Indoor microclimate shapes resting patterns of Anopheles funestus in south-eastern Tanzania

Inside the walls of a home, a hidden world of air exists that is distinct from the weather outside. This invisible layer, known as the indoor microclimate, is shaped by the materials used to build a house and the way heat and moisture move through it. For the mosquitoes that carry malaria, these tiny differences in temperature and humidity are not just background noise; they are the deciding factors for where the insects choose to rest after feeding. Understanding these preferences is critical because the most common tools used to stop malaria, such as insecticide sprays applied to walls, only work if the mosquitoes land on those specific surfaces. If the insects are hiding in different spots because the air feels better there, the sprays may miss them entirely, leaving the disease to spread.

In the south-eastern corner of Tanzania, where rice fields and livestock define the landscape, a team of researchers set out to map this invisible world and see how it guides the behavior of Anopheles funestus, a primary mosquito responsible for transmitting malaria in the region. They focused on thirteen houses that represented the most common building styles in the area: some with mud walls and grass-thatched roofs, others with brick walls and metal roofs. Over a period spanning from late 2024 into early 2026, the team visited these homes three times a week, collecting mosquitoes from three distinct zones within each house: the floor and lower walls, the middle of the walls, and the area just under the roof. To understand the environment these insects were choosing, the researchers placed small sensors on the floor, the middle of the walls, and under the roof to record the temperature and humidity every thirty minutes.

The results revealed a clear pattern in where the mosquitoes chose to hide. Contrary to the assumption that they would be evenly distributed or clinging to the middle of the walls where sprays are often applied, the majority of the mosquitoes, about 40 percent, were found resting near the bottom of the house. Only about 27 percent were found in the upper section near the roof. The type of house also mattered significantly. Mosquitoes were far more abundant in houses with brick walls compared to those with mud walls, and they preferred houses with grass-thatched roofs over those with metal roofs. The researchers found that the indoor environment was not uniform; the air near the floor was consistently cooler and more humid, while the air near the roof was warmer and drier. This vertical difference was most pronounced in houses with metal roofs, which trapped heat and created a much hotter, more variable environment than the grass-thatched roofs, which acted as a natural insulator.

When the researchers analyzed the relationship between the weather inside the house and the number of mosquitoes, they discovered that the insects were not just looking for a specific temperature or a specific amount of moisture, but a very specific combination of both. They found that the mosquitoes thrived when the average temperature was around 32.7 degrees Celsius and the amount of water vapor in the air, known as absolute humidity, was about 16.4 grams per cubic meter. Crucially, the mosquitoes avoided environments where the temperature changed rapidly throughout the day. They preferred spaces where the temperature remained stable, avoiding the sharp swings in heat that occurred in the upper parts of metal-roofed houses. The study showed that relative humidity, a common way to measure moisture, was not a good predictor on its own because it changes with temperature; instead, the actual amount of water in the air was the key factor.

The study also looked at the mosquitoes themselves to ensure they were the right species and to check if they were carrying the malaria parasite. Genetic testing confirmed that the vast majority of the mosquitoes collected were indeed Anopheles funestus, and a small number of them were found to be carrying the parasite. Most of the mosquitoes had fed on human blood, confirming their role in transmitting the disease. The researchers noted that the study was observational, meaning they watched and measured what was happening naturally without changing the houses, so they could not prove that the temperature caused the mosquitoes to move, but the strong link between the cool, stable air near the floor and the high number of mosquitoes suggests a clear preference.

These findings suggest that the way a house is built directly influences where malaria mosquitoes rest. By choosing cooler, more stable microclimates near the floor, these insects may be protecting themselves from the stress of heat and drying out. This behavior has important implications for how malaria is fought. If the mosquitoes are resting on the floor or in the lower parts of the walls rather than the middle sections, standard spraying methods might be less effective. The study indicates that improving housing, perhaps by using materials that create hotter or more unstable indoor environments, could make a home less suitable for mosquitoes to rest and survive. By understanding the specific thermal and moisture needs of these insects, health officials can better design interventions that target the actual places where the mosquitoes hide, rather than relying on assumptions about where they might be.