Temperature during larval development and adult maintenance influences the survival of Anopheles gambiae s.s.

Background: Malaria transmission depends on vector life-history parameters and population dynamics, and particularly on the survival of adult Anopheles mosquitoes. These dynamics are sensitive to climatic and environmental factors, and temperature is a particularly important driver. Data currently exist on the influence of constant and fluctuating adult environmental temperature on adult Anopheles gambiae s.s. survival and on the effect of larval environmental temperature on larval survival, but none on how larval temperature affects adult life-history parameters. Methods: Mosquito larvae and pupae were reared individually at different temperatures (23 +/- 1 degrees C, 27 +/- 1 degrees C, 31 +/- 1 degrees C, and 35 +/- 1 degrees C), 75 +/- 5% relative humidity. Upon emergence into imagoes, individual adult females were either left at their larval temperature or placed at a different temperature within the range above. Survival was monitored every 24 hours and data were analysed using non-parametric and parametric methods. The Gompertz distribution fitted the survivorship data better than the gamma, Weibull, and exponential distributions overall and was adopted to describe mosquito mortality rates. Results: Increasing environmental temperature during the larval stages decreased larval survival (p < 0.001). Increases of 4 degrees C (from 23 degrees C to 27 degrees C, 27 degrees C to 31 degrees C, and 31 degrees C to 35 degrees C), 8 degrees C (27 degrees C to 35 degrees C) and 12 degrees C (23 degrees C to 35 degrees C) statistically significantly increased larval mortality (p < 0.001). Higher environmental temperature during the adult stages significantly lowered adult survival overall (p < 0.001), with increases of 4 degrees C and 8 degrees C significantly influencing survival (p < 0.001). Increasing the larval environment temperature also significantly increased adult mortality overall (p < 0.001): a 4 degrees C increase (23 degrees C to 27 degrees C) did not significantly affect adult survival (p > 0.05), but an 8 degrees C increase did (p < 0.05). The effect of a 4 degrees C increase in larval temperature from 27 degrees C to 31 degrees C depended on the adult environmental temperature. The data also suggest that differences between the temperatures of the larval and adult environments affects adult mosquito survival. Conclusions: Environmental temperature affects Anopheles survival directly during the juvenile and adult stages, and indirectly, since temperature during larval development significantly influences adult survival. These results will help to parameterise more reliable mathematical models investigating the potential impact of temperature and global warming on malaria transmission.