Biochemical mechanisms of acaricidal activity of 2,4-di-tert-butylphenol and ethyl oleate against the carmine spider mite Tetranychus cinnabarinus
Tetranychus cinnabarinus (Boisduval) is one of the most economically important and highly polyphagous herbivorous pests in fields and greenhouses worldwide. We previously reported that 2,4-di-tert-butylphenol (DTBP) and ethyl oleate (EO) showed significantly acaricidal, repellent and oviposition deterrent properties against T. cinnabarinus via an unknown mechanism. In this study, the acaricidal activities of DTBP and EO and their biochemical mechanisms in controlling T. cinnabarinus were investigated at different time points by assessing the associated changes in toxic symptoms, potential target-related enzyme activities and seven neurotransmitters belonging to the biogenic amines (BAs). The results showed that the median lethal times (LT50) for DTBP and EO were 8 and 15 h after treatment, respectively. Using dynamic symptomatology observations, typical neurotoxic symptoms including excitation, convulsion and paralysis were observed in the mites treated with DTBP and EO. Furthermore, the two compounds exerted significant inhibitory activity on monoamine oxidase (MAO) in adult T. cinnabarinus females in vitro and in vivo and had little effect on acetylcholinesterase (AChE) activity. The content levels of the seven BAs analyzed by UPLC-3QMS were higher in the mites treated with DTBP and EO than in the controls, except for phenethylamine (PEA) (for DTBP and EO) and octopamine (OA) (for EO). These results demonstrate that both DTBP and EO exert effects on T. cinnabarinus that are possibly consequences of their preventive effects on the deamination of BAs in the nervous system, most likely through inhibitory effects on MAO or MAO-like enzymes and/or interactions with certain special biogenic amine G protein-coupled receptors.