To examine the permethrin resistance of Amblyseius womersleyi (Hiranuma-1 strain), 10 successive permethrin selections were carried out under laboratory conditions by spraying the pesticide solution on the mites on a leaf disc. During six selections with wettable-powder (WP) permethrin concentrations of 400ppm, the survival rates increased from 35.5% to 61.6%. Since the emulsifiable concentrate (EC) of permethrin is more toxic than the WP formulation, the next four selections were switched to

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... permethrin concentrations of 50ppm. The survival rates increased from 8.7% to 31.6% during these selections. The WP permethrin LC50 of this selected strain (SEL10) was 522.6ppm. Although this LC50 is only twice as high as that of the initial strain (Hiranuma-1), it is much higher than the recommended permethrin concentration for tea growers. The resistance stability of permethrin, methidathion and methomyl of the SEL10 strain was monitored after the selection. The permethrin resistance level remained stable for 20 months, and then decreased greatly. The methidathion and methomyl resistance level decreased slightly over the monitored period.