Development of larval thermotolerance and its impact on adult susceptibility to malathion insecticide and Plasmodium vivax infection in Anopheles stephensi

The effect of the range of temperature on the thermal adaptation in Anopheles stephensi Liston 1901 was evaluated in the laboratory. Late third instar larvae of An. stephensi were exposed to variable temperatures viz. 37°C, 39°C, 41°C, 43°C and 45°C, and their lethal time to cause 50% mortality (LTM₅₀) values were calculated. All larvae survived up to 39°C of exposure. However, at 45°C, they died within 30 min of exposure. Pre-exposure to variable temperatures and re-exposure to higher temperatures conferred adaptive thermotolerance. The larvae pre-adapted at 41°C that were re-exposed to 43°C, and larvae pre-adapted at 39°C that were re-exposed to 45°C, were found more thermotolerant than the thermally non-adapted larvae. Adaptive cross-tolerance to malathion was also induced by pre-exposing them to 37°C and 39°C. It suggests that temperature stress also play an important role in the development of adaptive cross-tolerance to other stress conditions. Similarly, the oocyst rate was relatively more in adults that emerged from larvae pre-adapted at 40°C as compared to adults that emerged from thermally non-adapted larvae and with lower oocyst load.