Evaluation of the time–concentration–mortality responses of Plutella xylostella larvae to the interaction of Isaria fumosorosea with the insecticides beta‐cypermethrin and Bacillus thuringiensis

BACKGROUND Entomopathogenic fungi are potential candidates for controlling Plutella xylostella , a cosmopolitan pest of crucifers. In this study, bioassays were conducted to evaluate the interaction between Isaria fumosorosea and sublethal doses of two insecticides, beta‐cypermethrin and Bacillus thuringiensis , against P. xylostella . RESULTS Data of each assay were in good agreement with the time–concentration–mortality model, indicating a strong dependence of the fungus and insecticide interaction on both concentration and post‐exposure time. Using beta‐cypermethrin 58–116 µg mL −1 or B. thuringiensis 222.5–890 µg mL −1 with the fungus significantly enhanced fungal efficacy. The LC 50 values of the fungus declined over a 1–7 day period after exposure, and the LT 50 values decreased with increasing concentration. Based on LC 50 or LC 90 estimates, synergism between the fungus and beta‐cypermethrin resulted in a 2.7–28.3‐fold reduction in LC 50 values and a 12.1–19.6‐fold reduction in LC 90 values, while synergism of the fungus with B. thuringiensis led to a 2.4–385.0‐fold reduction in LC 50 values and a 4.4–151.7‐fold reduction in LC 90 values. CONCLUSION Results show that sublethal doses of B. thuringiensis and beta‐cypermethrin can synergise I. fumosorosea activity on P. xylostella , suggesting that combination of I. fumosorosea with the two insecticides might offer an integrated approach to controlling P. xylostella in practice. © 2014 Society of Chemical Industry