Protecting trees might not seem like a public health strategy, but new research suggests it could be especially in the tropics. The Stanford University investigation shows that in Costa Rica, even modest patches of tree cover can reduce the presence of invasive mosquito species known to transmit diseases like dengue fever.

The illness often brings flu-like symptoms and can escalate to severe bleeding, organ failure, and even death without proper medical care. The findings can inform land use decisions and tree preservation strategies in rural areas, according to the researchers.

“We already knew that small patches of tree cover support biodiversity for a wide range of plants and animals in this region," said study lead author Johannah Farner, Ph.D. Student in Biology at the Stanford School of Humanities and Sciences. "It turns out to be true for mosquitoes too, and has the upside of keeping out a disease-carrying invasive species.”

According to Farner, it may sound counterintuitive to suggest that humans should protect the habitat for mosquitoes. "But making sure that the many native mosquito species that do not spread disease can stick around can help prevent dangerous invasive species from moving in," she added.

Using field observations and satellite data on land cover for a patchwork of forests, farms, and residential areas in southern Costa Rica, the researchers found the presence of the Aedes albopictus mosquito, a dengue vector, decreased in areas with more tree cover while the total number of mosquito species increased.

Costa Rica has numerous mosquito-borne diseases and two invasive mosquito species serving as vectors. The forests surveyed in the study hosted a high diversity of mosquito species, none of which were the dengue vector Aedes albopictus.

Residential areas, by contrast, had lower overall diversity and were far more likely to harbor the invasive, disease-spreading species. Agricultural areas fell somewhere in between, with outcomes seemingly tied to the intensity and type of land use.

Natural habitats exist alongside agriculture and development in rural areas throughout the world. In Costa Rica and beyond, these areas can provide pathways to conserving biodiversity. The study’s findings offer a potential win-win strategy: protecting trees can help conserve biodiversity while also reducing the likelihood of disease transmission. That is good news in the face of warmer temperatures, changes in rainfall, and human activity that are enabling the spread of mosquito-borne illnesses to new places often unprepared to deal with them.

The researchers emphasize the need to do more research to understand how other vector species respond to increased tree cover and what factors beyond tree cover contribute to dengue transmission.

The study underscores the continued importance of forest reserves, which remain critical biodiversity strongholds and natural buffers against disease. Still, researchers caution that planting trees outside of forests should be viewed as a complement, not a replacement, for conserving larger natural areas.

"We need to know more about what drives dengue in rural tropical areas,” said study senior author Erin Mordecai Associate Professor of Biology, at Stanford School of Humanities and Sciences. “This work shows that forests and tree cover can reduce risk, but identifying other land use types that sustain vector populations is the next frontier for controlling this rapidly expanding disease.”