Distribution and abundance of larval habitats in different ecological zones and seasons
A total of 383 breeding habitats made up of 11 different habitat types were encountered and recorded during the study. Most of the habitats were man-made (69.71%, 267/383) and the others (30.29%, 116/383) were natural. The most abundant habitat type was man-made ponds (27.15%, 104/383), which were present throughout the rainy and dry seasons mostly on farmlands. This was followed by natural ponds (12.01%, 46/383), swamps (11.49%, 44/383), dug-out wells (10.97%, 42/383), and concrete wells (9.92%, 38/383). Other habitat types included tyre tracks (7.83%, 30/383), puddles (6.27%, 24/383), and drainage ditches (6.01%, 23/383). The less abundant habitat types were furrows (4.18%, 16/383), hoofprints (2.87%, 11/383), and footprints (1.31%, 5/383) (Table 1). The distribution of mosquito larval habitat types and their abundance varied among the study sites (ꭓ2 = 498.2658; df = 40; p = 0.0001) as well as ecological zones (ꭓ2 = 369.5865; df = 20; p = 0.0001) (Additional file 1). In Anyakpor in the coastal savanna zone, only 4 habitats types were encountered; man-made ponds (57%, 97/168), dug-out wells (20.83%, 35/168), concrete wells (20.83%, 35/168), and a natural pond (0.60%, 1/168). In Duase, in the forest, area 8 habitat types were found; natural ponds (29.03%, 18/62), puddles (14.52%, 9/62), tyre tracks (12.90%, 8/62), man-made ponds (11.29%, 7/62), drainage ditches (11.29%, 7/62), dug-out wells (11.29%, 7/62), swamps (6.455, 4/62), and concrete wells (3.23%, 2/62). In Kpalsogu, in the sahel savanna area also, 8 habitat types were encountered. These were swamps (31.50%, 30/96), tyre tracks (21.88%, 21/96), hoofprints (11.46, 11/96), natural ponds (11.46, 11/96), puddles (10.42%, 10/96), furrows (7.29%, 7/96), footprints (4.17%, 4/96), and drainage ditches (2.08%, 2/96). Only 4 habitat types; furrows (28.13%, 9/32), drainage ditches (28.13%, 9/32), natural ponds (21.88%, 7/32), and concrete well (3.13%, 1/32) were encountered in Libga, in the sahel savanna area. In Pagazaa, also in the sahel savanna area, the 6 habitats encountered were natural ponds (36%, 9/25), puddles (20%, 5/25), drainage ditches (20%, 5/25), swamps (16%, 4/25), tyre track (4%, 1/25), and footprints (4%, 1/25). Larval habitats were significantly more abundant in the rainy season (63.45%, 243/383) than in the dry season (36.55%, 140/383) (ꭓ2 = 91.3295; df = 10; p =0.001) (Additional file 1). Dug-out wells were the most abundant habitat type during the dry season while man-made ponds were the most abundant during the rainy season. The presence of larval habitat type was associated with the land use type (ꭓ2 = 587.4192; df = 60; p = 0.0001). Larval habitats were mostly found on farmlands (58.75%), followed by pastures (16.19%), and on roads (13.05%). The rest were present in forested areas (4.70%), around homes or compounds (4.18%) by streams and (1.83%), and rivers in swamps (1.31%). Most of the habitats had a vegetation cover of less than 24% (ꭓ2 = 215.1340; df = 40; p = 0.0001) (Additional file 1).
Larval habitat types, the presence, and densities of Anopheles larvae
The presence of Anopheles larvae was dependent on the type of habitat present (ꭓ2 = 41. 3651; df = 10; p < 0.0001). Even though there was a significant increase in the number of habitats during the rainy season (ꭓ2 = 91.3295; df = 10; p < 0.0001) (Additional file 1) compared to the dry season, the proportion of habitats that had the presence of Anopheles larvae were not significantly different between the two seasons (ꭓ2 = 0.0051; df = 1; p = 0.943) (Additional file 2). In the dry season, 29.29% (41/140) of the habitats had Anopheles larvae present while in the rainy season, 29.63 % (72/243) of the habitats were positive for Anopheles larvae. In all, dug-out wells were the most inhabited with Anopheles larvae during the dry season (39.02%, 16/41) followed by man-made ponds (21.95%, 9/41) whereas during the rainy season man-made ponds were the most inhabited habitat (30.56%, 22/41) followed by swamps (16.67% 12/72) (Table 2).
