According to new research published in the Journal of Medical Entomology, low-income urban neighborhoods in Baltimore and Washington, D.C., have higher numbers of mosquitoes, partly because of the trees growing on their streets and vacant lots.The study, co-authored by scientists at the Cary Institute of Ecosystem Studies and led by Dr. Sarah Rothman, a former postdoctoral researcher at the University of Maryland, shows that rotting leaves of invasive, non-native plants commonly found in poor areas provide mosquito larvae with a rich food source. This diet helps young insects live longer, grow faster and, in some cases, develop larger bodies as adults, making them better able to spread dangerous viruses.
money gap in tree cover
For years, disease experts have noticed a puzzling pattern in Baltimore. Wealthy areas had dense, diverse trees, yet poor areas were constantly plagued by large swarms of aggressive, disease-carrying mosquitoes.The gap comes from years of public and state underinvestment. Slow-growing native trees such as elm and maple are regularly planted and cared for in prosperous areas. On the other hand, vacant lots and neglected sites in low-income areas are often overgrown with fast-growing, non-native weeds and trees.“We wanted to understand whether differences in tree canopy might help explain why we found more abundant and larger mosquitoes in some neighborhoods,” said co-author Dr. Shannon LaDeau, a disease ecologist at the Cary Institute who has studied the environment of Baltimore since 2011.“
Mosquitoes reared on leaf litter were more likely to survive in low-income areas (Credit: Sarah Rothman)
How does the water in leaves give birth to big insects?
Mosquito larvae do not eat leaves directly. Instead, they eat bacteria that break down plant fragments that sit in water-filled containers, such as old tires, buckets and trash cans.To test how local plants alter this food chain, the team collected fallen leaves from common trees in Baltimore, Washington, DC, and Capitol Heights, Maryland. They created three different “leaf teas” in the laboratory:
- Mix of low-income neighborhoods: Leaves of two common non-native species, tree of heaven (Alanthus altissima) and princess tree (Paulownia tomentosa).
- Mix of high-income neighborhoods: Leaves from two native trees common in wealthy areas, American elm (Ulmus americana) and red maple (Acer rubrum).
- General Mixture: Leaves of species found in both areas, white mulberry (Morus alba, non-native) and black walnut (Juglans nigra, native).
The researchers soaked the leaves for five days before they laid mosquito eggs. They tested two invasive species: the Asian tiger mosquito (Aedes albopictus), which spreads dengue, Zika and yellow fever, and the common house mosquito (Culex pipiens), which spreads West Nile virus.“The hardest part of doing this type of laboratory experiment was preparing hundreds of mosquito eggs at once so that we could start multiple containers at the same time for comparison,” Rothman said.
unexpected lab results
The results showed a clear link between non-native leaves and strong pest growth. Larvae reared in a mixture of leaves with at least one non-native species grew faster and were more likely to survive.The non-native leaf diet also reduced competition between the two mosquito species, allowing both to live together in the same water container. Female Asian tiger mosquitoes reared on non-native leaf tea also had longer wings, reflecting larger body size. Larger adult mosquitoes generally live longer and bite more often, making them better at spreading disease.Researchers believe there are two main reasons for this increase. First, invasive plants often contain excess nitrogen, causing their leaves to break down early and leaving nutrients for the bacteria early in the larvae’s life. Second, because both these invasive trees and the mosquitoes come from similar habitats in Asia, the insects may naturally fare better by eating those specific leaves.
New ideas for mosquito control
In the past, public health officials believed that high numbers of mosquitoes in low-income areas were mostly caused by physical problems, such as excess garbage lying around or more stray animals that provide food for mosquitoes.“Scientists have found a number of reasons why mosquito infestations are worse in low-income areas, often related to their quantity – for example, more containers for breeding or more rats to eat,” Rothman said. “We’re showing that quality also matters. Municipal governments and residents can use this information when deciding which trees to plant along roadsides or in yards.”Research shows that simple city planting schemes can bring huge public health benefits. City councils can help cut mosquito numbers by replacing invasive plants with native young trees.Looking ahead, the research team plans to test other tree species to see if the findings hold true in larger urban areas. Co-author Dr. Jane Lucas, a community ecologist at the Cary Institute, said that while total bacteria levels were similar in the test cups, future work will look at the specific types of bacteria that thrive in different leaf mixes.“Our findings suggest that mosquito control agencies can better predict where infestations may occur by surveying local vegetation,” Ladeau said. “They also provide hope that governments and city residents can protect public health in low-income communities by replacing non-native plants with native plants.”The title of the research is Leaf litter characteristics of low-income urban blocks improve population performance of Aedes albopictus (Diptera: Culicidae) and Culex pipiens mosquitoes.