ENTOMOLOGY R. Ay and M. Oktay G¨ urkan (2005) Phytoparasitica 33(3):237-244 Resistance to Bifenthrin and Resistance Mechanisms of Different Strains of the Two-Spotted Spider Mite (Tetranychus urticae) from Turkey Recep Ay ∗, 1 and M. Oktay G ¨ urkan 2 Nine different strains of the two-spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae) were collected on cotton from Adana, Antalya, Izmir, Manisa and Urfa in Turkey. Their responses to bifenthrin were investigated using conventional bioassay and biochemical assays. LC 50 and LC 90 values of bifenthrin were determined for all strains by using a residual bioassay with a petri dish–spray tower. Resistance ratios were determined by comparing the samples with a standard susceptible strain, GSS. The resistance ratios of the strains ranged from <1 to 669–fold (at LC 50 ). Of the investigated field strains, only three (two from Adana and one from Urfa) were resistant to bifenthrin. There was a correlation between esterase enzyme activity and bifenthrin resistance according to polyacrylamide gel electrophoresis and microtiter plate assays in the three resistant strains. KEY WORDS: Tetranychus urticae; two-spotted spider mite; resistance; bifenthrin; esterase; cotton. INTRODUCTION The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), a worldwide pest of many plant species, after having been exposed to many insecticides and acaricides for many years has developed resistance to a large variety of compounds (21). This resistance may develop quickly because of the mite’s numerous annual generations and the high frequency of spray applications. Instead of using integrated pest management for controlling pests in cotton areas, Turkish producers prefer to use broad-spectrum insecticides that are effective against all pests. Such insecticides, when used against major pests, eliminate them as well as their natural enemies. As a result, secondary pests such as two-spotted spider mites became a major problem in cotton-growing areas. The evolution of resistance depends on gene flow among populations that colonize different habitats in a given area (21). Excessive use of broad-spectrum pesticides with the same mechanism of action promotes the development of pest resistance in rapidly reproducing species, such as spider mites and aphids. It has often been reported that T. urticae developed resistance to many insecticides (9,12,14,20), making it imperative to choose suitable and effective methods for monitoring the development of pest resistance. The petri dish bioassay method was initially used by Dennehy et al. (4) to monitor spider mite resistance to miticides. When using this method, pesticide residues retain their Received Nov. 10, 2004; accepted Feb. 7, 2005; http://www.phytoparasitica.org posting May 17, 2005. 1 Plant Protection Department, Faculty of Agriculture, S¨ uleyman Demirel University, 32260 Isparta, Turkey. * Corresponding author [e-mail: recepay@ziraat.sdu.edu.tr; fax: +90-246-237-1693]. 2 Plant Protection Department, Faculty of Agriculture, Ankara University, 061110 Ankara, Turkey. Phytoparasitica 33:3, 2005 237