Monitoring and mechanisms of organophosphate resistance in San Jose scale, Diaspidiotus perniciosus (Hemiptera: Diaspididae)

San Jose scale, Diaspidiotus perniciosus (Comstock), is a serious pest in Chilean apple tree orchards, and a number of organophosphate insecticides were used to control them for decades. Recently, control failures with these insecticides were reported and linked to insecticide resistance development. In this study, 40 San Jose scale field populations were collected and their susceptibility to two commonly used organophosphate insecticides, that is chlorpyrifos and methidathion, was assessed. The obtained bioassay data suggest moderate levels of resistance to both insecticides when compared to a reference susceptible strain. The highest resistance ratio (RR) detected for chlorpyrifos and methidathion was 31‐fold and 11‐fold, respectively. The bioassay results suggest the occurrence of a significant cross‐resistance between both compounds. Biochemical measurements revealed a role for esterases in conferring resistance to organophosphates, but not modified acetylcholinesterase. The spatial spread and extend of insecticide resistance were also evaluated. Our result shows that no autocorrelation can be assumed, and then, insecticide resistance is caused by random factors.