Effect of photoperiods on development and acaricide susceptibility in the two-spotted spider mite, Tetranychus urticae.

Demographic analysis of Tetranychus urticae under photoperiods of 12L:12D, 14L:10D and 18L:6D at 75% relative humidity and 25 °C showed that the developmental time and oviposition per female declined with increasing light period. The nymph, oviposition and adult stages were significantly shortened, resulting in shorter generation duration and faster population decline, but there was no effect on egg and larval stages of T. urticae. Kaplan-Meier survival analysis followed by a log-rank test indicated that the mean and median survival times were 40.4 (12:12), 39.1 (14:10), and 38.1 (18:6) days, and 42.0, 40.0, 39.0 days, respectively-this difference among photoperiods was significant. The total number of eggs per female under the three photoperiods was 69.63 (12:12), 77.44 (14:10) and 42.17 (18:6), respectively, and the sex ratios were 70.0, 81.6 and 71.6% female offspring. Under 14 h light, T. urticae experienced its highest net reproductive rate (R0 = 83.0577), intrinsic rate of increase (rm = 0.2740), finite rate of increase (λ = 1.3153), lowest mean generation time (T = 16.1277 days) and population doubling time (Dt = 2.5294 days). All demographic parameters displayed a decreasing relationship with the light phase under the three photoperiods. No significant difference in susceptibilities to the acaricides diafenthiuron and propargite was shown among the three photoperiods. The results of this study indicated that the 14L:10D photoperiod was optimal for the development and reproduction of T. urticae, and the 18L:6D period was disadvantageous for spider mite development.

Molecular cloning and expression of glutathione S-transferases involved in propargite resistance of the carmine spider mite, Tetranychus cinnabarinus (Boisduval).

The carmine spider mite (CSM) Tetranychus cinnabarinus has become a serious pest in China and has developed resistance to acaricide propargite as it is used to control mites worldwide including T. cinnabarinus. In this study, a resistant colony of T. cinnabarinus, PRR34 (37.78-fold resistant ratio), was established after 34 generations of propargite selection, and cross-resistance patterns of 7 other acaricides were determined in comparison with a susceptible strain (SS). The contribution of detoxification enzymes to propargite tolerance were investigated using biological, biochemical and molecular approaches. Enzyme inhibitor synergist tests suggested glutathione S-transferases (GST) involvement in propargite-resistance of PRR34, and GST activity against 1-chloro-2,4-dinitrobenzene (CDNB) was correlated with the development of resistance. Eight novel GST genes (TcGSTd1, TcGSTd2, TcGSTm1, TcGSTm2, TcGSTm3, TcGSTm4 and TcGSTm5) were cloned, and phylogenetic analysis showed that the eight GST genes were most closely related to GST family delta and mu from Tetranychusurticae. Quantitative RT-PCR revealed that the expression level of GSTs in PPR34 strain increased in larvae, nymphs and adults, while decreased in eggs compared with that of SS. Collectively, these results support a role of GSTs in mediating resistance to propargite in the PRR34 strain. TcGSTd1,TcGSTd2 and TcGSTm2 genes might play significant roles in propargite resistance of CSM, especially at adult stage. This is the first attempt to define specific genes involved in GST mediated propargite resistance of T. cinnabarinus at the transcriptional level.