Diversity and spatiotemporal distribution of mosquitoes (Diptera: Culicidae) with emphasis on disease vectors across agroecological areas of Kerala, India

Mosquito species composition and abundance in the study districts of Kerala, India

This study elaborates on the results of the extensive mosquito survey undertaken, indicating a rich and diverse mosquito fauna, with 108 species identified across 28 genera (Supplementary file no. 1) from a total mosquito sample of 12,535, highlighting significant biodiversity within the districts. The sunburst chart illustrates the hierarchical distribution of species within each genus, with vectors and non-vectors (Fig. 1). A total of 14 vectors²⁷ of different MBDs were collected during this study. From the genera collected, Culex exhibited the highest species richness (25.0%), contributing a substantial proportion to the total diversity, followed by other genera such as Anopheles (12.9%), Stegomyia (10.2%) and Uranotaenia (9.3%).

Distribution of mosquito and vector species status: the chart illustrates the hierarchical structure of mosquito species, categorizing them by genus and species and also highlighting with distinct colours for vectors and non-vectors.

The majority of specimens (67.92%) were obtained by immature collection from a total of 777 habitats, and the remaining 4,021 (32.1%) were adult mosquitoes collected from 422 sites using various adult collection techniques. Of the total mosquitoes collected (adults and emerged mosquitoes) from all five districts, 49.6% were male and 50.4% were female (Supplementary file no. 3). From these collections, 47 (43.51%) species were collected exclusively through immature sampling techniques, suggesting that these species may be more prevalent and easy to collect in their immature stages rather than in the adult stages, when they would rest in dark and damp, cryptic hideouts, as only 25 (23.15%) species could be collected using adult collection methods. Notably, 36 (33.33%) species were captured using both methods, indicating their presence across multiple life stages and potentially suggesting a more stable or extensive population within the study areas. This dual sampling approach allowed a more comprehensive representation of the mosquito community, evidenced by the higher number of species identified through immature sampling (83 species) compared to adult collection methods (71 species).

Of the 12,535 mosquitoes collected by both immature (emerged to adult specimens) and adult sampling techniques, Stegomyia albopicta was the predominant species (54.82%), followed by Culex quinquefasciatus (6.92%), Hulecoeteomyia chrysolineata (6.33%), and Armigeres subalbatus (5.03%). A relatively low number of primary disease vectors was observed among the collected mosquito species, except St. albopicta. St. aegypti, a primary vector which transmits dengue, chikungunya, Zika, and yellow fever, comprised only 1.43% of the total collection, mostly in towns/cities. Cx. tritaeniorhynchus, the vector of Japanese encephalitis, represented even a smaller proportion (0.85%). Anopheles stephensi, and An. culicifacies, the primary malaria vectors, were found in extremely low numbers, accounting for 0.06% and 0.01% of the collections, respectively. Mansonia uniformis, the lymphatic filariasis vector in the study area, was rarely encountered (0.03%).

District-wise abundance also revealed that St. albopicta was the predominant species in all surveyed districts, with a very high proportion in the Thiruvananthapuram district, accounting for 77.29% of the collected mosquitoes. However, the second dominant species varied in the districts. Cx. quinquefasciatus was found to be the second most prevalent species in the Ernakulam, Idukki, and Thiruvananthapuram districts, whereas Fredwardsius vittatus was found in Pathanamthitta and Hl. chrysolineata in Wayanad (Fig. 2). The prevalence of these vectors underscores the importance of deploying selective and tailored vector control strategies, as the same vector control method may not be effective in tackling them.

Heatmap showing the distribution of prevalent mosquito species across five districts of Kerala. The colour gradient indicates the species count, with darker shades representing high abundance. The x-axis lists various mosquito species, while the y-axis represents the districts.

