experts in the ecology of: forests, freshwater, soils, cities, and disease.
