Effect of relative humidity on the efficacy of entomopathogen Beauveria bassiana-based mycopesticide against red spider mite Tetranychus macfarlanei

Abstract

Tetranychus macfarlanei is a potential threat to various economically important crops. Chemical control is widely used to control this spider mite, but the exclusive reliance on chemical acaricides is now being questioned, and entomopathogens are emerging as a promising alternative. The efficacy of the entomopathogens largely depends on abiotic factors like temperature, humidity, rainfall etc. The experiments were conducted to determine the effect of relative humidity on the efficacy of the entomopathogen Beauveria bassiana (strain GHA), marketed as BotaniGard ES, against the adult female and egg stages of T. macfarlanei under laboratory conditions. Adult females and eggs of T. macfarlanei were treated with three concentrations of BotaniGard (1×106, 1×107, and 1×108 conidia/ml), and virulence was assessed on mites held at three relative humidity levels (55, 75, and 95 ± 2% RH) at 25 ± 1 °C. The results showed that the mortalities of adult females and eggs of T. macfarlanei were dose-dependent, and there is certainly a synergistic effect of relative humidity on the efficacy. When the eggs were treated under the above laboratory conditions, highest egg mortalities of 86.4 and 91% were recorded at a concentration of 1×108 conidia/mL at 75 and 95 ± 2% RH, respectively, after six days of treatment. The LT50 values on adult females were 28.7 h and 33.4 h at 95 and 75 ± 2% RH in a concentration of 1×108 conidia/ml, respectively, which was significantly lower than values for other concentrations and RH levels. This study identified 1×108 conidia/ml of B. bassiana at 75 to 95 ± 2% RH as the best possible combination than other lower RH to manage the spider mite.