Wolbachia bacteria affect rice striped stem borer (Chilo suppressalis) susceptibility to two insecticides

The striped stem borer (SSB), Chilo suppressalis (Walker) (Lepidoptera Crambidae), is one of the most economically important rice pests in Asia, the control of which primarily relies on chemicals. As such, C. suppressalis has developed resistance to many insecticides in most rice growing areas. Wolbachia are obligate intracellular bacteria known to manipulate arthropod host biology, and are thus considered a promising tool for pest control. In this study, we investigated the effects of Wolbachia and their density on the susceptibility of C. suppressalis to fipronil and avermectin insecticides under laboratory conditions. Specifically, fipronil and avermectin toxicities against Wolbachia-infected and Wolbachia-uninfected C. suppressalis were tested by topical application. Furthermore, Wolbachia densities in C. suppressalis over different survival times following fipronil treatment (0.2 mg/L) were evaluated by quantitative polymerase chain reaction. Results showed that average mortality of Wolbachia-infected C. suppressalis was lower than that of Wolbachia-uninfected C. suppressalis under fipronil and avermectin treatment. In addition, the 50% lethal concentration (LC50) of fipronil and avermectin to Wolbachia-infected strains was 9.74 times and 5.32 times higher, respectively, than that of Wolbachia-uninfected strains, indicating that Wolbachia reduce the susceptibility of C. suppressalis to fipronil and avermectin. Under fipronil treatment (0.2 mg/L), Wolbachia content in surviving C. suppressalis at 72 hours was significantly higher than that at 24 and 48 hours (P < 0.05); thus, Wolbachia density increased with the length of C. suppressalis life. Therefore, C. suppressalis susceptibility was negatively correlated with Wolbachia density.