Role of thermal stress on development of cross tolerance to toxins of Bacillus thuringiensis in wild Culex quinquefasciatus

Abstract

Impact of thermal stress on survival and development of cross tolerance to toxins of Bacillus thuringiensis israelensis (Bti) in larvae of field collected Culex quinquefasciatus was evaluated. To assess the thermal stress, late third instar larvae were exposed to variable temperatures viz. 39°C, 40°C, 41°C, 42°C, 43°C, 44°C and 45°C. All larvae survived up to 60 minutes of exposure at 39°C whereas hundred percent larval mortality was observed after 180 minutes of exposure at 40°C, followed by 135 min exposure at 41°C, 70 min exposure at 42°C, 35 min exposure at 43°C, 15 min exposure at 44°C and 10 min exposure at 45°C respectively. To understand the efficacy, larvae were treated with various concentrations of Bti as per WHO protocol and it was observed that 5%, 23%, 70%, 73%, 81% and 100% larval mortality found after treatment at 0.5 ppm, 1.0 ppm, 1.5 ppm, 2.0 ppm, 2.5 ppm and 3.0 ppm respectively. Further, in order to assess the cross tolerance level to Bti, larvae pre-adapted at 39°C for 60 min, 90 min and 120 min duration were re-exposed to Bti solution of 1.0 and 1.5 ppm. Data indicated that the temperature pre-adapted larvae showed 2.6 and 1.5 fold less larval mortality than the non-adapted (control) larvae revealing thermal stress do play an important role in the development of cross tolerance to toxins of Bti in wild Culex quinquefasciatus.