Interaction between Short-Term Heat Pretreatment and Fipronil on 2nd Instar Larvae of Diamondback Moth, Plutella Xylostella (Linn)

Based on the cooperative virulence index (c.f.) and LC50 of fipronil, the interaction effect between short-term heat pretreatment and fipronil on 2nd instar larvae of diamondback moth (DBM), Plutella xylostella (Linnaeus), was assessed. The results suggested that pretreatment of the tested insects at 30 °C for 2, 4 and 8h could somewhat decrease the toxicity of fipronil at all set concentrations. The LC50 values of fipronil increased after heat pretreatment and c.f. values in all these treatments were below zero. These results indicated that real mortalities were less than theoretical ones and antagonism was found in the treatments of fipronil at 0.39 and 0.78 mg/L after heat pretreatment at 30°C at 2, 4 and 8 h. However, pretreatment at 30°C for 12h could increase the toxicity of fipronil at all set concentrations, the LC50 of fipronil decreased after heat pretreatment and c.f. values in all these treatments were above zero, which indicated real mortalities were higher than theoretical ones. Pretreatment of the tested insects at 35 °C for 2, 4, 8 and 12h was found to increase the toxicity of fipronil at all set concentrations which resulted in the decrease of LC50 values of fipronil and c.f. above zero in all treatments with only one exception. Most interactions were assessed as synergism. The results indicated that cooperative virulence index (c.f.) may be adopted in hormetic effect assessment.