The involvement of acetylcholinesterase in resistance of the California red scale Aonidiella aurantii to organophosphorus pesticides

Laboratory toxicity bioassays using chlorpyrifos (Dursban) confirmed the notion that development of resistance is responsible for widespread failures to control the California red scale, Aonidiella aurantii (Mask.) by applying organophosphorus (OP) compounds in citrus groves in Israel. Higher Vmax values of acetylcholinesterase (AChE) activity (9–13 fold) were measured in resistant strains collected from the field as compared to a susceptible line. No differences were found with respect to Km values using acetylthiocholine iodide as a substrate, or degree of inhibition (expressed by IC50 values) by the OP compounds chlorpyrifos‐oxon and paraoxon and the carbamate pirimicarb. We suggest that resistance of the California red scale is caused by excess of AChE molecules able to bind and thus scavenge inhibitory OP compounds. This scavenging mechanism related to AChE may be similar in other insect species where elevated levels of detoxifying esterases were implicated in conferring OP resistance.