Adult female sensitivity to day-length conditions in terms of winter-egg production in Schizotetranychus brevisetosus Ehara (Acari: Tetranychidae)

Schizotetranychus brevisetosus Ehara lives on the leaves of Quercus glauca L. In Tosayamada, Kochi Prefecture, Japan, females in the field stop producing yellowish-white summer eggs by early November, and after several weeks of reproductive arrest, they produce bright orange winter eggs, from late November or early December until March, when they die. In this study, adult female sensitivity to day-length conditions was evaluated at the beginning of the season in which they produce winter eggs. Females collected from the field in late November and early December were reared under one of two sets of day-length conditions (10L:14D or 15L:9D) for 16 days at either 15°C or 25°C. Both groups of field-collected females increased their winter-egg production under long day-length conditions, and December females laid significantly more eggs than November females at 25°C. In contrast, females from the laboratory strain produced summer eggs at the higher temperature, but were not sensitive to day length. The trend whereby long daylength conditions promote the production of winter eggs might partly explain the field observation that the number of winter eggs increases during the period over which day length increases, towards the vernal equinox (late March).