Identification and characterization of new mutations in mitochondrial cytochrome b that confer resistance to bifenazate and acequinocyl in the spider mite Tetranychus urticae

Background: In spider mites, mutations in the mitochondrial cytochrome b Q_o pocket have been reported to confer resistance to the Q_o inhibitors bifenazate and acequinocyl. In this study, we surveyed populations of the two-spotted spider mite Tetranychus urticae for mutations in cytochrome b, linked newly discovered mutations with resistance and assessed potential pleiotropic fitness costs.

Results: We identified two novel mutations in the Q_o site: G132A (equivalent to G143A in fungi resistant to strobilurins) and G126S + A133T (previously reported to cause bifenazate and acequinocyl resistance in Panonychus citri). Two T. urticae strains carrying G132A were highly resistant to bifenazate but not acequinocyl, whereas a strain with G126S + A133T displayed high levels of acequinocyl resistance, but only moderate levels of bifenazate resistance. Bifenazate and acequinocyl resistance were inherited maternally, providing strong evidence for the involvement of these mutations in the resistance phenotype. Near isogenic lines carrying G132A revealed several fitness penalties in T. urticae; a lower net reproductive rate (R₀ ), intrinsic rate of increase (rm) and finite rate of increase (LM); a higher doubling time (DT); and a more male-biased sex ratio.

Conclusions: Several lines of evidence were provided to support the causal role of newly discovered cytochrome b mutations in bifenazate and acequinocyl resistance. Because of the fitness costs associated with the G132A mutation, resistant T. urticae populations might be less competitive in a bifenazate-free environment, offering opportunities for resistance management. © 2019 Society of Chemical Industry.