| Summary: | Abstract Background In the present study, the efficacy of two isolates of Beauveria bassiana namely, BKN20 and BKN1/14 was evaluated against the tea mosquito bug (TMB), Helopeltis theivora, Waterhouse (Hemiptera: Miridae) damaging harvestable shoots of tea plants in the Dooars and Darjeeling regions of West Bengal, India. Results Laboratory study revealed that, in both isolates, BKN20 was more pathogenic than BKN1/14, exhibiting 76% mortality of the test insect. The BKN20 isolate was formulated as an aqueous suspension (5%AS), and evaluated against TMB through micro-plot trials in tea plantations. The Micro-plot field study revealed a maximum of 72.19% reduction in the shoot damage due to TMB in plots sprayed with a 1000 ml/ha concentration of BKN20 5%AS containing 2 × 107 conidia/ml, as compared to the synthetic insecticide (Thiamethoxam 25%WG), where a 63.12% reduction in the shoots was recorded. Furthermore, different concentrations of the formulated BKN20 5%AS were evaluated against the test insect at 2 locations of tea gardens in the Dooars and Darjeeling regions. The results from both locations revealed that 1000 and 1200 ml/ha concentrations of BKN20 5%AS (each concentration containing 2 × 107 conidia/ml) significantly (p < 0.05) reduced the TMB population and they were more effective than Thiamethoxam 25%WG (120 g/ha). However, non-significant differences in crop yields were recorded. The formulation BKN20 5%AS was found to be non-pathogenic to non-target insects, i.e. natural enemies present in the tea ecosystem. BKN20 5%AS had no phytotoxic effect on the tea leaves, with acceptable organoleptic attributes. Conclusion The BKN20 isolate could be commercialized as an alternative microbial insecticide to reduce the load of chemical insecticides in the tea ecosystem.
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