Overwintering at multiple life stages in Schizotetranychus shii (Acari: Tetranychidae), a specialist of evergreen chinquapin

Spider mites (Acari: Tetranychidae) overwinter as eggs or adult females, but some do so as multiple life stages on evergreen hosts. However, proximate factors influencing such overwintering stages remain poorly understood. This study investigated photoperiodic responses and life-stage compositions during winter in a population of Schizotetranychus shii, a specialist of Japanese chinquapin (Fagaceae). The proportion of non-ovipositing females at 20 °C followed a sigmoid curve with increasing photoperiod. The critical day length was estimated as a photoperiod of 11.8 h light and 12.2 h dark (11.8L), which was similar to the natural photoperiod from late September to early October. Although females grown under 10L and 11L conditions never oviposited within 7 days, 90–96% of them started oviposition within 30 days without chilling, indicating shallow reproductive diapause. In the field, all life stages occurred throughout winter, but their proportions varied considerably. The proportion of eggs declined from early October (62%) to early December (12%), as predicted by the critical day length, but steeply increased toward late February (96%), in which only adult females and eggs remained. These findings suggest that short-day conditions in the field do not maintain adult diapause as predicted by the above experiment. In summary, a short photoperiod in October or November arrests oviposition in emerging females, but they commence oviposition during the short-day season when immature stages are still growing. As a result, individuals at all life stages coexist until all immature stages mature in February.