We investigated the effect of temperature on development and demographic parameters such as the intrinsic rate of natural increase (r (m)) of the two spider mite species Tetranychus merganser Boudreaux and T. kanzawai Kishida at eleven constant temperatures ranging from 15 to 40°C at intervals of 2.5°C. Both male and female T. merganser and T. kanzawai completed development from egg to adult at temperatures ranging from 15 to 37.5°C. The longest developmental duration of immature stages was found at 15°C and the shortest developmental duration was found at 35°C for both species. Using linear and non-linear developmental rate models, the lower thermal thresholds for egg-to-adult (female and male) and egg-to-egg development were estimated as 12.2-12.3°C for T. merganser and as 10.8°C for T. kanzawai. The highest developmental rates were observed at around 35°C, whereas the upper developmental thresholds were around 40°C for both species. In fact, at 40°C, a few eggs of either species hatched, but no larvae reached the next stage. The r (m)-values of T. merganser ranged from 0.072 (15°C) to 0.411 day(-1) (35°C), whereas those of T. kanzawai ranged from 0.104 (15°C) to 0.399 (30°C). The r (m)-values were higher for T. kanzawai than for T. merganser at temperatures from 15 to 30°C, but not at 35°C (0.348 day(-1)). Total fecundity of T. merganser was also higher than that of T. kanzawai at 35°C. These results indicate that higher temperatures favor T. merganser more than T. kanzawai.

The objective of this study was to investigate whether four spider mite species, Tetranychus ludeni, T. phaselus, T. piercei and T. truncatus, currently with insignificant economic impact, have the potential to achieve the same status as T. urticae, which until now has been considered as the most serious tetranychid pest species in orchards and greenhouses. We investigated the effect of temperature on development, survival and oviposition at 11 constant temperatures ranging from 15 to 40 °C at intervals of 2.5 °C and estimated demographic parameters, such as the intrinsic rate of natural increase (rm), for these five species at five constant temperatures. Developmental time from egg to adult (female and male) decreased with increasing temperature from 15 to 32.5 °C in all five species, but increased slightly at 35 °C or higher, especially in T. ludeni and T. urticae. Using linear and non-linear developmental rate models, the lower thermal thresholds for egg-to-adult (female and male) and egg-to-egg development were found to range from 9.8 to 11.7 and from 9.8 to 11.4 °C, respectively. The intrinsic optimal temperature (TΦ) ranged from 18.0 to 27.4 °C for egg-to-female adult and from 23.9 to 27.2 °C for egg-to-egg development. The oviposition period and adult longevity were strongly affected by temperature. The rm-values increased with increasing temperature from 15 to 30 or 35 °C in all five species. The highest rm-values at each temperature were 0.114 day(-1) at 15 °C for T. ludeni, 0.199 day(-1) at 20 °C for T. urticae, 0.314 day(-1) at 25 °C for T. ludeni, 0.451 day(-1) at 30 °C for T. ludeni and 0.433 day(-1) at 35 °C for T. truncatus. The total fecundity, net reproductive rate (R0) and rm of T. ludeni were higher than those of T. urticae at all temperatures. T. piercei and T. truncatus showed higher rm-values at 30 and 35 °C than T. urticae. The results indicate that the former three species are better adapted to hot weather than T. urticae and have a high potential to become serious pests.

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