Biological activity and field efficacy of Bacillus thuringiensis kurstaki strains with protein film adjuvants (PFAs) against Plutella xylostella (Lepidoptera: Plutellidae)

Abstract Background Diamondback moth (DBM), Plutella xylostella L. is a globally distributed insect pest with developed resistance to many insecticides such as Cry1Ac proteins of Bacillus thuringiensis (Bt). It is therefore urgent to develop novel Bt bacterial strains with biocontrol activity against the DBM population. Results Nine Bt var. kurstaki (Btk) strains with high insecticidal activity against DBM larvae were selected, and three of them (B6, P2, and P6) had middle lethal concentrations (LC 50 ) of 39.23 μg/mL, 3715.08 μg/mL, and 121.62 μg/mL, respectively, against 3rd instar DBM at 48 h. The three Btk strains were formulated with three different protein film adjuvants (PFAs) to perform insecticidal activity bioassays on laboratory potted cabbages. B6 and P6 formulations showed a faster insecticidal rate than the control insecticide (Emamectin benzoate, EB), and all caused approximately 100% mortality at 24 h after spraying. Interestingly, B6 demonstrated enhanced adhesion capacity to the leaf surface with the help of PFAs, while P6 showed reduced adhesion. Field control assays showed that B6 and P6 can effectively control the DBM population. However, they were not particularly effective against Pieris rapae larvae. Conclusion The present study identified two Btk strains with efficient insecticidal activity against DBM in both laboratory and field, as well as three PFAs allowing insecticide dose reduction while retaining efficacy. © 2025 Society of Chemical Industry.