Year-round Anopheles funestus (s.l.) larval habitat dynamics: combining drone, pictorial, and conventional surveys in south- eastern Tanzania

Abstract Introduction : Effective larval source management (LSM) requires detailed knowledge of the aquatic habitats supporting dominant malaria vector. This study investigated intra-annual variations in Anopheles funestus sensu lato habitats in south-eastern Tanzania, where this species drives malaria transmission, and evaluated pictorial and drone-based methods to complement standard entomological surveys. Methods Monthly entomological surveys (July-2022 to June-2023) quantified larval productivity in 184 An. funestus sensu lato habitats and characterized their environmental features (vegetation cover, shading, turbidity, water movement, size, and depth) and physicochemical properties (temperature, pH, dissolved oxygen, and total dissolved solids); temporal variation was analysed using generalised additive mixed models. Pictorial surveys of a habitat’s subset yielded 132 standardized monthly images, independently assessed by five trained raters and inter-rater reliability quantified. A 6 km² drone survey conducted in May 2023 was validated against concurrent ground-truth data. Key informant interviews assessed stakeholder perceptions of pictorial and drone approaches, analysed thematically. Results Large ground pools and rivers sustain An. funestus sensu lato larvae year-round, human-made habitats like ditches declined sharply during the dry season. Physicochemical and environmental characteristics varied significantly over time, but larval densities fluctuating primarily by environmental characteristics. Habitats with partial shading, moderate vegetation cover, and limited water disturbance, conditions most common after the rains and during the early dry season, consistently supported the highest An. funestus sensu lato larval densities. Pictorial surveys reliably tracked temporal habitat changes, showing good inter-rater reliability for water coverage (ICC = 0.809), vegetation quantity (ICC = 0.761), and water presence (κ = 0.839), and were viewed by stakeholders as a practical for documenting habitat conditions. Drone imagery underperformed, detecting only 35.7% of confirmed habitats with 60% classification accuracy (κ = 0.295), limited by vegetation canopy cover. Conclusion The main An. funestus sensu lato habitats in south-eastern Tanzania persisted year-round, though their characteristics varied significantly by month. Environmental, not physicochemical, factors drove intra-annual variations in larval density, suggesting scope for seasonally adapted larval source management. Ground-based pictorial monitoring offers a practical supplement to entomological surveys, while drone-based mapping needs validations.