Combined detoxification mechanisms and target mutation fail to confer a high level of resistance to organophosphates in Cydia pomonella (L.) (Lepidoptera: Tortricidae)

Despite the frequent and widespread applications of organophosphates against Cydia pomonella this species has developed low levels of resistance to this chemical group. Investigations concerning the mechanisms involved in resistance are scarce, and usually consider only one of the potential mechanisms. With the aim of a better understanding the resistance mechanisms and their possible interaction, four of these mechanisms were investigated simultaneously in one sensitive (Sv) and two resistant strains (Raz and Rdfb) of this insect. Resistant strains displayed an increased mixed function oxidase activity, whereas carboxylesterase activity varied upon the substrate used. The three strains had similar β-naphtyl acetate activity, and the hydrolysis of α-naphthyl acetate and p-nitrophenyl valerate was higher in the Sv strain. The p-nitrophenyl acetate activity was highest in the resistant strains and was strongly inhibited by azinphos and DEF. The Raz strain has a modified acetylcholinesterase (AChE), which resulted in a 0.7-, 3.2- and 21.2-fold decrease in the susceptibility to chlorpyriphos-ethyl-oxon, azinphos-methyl-oxon, and paraoxon-methyl, respectively. These combined resistance mechanisms only conferred to Raz a 0.6-, 7.9- and 3.1-fold resistance to the related insecticides. Organophosphates resistance in C. pomonella results from a combination of mechanisms including modified affinities to carboxylesterase substrates, and increased metabolisation of the insecticide. The apparent antagonism between increased functionalisation and reduced sensitivity of the AChE target is discussed.