Invasive trees are boosting mosquitoes in low-income neighbourhoods of Baltimore and Washington, D.C.

Low-income urban neighbourhoods often experience heavier mosquito burdens than wealthier areas, which can leave local people more exposed to mosquito-borne illness.

A new study highlights a frequently missed contributor: invasive trees. These nonnative species occur far more often in low-income communities and appear to be actively increasing mosquito numbers in Baltimore and Washington, D.C.

The research was carried out by a team that includes scientists from the Cary Institute of Ecosystem Studies.

“Leaves that decay in water-holding containers provide the resources that mosquito larvae eat,” said lead author Sarah Rothman, a former postdoctoral researcher at the University of Maryland.

“Leaves from canopy trees – more commonly found in low-income neighborhoods – helped boost mosquito growth in ways that were measurable and significant.”

Invasive trees give mosquitoes an edge

The team found that mosquitoes reared on leaves from nonnative trees were more likely to make it through to adulthood. They also developed into adults more quickly than mosquitoes raised on leaves from trees more typical of wealthier neighbourhoods.

Under these conditions, one of the two mosquito species in the study also produced adults with longer wings.

Wing length is a well-established proxy for mosquito body size. Bigger mosquitoes are often better at spreading disease, so this result carries meaningful public health implications.

Taken together, the findings suggest a practical route for action. By choosing to plant native tree species rather than nonnative varieties, councils and residents could potentially lower local mosquito infestations.

At present, nonnative trees are the dominant species in many low-income neighbourhoods.

Part of a broader project

This study extends research begun in 2011 by Cary Institute disease ecologist and co-author Shannon LaDeau in Baltimore, as part of the wider Baltimore Ecosystem Study.

Across the years, LaDeau and colleagues have examined how social conditions and ecological factors shape people’s experiences of invasive mosquitoes in urban settings.

Their research maps links between socioeconomic context, vegetation and mosquito behaviour.

In cities, mosquito larvae typically rely on bacteria that colonise decaying plant material-often leaf litter that collects in water-filled containers such as discarded tyres or rubbish bins.

Where you live can matter

Some years ago in Baltimore, LaDeau’s team observed a pattern that did not initially make sense. Affluent neighbourhoods tended to have tree canopies that were both denser and more diverse, yet lower-income neighbourhoods still had more mosquitoes overall.

Those lower-income areas also hosted more of the larger mosquito species, which are stronger disease vectors.

“We wanted to understand if differences in tree canopy might help explain why we found more abundant and larger mosquitoes in some neighborhoods,” LaDeau said.

“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 what mosquitoes thrive on

To find out how different tree species influence mosquito survival and development, the researchers created several leaf mixtures using the most common local trees.

They gathered leaves from low- and high-income areas across Baltimore, Capitol Heights (Maryland) and Washington, D.C.

One mixture paired tree-of-heaven and princess tree, the two most widespread tree species in low-income areas. Both are introduced, nonnative species.

A second mixture used American elm and red maple-native species that are more dominant in wealthier neighbourhoods.

A third mixture combined white mulberry (nonnative) with black walnut (native). Unlike the other pairings, these trees appear in both high- and low-income areas.

Mosquitoes thrived on invasive trees

The team steeped the different leaf mixtures in water for five days. They then added larvae from two invasive mosquito species: the Asian tiger mosquito and the common house mosquito.

These mosquitoes can transmit diseases such as dengue, Zika, yellow fever and West Nile virus.

Some containers held larvae from only one species, while others contained both species together. This design let the researchers assess how each leaf mixture influenced competition between the two mosquitoes.

As the larvae fed on bacteria growing on the decomposing leaves, the researchers monitored survival and growth.

“Mosquito growth and survival were significantly enhanced when reared on leaf mixtures that contained at least one nonnative tree species,” LaDeau said.

“We also found that the nonnative vegetation may reduce competition between the two mosquito species, allowing them to coexist in low-income areas.”

Why some trees help mosquitoes

The researchers conclude that introduced plants are far more common in low-income neighbourhoods. In turn, these plants are likely helping to maintain higher mosquito populations, increasing residents’ risk of mosquito-borne disease.

“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,” Rothman said.

“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.”

One potential explanation is that nonnative plants may contain relatively more nitrogen.

That could make the leaves easier for larvae to digest while also speeding decay. In that case, nutrients would reach microbes sooner than they typically would from native leaves.

Another possible mechanism relates to shared evolutionary history. The invasive plants and mosquito species examined in this work originally overlapped in China.

As a consequence, the mosquitoes may simply be particularly well adapted to doing well alongside these specific plants.

The mosquito puzzle is not solved

Next, the researchers plan to repeat the experiments using a wider range of tree species to test whether the pattern holds more generally.

One aspect of the current results raised a further question.

Cups containing mixtures of native and nonnative leaves produced similar overall levels of microbial abundance. However, the study did not identify which microbes were present under each treatment.

“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 co-author Jane Lucas, a community ecologist at Cary Institute.

Future work should also examine whether the effects seen here translate into improved survival and fitness for mosquitoes under real neighbourhood conditions.

The researchers also suggest that mosquito-control agencies could improve predictions of where outbreaks will occur by surveying local vegetation.

They add that replacing nonnative plants with native species could help safeguard public health in low-income communities.