Using microtitration method, the relationship between Phenoloxidase activity and the resistance of the diamondback moth Plutella xylostella (Linnaeus) to the novel insecticide butane-fipronil was determined in vitro. After selection of the tenth-generation by butane-fipronil, the resistance of the fourth instar larvae was increased 83.80-fold as compared to the susceptible strain. Phenoloxidase activity of the resistant strain (POr) was 1.29-fold higher than the susceptible one (POs). However, the Km and optimum pH values were similar in resistant and susceptible strains, which were 1.11 mM and 6.5, respectively. Both POr and POs have maximum stability at pH values less than 7.0, although POs was less stable at lower pH values than POr. In addition, the thermal stabilities of the two phenoloxidase were very similar. It is suggested that PO may play an important role in the increasing resistance of pests to pesticides
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