Daily temperature profiles in and around Western Kenyan larval habitats of Anopheles gambiae as related to egg mortality

Background: Anopheles gambiae eggs are more frequently found on soil around puddle habitats of the larvae, than on the water surface itself in Western Kenya. Thus, eggs can experience temperatures more wide-ranging and lethal than those experienced by larvae or pupae confined within puddles. Methods: Small batches of eggs from house-collected An. gambiae as well as from the Kisumu laboratory strain were bathed for defined times in water whose temperature was precisely controlled. Daily temperature profiles were recorded by an infrared thermometer on seven different days in and around three types of typical An. gambiae larval habitats at Kisian. Results: For wild eggs, significant mortality occurred upon brief heating between 42 - 44 degrees C. Few eggs hatched after 10 min at 45 degrees C and none hatched above this temperature. A similar pattern occurred for eggs of the Kisumu strain, except it was shifted downwards by 1 degrees C. Egg mortality was time-dependent above 40 degrees C. Temperatures of water in the three types of larval habitats never exceeded 35 degrees C. Wet or damp mud rarely and only briefly exceeded 40 degrees C; thus, water and mud would be highly conducive to egg survival and development. However, dry soils frequently reached 40 - 50 degrees C for several h. Eggs stranded on dry surfaces would experience substantial mortality on hot, sunny days. Conclusion: Moist mud around puddles constitutes suitable habitat for An. gambiae eggs; however, eggs on the surface of dry soil under direct sunlight are unlikely to survive for more than a few hours.