Citrus species contain toxic furanocoumarins, terpenoids and so on. Unlike other spider mites that cannot develop on citrus plants, Panonychus citri is able to flourish on these plants, and on other nontoxic hosts such as Japanese pear. We hypothesized that P. citri adapted to citrus plants by evolving a detoxification system against the allelochemicals contained in citrus leaves. Here, we examined the efficacy of the detoxification system in relationship to P. citri survival on citrus plants using piperonyl butoxide, a well-known inhibitor of cytochrome P450s. The addition of critical concentrations of piperonyl butoxide had a lethal effect on P. citri larvae and adults reared on citrus plants, but did not affect larvae or adults reared on Japanese pear. This implies that P. citri adapted to citrus plants by evolving detoxification systems composed of cytochrome P450s and/or other detoxifying enzymes.