Effect of ultraviolet radiation on spore viability and mosquitocidal activity of an indigenous ISPC-8 Bacillus sphaericus Neide strain

Abstract

Effects of UV-A, UV-B and their combination on spore viability and larvicidal activity of an indigenous isolate of Bacillus sphaericus Neide, ISPC-8 were studied under laboratory conditions. The UV sensitivity of ISPC-8 was compared with standard strain 1593 and larvicidal activity was tested against third instar larvae of mosquito, Culex quinquefasciatus Say. No significant adverse effects on viability as well as larvicidal activity of both strains were observed when spores were exposed to UV-A for 6 h. However, exposure to UV-B for a few minutes adversely affected the spore viability as well as larvicidal activity and this adverse effect was more pronounced on spore viability. In both strains about 50% larvicidal activity was retained after exposure of the spores to UV-B for 8 h. However, spore viability at this exposure of time was drastically reduced to 2.5% in ISPC-8 and 0.3% in 1593. The spore viability and larvicidal activity patterns were found to be similar to UV-B treatment when spores were exposed to a combination of UV-A and UV-B. Our study hence, shows the adverse effect of UV radiation on ISPC-8 and 1593 indicating the need to incorporate eco-friendly UV protectants in formulations so that the efficacy of biopesticides based on these entomopathogens can be prolonged under field conditions, especially in tropical countries.