District-wise distribution of mosquito species

The distribution of mosquito species across the five surveyed districts in Kerala, India, revealed a complex spatiotemporal distribution pattern with varying levels of biodiversity (Fig. 3). Wayanad district was observed to be the hotspot of mosquito species diversity, hosting 14 unique species, which can be attributed to its diverse ecological niches, including forests, plantation areas with more vegetation coverage, comparatively with less human interference, and high altitude. This rich biodiversity is followed by Ernakulam and Pathanamthitta districts, with 11 and 9 unique species, respectively, suggesting that these districts also possess favourable geo-environmental conditions for supporting a wide range of mosquito species. However, Thiruvananthapuram district showed moderate diversity with five mosquito species, whereas Idukki had the lowest diversity, comprising only one species. The presence of 19 species across all five districts indicated eco-environmental determinants that favour the distribution pattern with more or less similar mosquito breeding habitats. A strong species overlap was noticed between Ernakulam and Pathanamthitta (5 species) and between Wayanad and Thiruvananthapuram districts (6 species), suggesting similar habitat profiles and niche characteristics between these district pairs.

Venn diagram depicting mosquito species distribution and overlap across the five districts of Kerala, India. The numbers in each section represent the count of mosquito species unique to or shared among the respective districts. The central overlapping region indicates mosquito species found in all five districts, while non-overlapping sections show species unique to each district.

Habitat preference of different species

This study revealed extensive mosquito breeding in various intra- and peri-domestic habitats, which were classified into natural and artificial habitats based on the breeding type characteristics (Supplementary file no. 2). Natural habitats are water bodies formed without human intervention that include freshwater bodies, tree holes, leaf axils of plants, rockpools and ground depressions that collect rainwater. Artificial habitats are anthropogenic or human-influenced water collections such as agricultural water sources, discarded plastics, household items, etc. Artificial habitats were observed to have greater diversity (77.7%) of mosquito species than natural habitats (22.3%). St. albopicta, a prevalent species across districts, was found breeding in 77 different types of habitats. These habitats ranged from discarded habitats such as tyres and plastic containers, as well as latex collection cups, to natural habitats like tree holes and the leaf axils of pineapple and Colocasia plants. The primary breeding sites for St. albopicta were discarded tyres, containers, and other household items such as barrels, plastic buckets, etc., collectively accounting for 43% of the observed habitats. Additionally, the overall collection revealed that St. albopicta predominantly bred in discarded tyres (20.8%), latex collection cups (10.57%), and tree holes (10.9%). This highlights the species’ preference and remarkable adaptability to various peri-domestic and sylvatic environments. Hl. chrysolineata was observed in 36 different types of habitats, indicating its diverse breeding pattern and adaptability to various habitats. Other mosquito species exhibited more specific habitat preferences. For instance, St. aegypti was primarily found in water storage containers and discarded items in urban areas. Nevertheless, Ar. subalbatus, a nuisance mosquito, prefers polluted water habitats such as coconut shells, cocoa pods, and discarded items. Cx. quinquefasciatus was often found breeding alongside St. albopicta in water storage habitats and semi-polluted discarded containers. Quite surprisingly, some species showed signs of adaptation to urbanization, such as Hz. chandi, and St. subalbopicta which used to breed in tree holes but was found in a few discarded tyres. This study also recorded the presence of a biocontrol mosquito species (Toxorhynchites splendans and Tx. minimus), limited to natural habitats such as tree holes and water accumulation in reed or bamboo stump plants, and occasionally found in discarded tyres. Similar to Hl. chrysolineata, the breeding of Collessius pseudotaeniatus, was also observed in various habitats, including water storage containers, rainwater accumulations, rock pools, and discarded habitats.

Species richness and diversity indices

The analysis of mosquito diversity across different districts revealed distinct patterns in species richness (Fig. 4). From adult collection, Thiruvananthapuram exhibited the highest adult mosquito species richness, with an observed richness of 38 and a Chao1 estimate of 104.618 (CI: 56.23–281.448), suggesting a high likelihood of undetected species. Similarly, Pathanamthitta had a Chao1 richness of 47.963 (CI: 29.71–124.569) with an observed richness of 24. In contrast, Idukki recorded the lowest adult mosquito diversity, with an observed richness of 15 and a Chao1 estimate of 22.978 (CI: 15.985–79.644), suggesting a limited range of species.

