Studies on cholinesterase of the mitesSancassania berlesei(Tyroglyphidae) andTetranychus urticae(Tetranychidae): Substrate-activity relationships and organophosphorus acaricide inhibition

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Abstract
The activity of cholinesterase from crude mite homogenates of Sancassania berlesei (Michael) and Tetranychus urticae (Koch) was investigated using a number of substituted choline substrates and organophosphate inhibitors, and a colorimetric technique. The cholinesterase of S. berlesei was found to be less sensitive to organophosphate inhibitors than that of T. urticae, and differences in sensitivity could be correlated with differences in toxicity. The substrateactivity relationships between S. berlesei and T. urticae were found to be similar to those relationships between resistant and susceptible strains of Acarina species. In this context S. berlesei may be regarded as the ‘resistant’ and T. urticae as the ‘susceptihle’ species. Homogenates of both mites contained separate enzymes capable of hydrolysing acetylcholine and acetyl β-methylcholine. Organophosphorus acaricides were more potent inhibitors of acetyl β-methylcholinesterase than of acetylcholinesterase. The involvement of the former enzyme in the mechanism whereby S. berlesei is tolerant to these organophosphorus compounds is thus unlikely.