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.