A longitudinal study conducted between February 2021 and June 2022 in Bengaluru city has found how urbanisation affects mosquito ecology and how mosquito species diversity and abundance changes across macro and microhabitats. The study found that the quality of breeding sites determines the abundance and distribution of mosquito species, specifically the ones that cause dengue.
While water containers, discarded grinding stones, tyres and plastic containers served as perfect man-made larval habitats, the way mosquitoes interact with larval habitats in different macrohabitats did play an important role in determining mosquito diversity and abundance. Whereas plantations, owing to diverse habitat types, proved to have high mosquito species diversity and richness compared with high-density populated areas in the city, even barren lands did have microhabitats conducive for breeding.
The results of the study, posted in the medRxiv preprint server, are yet to be peer-reviewed.
In all, six macrohabitats were studied. These include: barren lands, lakes and their surroundings, plantations, and high dense, medium dense, and low dense urban areas. A total of 48 index grids (fixed) were selected with eight replicates of each macrohabitat type, which were surveyed during all four seasons — dry (January-March), pre-monsoon (April-June), monsoon (July-September), post-monsoon (October-December). In addition, 98 random grids representing a macrohabitat type were surveyed during two seasons (April-June and July-September). This allowed sampling mosquitoes in varied ecological niches in the city while surveying fixed grids on a seasonal basis. In each macrohabitat (such as grinding stone, tyres, discarded plastic containers, tree holes) the researchers visually inspected for the presence of mosquito larvae.
A total of 8,717 container breeding sites were inspected by the researchers and 1,619 mosquito larvae were collected from six macrohabitats and nine microhabitats. Of these nearly 1,300 mosquitoes emerged comprising 16 species from five genera. Aedes aegypti was the most dominant species (55%), followed by Aedes albopictus (28%).
While the WHO protocols are restricted to door-to-door larval surveillance and looking for mosquito larvae in containers, the study has demonstrated that urban environments with non-residential locations too harbour ideal breeding sites. Any programme aimed at reducing dengue transmission should also factor in neighbourhood surveillance to prevent and control the rising threat of Aedes species. “We found landscape ecology drives mosquito diversity and abundance even at a small spatial scale, and this could be affecting the localised dengue outbreaks,” says Dr. Farah Ishtiaq from the Bengaluru-based Tata Institute for Genetics and Society, and the corresponding author of the preprint.
“We found man-made artificial containers accounted for over 90% of larval habitats. Water storage containers came out as the most common breeding habitat for Ae. aegypti, but since the containers are actively in use, they serve only as ephemeral habitats due to high disturbance,” she says. The surprise finding was not just the preference for discarded grinding stones as breeding sites but these microhabitats containing the highest prevalence of Aedes larvae. “Discarded grinding stones showed high prevalence of Ae. aegypti and Ae. albopictus larvae. But the prevalence in stagnant water was less as these are open water bodies and more polluted,” Dr. Ishtiaq says.
While plastic containers such as cups and coconut shells were abundant, discarded tyres were not. For Ae. aegypti, tyres and coconut shells did serve as ideal habitats but were limited in numbers. In the case of Ae. Albopictus, albeit at low frequency, plant axils and tree holes served as breeding sites while coconut shells did not.
The heterogeneity in macrohabitat types was masked by the man-made microhabitats, which were ubiquitous in all the six landscapes studied, and these microhabitats played key ecological roles. Habitat preference by the two Aedes species appeared to be driven at the microhabitat level.
Body size of both Aedes species showed a variation in wing length across microhabitat and macrohabitat types. Mosquitoes that emerged from the discarded grinding stones had significantly larger wing length than discarded containers or coconut shells. While large body size females can fly long distances, smaller females cannot fly farther and require multiple meals. The study found that when both Aedes species co-existed in a microhabitat, Ae. albopictus always ended up with reduced wing length.
“Our study relies on outdoor surveillance which highlights the importance of ‘neighbourhood surveillance’ in public places which can help in real-time forecasting of dengue cases in urban areas,” she says. “Our findings show that sampling strategies for mosquito surveillance must include urban environments with non-residential locations. And dengue transmission reduction programmes should focus on ‘neighbourhood surveillance’ as well to prevent and control the rising threat of Aedes-borne diseases.”