From immature collection, Pathanamthitta exhibited the highest species richness, with an observed richness of 49 and a Chao1 estimate of 64.989 (CI: 52.017–133.735). Wayanad and Thiruvananthapuram also displayed high richness estimates, with observed values of 48 and 42, and Chao1 estimates of 53.059 (CI: 49.153–70.201) and 46.164 (CI: 42.673–67.756), respectively. Ernakulam followed with an observed richness of 38 and a Chao1 estimate of 42.498 (CI: 38.495–78.896). Idukki consistently exhibited the lowest richness for both adult and immature mosquitoes, with an observed richness of 31 and a Chao1 estimate of 31.666 (CI: 31.057–38.802).

Mosquito diversity indices

The analysis of adult mosquito diversity across districts did not show any significant variation in alpha diversity (F = 0.092, p = 0.983). Similarly, the diversity of immature mosquitoes also exhibited no significant differences across districts (F = 0.036, p = 0.997), indicating a relatively uniform distribution of mosquito diversity among the studied regions.

The rarefaction curve analysis of adults and immatures of mosquitoes across the five districts in Kerala provided valuable insights into species diversity and sampling efficiency (Fig. 5). Wayanad district emerged as the most biodiverse district for both adult and immature mosquito populations, with the steepest curves and highest species counts (55 for adults and 50 for immatures). This suggests a rich ecological landscape that supports a wide variety of mosquito species. Thiruvananthapuram district followed closely behind Wayanad in terms of diversity, particularly for the adult population. The steep initial rise in the curves for these two districts indicates the potential for the possible identification of more new species with further sampling, especially in unexplored or less explored habitats. In contrast, Ernakulam and Idukki districts observed lower species richness, with their curves plateauing earlier. This pattern is consistent across both adult and immature populations, suggesting that these districts have been adequately sampled but consist of fewer mosquito species. Pathanamthitta district occupied an intermediate position, showing moderate species diversity.

Diversity indices of adult and immature mosquitoes in surveyed districts of Kerala. Boxplots represent variation in Chao and observed richness, along with Shannon and Simpson indices for adult and immature. Coloured points indicate the district-wise distribution of data, with notable differences in richness and diversity indices reflecting spatial ecological variation.

Rarefaction and extrapolation curves showing the species diversity of adult (top) and immature (bottom) mosquitoes across five districts of Kerala, India. The figure depicts both observed (rarefaction) and predicted (extrapolation) species diversity.

District-wise mosquito composition

The mosquito richness map revealed a complex and varied distribution of mosquito species across the districts (Fig. 6). The Wayanad district has emerged as a hotspot for mosquito species diversity, having the highest species richness, with 64 different species identified. This was closely followed by the Thiruvananthapuram and Pathanamthitta districts, which exhibited similarly high levels of mosquito diversity, with 60 and 59 species, respectively. In contrast, the Idukki district represents a different pattern in species diversity. Despite adequate sampling efforts, the district observed significantly lower species richness, with only 34 mosquito species. However, Ernakulam district falls in between these two extremes, with 54 recorded mosquito species, indicating a moderate level of species diversity.

Map depicting mosquito species richness in five study districts of Kerala: The colour gradient represents species richness, with blue colour indicating higher and amber colour indicating lower species richness.

Species-area relationship

The species-area relationship analysis revealed that mosquito species richness was negatively associated with the logarithm of area (S = 80.46–3.92 log AREA, where 80.46 is the fitted intercept and − 3.92 is the fitted slope), which represented the best-fitting model for this relationship (ΔAICc = 0.00, Fig. 7). This model demonstrated a strong explanatory power (pseudo-R² = 0.83) and was statistically significant (p = 0.004), indicating a robust inverse relationship between mosquito richness and area. These findings emphasise the importance of spatial scaling in determining mosquito species richness across sampled geographical areas.

Species-area relationship between log area and mosquito richness found and mosquito species richness across different ecological districts of Kerala.