A novel approach: Beauveria bassiana granules applied to nursery soil for management of rice water weevils in paddy fields

BACKGROUND: Spraying of entomopathogenic fungi formulated as wettable powders and suspension concentrates based on oils or water has been used for the management of crop pests. However, the use of fungal granules to control paddy field pests has not been fully explored. Herein, several Beauveria bassiana isolates (ERL1170, ERL1578 and ERL836) were produced as granules using a millet-based solid culture. The granules were applied to the rice nursery 3 days before transplanting, and their control efficacy against rice water weevils was determined in paddy fields. RESULTS: The solid cultures produced ~1×10⁸ conidia g^-1 of millet grains 10 days after the inoculation. The granules were applied to the soil in the rice nursery 3 days before the rice seedlings were transplanted in the paddy fields. Rice in plots with granules of ERL1578 had 17.3% leaf damage (74% control efficacy) 14 days post-application, whereas rice plants in the untreated control had 66.5% damage. Rice plants treated in the nursery with ERL1170 and ERL836 had 52-54% damage. In the rice plots previously treated with ERL1578, the smallest numbers of larvae and adults were observed 38 days post-application. In laboratory conditions, ERL1578-treated larvae turned pink and were covered with mycelial mass. CONCLUSION: Applications of millet-based B. bassiana granules on rice nursery soil can be an effective and efficient biological control strategy for the management of rice water weevils. This method can be relatively inexpensive and requires less labor than practices involving the spraying of fungi directly on rice in paddy fields.