Problems associated with acaricide resistance in Tetranychus urticae Koch have been recorded in several countries including Brazil. The objective of this study was to characterize the resistance of T. urticae to the acaricide chlorfenapyr and to evaluate the resistance frequency in commercial fields of six crops in the State of São Paulo (SP). Selections for resistance and susceptibility to chlorfenapyr were performed in a population of T. urticae collected in 2002 from a commercial chrysanthemum field in Holambra county, SP. After six selections for resistance and five selections for susceptibility, susceptible (S) and resistant (R) strains of T. urticae to chlorfenapyr were obtained. The resistance ratio (R/S) at the LC50 reached values of 571-fold. A discriminating concentration of 37.4 mg L-1 of active ingredient (A.I.) was established for monitoring chorfenapyr resistance in T. urticae. Twenty one mite populations were collected from different crops (papaya, strawberry, bean, tomato, chrysanthemum, rose), in various counties in the State of São Paulo. Bean-leaf-disc arenas were infested with T. urticae mites and submitted to chlorfenapyr spraying at the discriminating concentration, using a Potter tower. The results showed significant differences among populations in their responses to chlorfenapyr. Populations with frequencies of resistance from 0,0 to 65.4% were detected. The highest frequencies of resistance were observed in populations obtained from chrysanthemum in Holambra.
Two-spotted spider mite; acaricide resistance; chemical control
