A. Soleymanzade - Department of Plant Protection, Faculty of Agriculture, Urmia University, Urmia, Islamic Republic of Iran.
O. Valizadegan - Department of Plant Protection, Faculty of Agriculture, Urmia University, Urmia, Islamic Republic of Iran.
G. Askari Saryazdi - Plant Protection Research Department, Yazd Agricultural and Natural Resources Research and Education Center, AREEO, Yazd, Islamic Republic of Iran.

Diamondback moth (Plutella xylostella) is a serious pest of cruciferous vegetables worldwide. In Iran, it is commonly controlled by using chlorpyrifos. Due to a range of biochemical and behavioral features, this pest can rapidly develop resistance to many insecticides from different groups. To achieve a better resistance management plan, a chlorpyrifos resistant strain of P. xylostella was selected under laboratory conditions and its cross resistance to five other insecticides and resistance characteristics were investigated. After ۱۵ generations of selection, the selected strain (CLRS) developed ۳۹.۶۱-fold higher resistance to chlorpyrifos in comparison with susceptible strain (AL). CLRS exhibited ۱۹.۶۲-, ۱۷.۸۴-, ۳.۴۳- and ۳.۳۳-fold cross resistance to hexaflumuron, indoxacarb, thiodicarb and flubendiamide, respectively, but showed no cross resistance to abamectin. Synergism and biochemical studies suggested potential involvement of Esterase (EST) in CLRS. However, no difference was seen for Glutahion-S-Transferase (GSTs) and Mixed Function Oxidase (MFO) in CLRS and AL strains. To determine the role of AcetylCholinEsterase (AChE) insensitivity in resistance mechanism, Kinetic parameters (Km and Vmax) and inhibitory effect of chlorpyrifos-oxon on this enzyme were evaluated. Affinities and hydrolyzing efficiencies of AChE in CLRS were higher than AL. This enzyme in CLRS was also less sensitive to inhibition by chlorpyrifos-oxon. Results indicated that chlorpyrifos resistance exhibited cross resistance to other insecticides from different classes and enhanced EST activity and AChE insensitivity were probably the main factors in chlorpyrifos resistance. These results can help the users of insecticides and can delay the resistance development of P. xylostella.

Acetylcholine esterase, Brassicaceae, Insecticide resistance, Resistance management plan, Synergism.