The synergistic effect of HycuNPV on four common chemical insecticides.

[Objectives] To compare the insecticidal activity of four commonly used insecticides; emamectin benzoate, beta-cypermethrin, chlorobenzuron and matrine, on the larvae of Hyphantria cunea (Drury), in order to determine which have a significant synergistic interaction with the Hyphantria cunea nuclear polyhedrosis virus (HycuNPV). [Methods] Groups of 3rd instar larvae of H. cunea infected with HycuNPV were treated with one of the four chemical insecticides above using the artificial feed surface coating method. The synergistic interaction between each insecticide and HycuNPV was then evaluated using the co-toxicity factor method. Those insecticides with a significant synergistic effect were identified, and the optimal pesticide: virus ratio and concentration was determined. [Results] The relative toxicity of the four insecticides tested was: emamectin benzoate >beta-cypermethrin>chlorobenzuron>matrine, and their median lethal concentrations (LC50) were 0.021, 2.149, 4.522 and 70.255 mg-L^-1, respectively. A combination of emamectin benzoate (0.021 mg-L^-1) and HycuNPV(1.18x10⁶ OBs mL^-1) had a significant synergistic effect. A pesticide: virus volume ratio of 1-9 resulted in a 29.06 fold increase in mortality and a 2.89 day reduction in the median lethal time (LT₅₀) compared to HycuNPV in isolation. [Conclusion] Among the insecticides tested, emamectin benzoate had both the highest toxicity to 3rd instar H. cunea larvae, and the greatest synergy with HycuNPV. Optimum toxicity was achieved at a concentration of 0.021 mg L^-1 emamectin benzoate and 1.18x10⁶ OBsmL^-1 HycuNPV and a volume ratio of 1:9. [ABSTRACT FROM AUTHOR]