The type of habitat present was associated with An.gambiae s.l larval density (ꭓ2 = 29.593; d.f =10; p < 0.001) (Additional file 3). Even though the mean An.gambiae s.l larval density in the rainy season (1.49 larvae/dip) was slightly higher than in the dry season (1.15 larvae/dip), the difference was not significant (z = -0.232; p = 0.8166). In Anyakpor, the most productive habitat types were dug-out wells during both the dry (1.6 larvae/dip) and rainy (11.28 larvae/dip) seasons (Table 5). The preferred breeding habitat in Kpalsogou were natural ponds during the dry season (0.89 larvae/dip) and swamps (2.57 larvae/dip) during the rainy season. In Libga, drainage ditches were the only habitat type present in the dry season with the mean larval density of 0.3 larvae/dip, whiles the only productive habitats were furrows in the rainy season (1.83 larvae/dip). There were no productive habitats in Pagazaa during the dry season, but in the rainy season, the most productive habitat type were puddles (1.44 larvae/dip). In Duase, the most productive habitat type was dug-out wells both in the dry (1.47 larvae/dip) and the rainy season (2.05 larvae/dip) (Table 5).
Abundance and distribution of Anopheles larvae in the different ecological zones
A total of 7,894 mosquito larvae were collected during this study. Out of this number, 2,152 (27.26%) were Anophelines whiles 5,742 (72.74%) were culicines. Of the Anopheline species, An. gambiae s.l. comprised 2,128 (98.88 %), An. rufipees were 16 (0.74 %), whereas An. pharoensis was 8 (0.37 %). During the rainy season 1,500 (70.49%) An. gambiae s.l were collected while 628 (29.51%) were collected in the dry season.
Anyakpor in the coastal savannah area had the highest abundance of An. gambiae s.l larvae (1,286) with 343 (26.67%) occurring in the dry season and 943 (73.33%) in the rainy season (Table 3). During the dry season in Duase, situated in the forest area 30.17% of An. gambiae s.l larvae were collected, while 69.83% were collected in the rainy season. Kpalsogou and Libga in the Sahel savannah recorded 39.00% and 35.79% of the An. gambiae s.l larvae in the dry season, while the rainy season contributed to 61.00% and 64.21%, respectively. In Pagazaa, also in the Sahel savannah An. gambiae s.l larvae were found only in the rainy season (Table 3) and none during the dry season.
The distribution of An. gambiae sibling species varied across the ecological zones (ꭓ2 = 45.9887 df = 8; p = 0.0001). Anopheles coluzzii was the most abundant species (53.44%) in each ecological zone. This was followed by An. gambiae s.s. (25.98%), and An. arabiensis (6.27%) which were found only in the Sahel savannah; Kpalsogu (19.30%), Libga (17.86%), and Pagazaa (35.71%). Anopheles melas were the least abundant species (4.19%) and were present only in Anyakpor in the Coastal savannah area. All the species were more abundant in the rainy season (ꭓ2 = 21.2510; d.f. = 2; p = 0.0001) than in the dry season. Anopheles gambiae s.s. and An. coluzzii were found in all the habitat types encountered in this study. An. arabiensis were predominantly found in swamps (52.38 %) and furrows (28.57 %) whereas An. melas were found in dug-out wells (55.56 %) and man-made ponds (44.44 %). Whiles An. rufipees were found only in Kpalsogu and Libga in the Sahel savannah area, An. pharoensis were found in Anyakpor in the coastal savannah area and Libga in the sahel savannah area Anopheles pharoensis were found only in man-made ponds (75.00 %) and furrows (25.00 %). An. rufipees were found in swamps (56.25 %), footprints (25.00 %), furrows (12.50 %), and puddles (6.25 %).
Habitat characteristics, the occurrence, and densities of Anopheles larvae
The type of habitat influenced the presence of Anopheles larvae (ꭓ2 = 41.3651; d.f. = 10; p = 0.0001) as well as their densities (p < 0.01). Significantly higher numbers (84.07% (95/113) of Anopheles positive habitats were less than 10 m2 in size (ꭓ2 = 11.9217; d.f. = 2; p = 0.0001). Land-use type influenced both the presence of Anopheles larvae (ꭓ2 = 26.5920; d.f. = 6; p = 0.0001) (Additional file 2) and their larval densities (ꭓ2 = 16.117; d.f. = 6; p = 0.013) (Additional file 3). Fifty-five percent (55%) of all larval habitats found around homes or compounds contained Anopheles larvae. The odds of finding Anopheles larvae in any habitat was twice higher if the vegetation cover was less than 24% (OR = 2.24 [1.02, 4.93]; p = 0.045) (Table 6). As the vegetation cover increases, the density of Anopheles larvae decreases (B= -0.016 [-0.28, 0.003]; p =0.015) (Additional file 4). The present study, again, showed that Anopheles larvae have the preference to co-habit with Culicine larvae. The likelihood of encountering Anopheles larvae in a breeding habitat was over three times higher when culicines are present (OR = 3.13 [1.75,5.59]; p < 0.01) (Table 6).