| The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), has become a major pest on vegetable crucifers due to its overlapping generations, high reproductive rate and high ability to develop resistance to insecticides, especially hi tropical and subtropical regions. Spinosad is a recently developed macrolide that has high efficacy hi killing Lepidoptera, Diptera, and Thysanoptera. However, field populations of the diamondback moth resistant to Spinosad were found in Hawaii, USA, and Wenzhou, China only after two to three years of large-scale application of this insecticide, hi this study, homozygotes of the diamondback moth with similar genetic background, susceptible (SS) or highly resistant (RR) to Spinosad, were obtained by crossing and selection using a susceptible strain originally collected from Wuhan, China hi 1989 and a field resistant strain originally collected from Wenzhou, China in 2001. Development, survival and reproduction were compared between the SS and RR strains at low, moderate, and high temperatures. Our objective was to investigate whether resistance to Spinosad is associated with fitness changes, and whether fitness changes are temperature-dependent.Eggs or pupae of SS and RR strains were kept at 6℃ for 0-28 days, or pupae of the two strains were kept -5℃ for 0-21 days. These low-temperature treated eggs or pupae were then moved to 25℃ to observe their subsequent development, survival or reproduction at this moderate temperature. The results showed that: (1) there was no or little difference between the two strains when they were not exposed to the low temperatures; (2) when eggs were kept at 6℃ for various periods of time, the subsequent percent hatch of eggs and percent pupation of larvae of RR strain were significantly lower, and development time of eggs of RR strain was significantly longer, compared to those of SS strain; (3) when pupae were kept at 6癈 for various periods of time, percent hatch of eggs in progeny of RR strain was significantly lower than that of SS strain; and (4) when pupae were kept at -5℃ for various periods of tune, the subsequent development time of pupae of RR strain was significantly longer and percent hatch of eggs in progeny of RR strain was significantly lower, compared to those of the SS strain.The development, survival and reproduction were also compared between RR and SSstrains at natural fluctuating low temperature regimes as well as at simulated natural fluctuating moderate and high temperature regimes. The results showed that: (1) at low temperature, in the 752 eggs of the SS strain initially inoculated 639 of them hatched and 15 of them developed to the pupal stage, while in the 823 eggs of the RR strain initially inoculated only 470 hatched and 1 of them developed to the pupal stage. Percent hatch of eggs (57%) and percent pupation of larvae (0.2%) of the RR strain were significantly lower than those (85% egg hatch, 2.4% pupation) of SS strain; (2) at moderate temperature, the two strains showed limited differences though statistically significant differences existed for some traits. Analysis using life and fertility tables showed that the intrinsic rate of increase was 0.2138 $/$/day for SS and 0.2112 day for RR, very similar between the two strains; and (3) at high temperature, percent pupation of larvae (12.4%) of RR strain was significantly lower than that of SS (21.2%), and mean pupae weight (3.77 mg) of RR was significantly lower than that of SS (4.25 mg). There were no significant differences in other traits. Life and fertility table analysis showed that the intrinsic rate of increase of RR (0.0793 day) was lower than that of SS (0.1153 day).It is concluded from these results that resistance to spinosad of the diamondback moth is associated with significant fitness costs in low temperature tolerance as well as some fitness costs in heat (>32) tolerance, but little or no fitness costs at moderate temperatures. In regions with a cold-winter (temperature frequently goes below<6) and a hot summ...
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