(Beyond Pesticides, September 16, 2020) Disease carrying mosquitoes are more likely to flourish in areas being altered by human activities, according to new research published by scientists at Oregon State University. With climate change facilitating the spread of mosquitoes into new regions throughout the world, it is critical to understand the drivers of mosquito-borne disease in order to establish effective mitigation measures. “People care a lot about what environment a lion needs to succeed in; we’ve researched that extensively. But people don’t do that with mosquitoes. We don’t understand them as a group of species and how their ecology differs between species,” said study co-author Brianna Beechler, PhD, a disease ecologist and assistant professor of research in Oregon State University’s Carlson College of Veterinary Medicine.
Authors of the study note that most mosquito collection occurs opportunistically, with samples taken at known mosquito breeding sites. To better understand mosquito spatial ecology, scientists conducted paired sampling at locations inside and outside South Africaâ€™s Kruger National Park, the largest nature preserve in the country. Each sample location inside the park was paired with another sample from a similar location (in terms of landscape and climatic conditions) in developing areas outside of the park. Human disturbance was measured by five factors, including (i) pesticide use, (ii) nutrient loading, (iii) human population density, (iv) biomass of grazing animals, and (v) loss of vegetation.
While these factors are all well known hazards for wildlife, researchers determined that disease vector mosquitoes are one important exception. Unsurprisingly, each of these impacts are significantly higher, by orders of magnitude, outside the park than inside. It followed that mosquito abundance outside the part is determined to be an average 2.9 times (ranging between 1.5 and 10 times) greater than paired sites of similar layout inside the national park.
Sheer numbers are merely half the story. Scientists also observed changes in the relative abundance of certain species of mosquitoes. Disease carrying mosquitoe populations are much higher outside of the park than inside, consistently accounting for roughly 80% of the difference in community composition between paired sites.
Although this research took place in South Africa, the authors indicate their findings are broadly applicable. â€śGiven the global extent and intensity of the investigated anthropogenic pressures, these results are likely relevant for a wide array of vector-borne pathogens and provides a mechanism for the association between ecosystem degradation and disease,â€ť the study reads.
In light of the results, certain impacts are more easily explained than others. Pesticide use, for example, can disrupt the trophic food web, killing of mosquito predators. A study published last year found that in the context of incessant pesticide use mosquitoes were able to out-breed predator damselflies. Nutrient loading and eutrophication can likewise result in the die-off of important species of aquatic predators. “It seems to suggest that disease-carrying mosquito species certainly did better in human-altered environments,” Dr. Beechler notes in a press release. The author further indicates that more study is needed to ascertain the ecological conditions that benefit various mosquito species.
This study has critical implications for mosquito management in states and communities throughout America. It underscores the importance of a science-based, ecological mosquito management program that takes local ecology into consideration and places and emphasis on habitat manipulation, a practice that naturally reduces mosquito populations through alterations to the landscape or built environment.
The City of Boulder, Colorado has long led the way in mosquito management, and has recently taken its approach to the next level in a way that appears aligned with the results of the study, and should act as a model for communities throughout the country. In 2019, the city began working with an ecologist to conduct wetland surveys and assess local biodiversity in mosquito breeding habitats. Preliminary results have already found when habitat supports fish populations, mosquito larvae are usually absent. And when damselflies and dragonflies are present, adult mosquitoes are often absent.
A sound approach to mosquito management is science-based and prioritizes preventive measures. These measures include surveillance, monitoring, public education on eliminating breeding sites and personal protective actions, consideration of local ecology, habitat manipulation, and larviciding with biological materials. Any pesticide use should always be considered a last resort. Not only because of the threats these chemicals pose to human health, but as the present study shows, because pesticide use has the potential to shift mosquito species composition toward increased populations of disease carrying mosquitoes. Community leaders must focus what is truly effective for long term health and safety, and resist the propensity to spray toxic pesticides to assuage resident concerns, and make it seem as though something is being done.
If youâ€™re concerned about spraying in your community, begin your education by reading Beyond Pesticides Public Health Mosquito Management Strategy for Decision Makers and Communities. See Beyond Pesticides Mosquito and Insect Borne Diseases program page for additional information. Reach out for additional assistance, and stay tuned for new resources aimed at encouraging safer mosquito management in local communities.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.