Annual Life Cycles of Populations of the Two-Spotted Spider Mite, Tetranychus urticae KOCH (Acari: Tetranychidae) in Four Japanese Pear Orchards

Abstract

The annual life cycles of Tetranychus urticae populations were studied in four different Japanese pear orchards in Ibaraki Prefecture, central Honshu. some mites overwintered on weeds in the orchards without entering diapause, and others entered diapause and passed through winter in bark crevices of pear trunks and within the strands of vinyl binding ropes used to bind the branches to a trellis. The seasonal prevalences of mite populations in the four pear orchards were similar, but their abundances varied greatly. Population peaks appeared in July and/or from September to early October. The population peak in July was formed by mites that migrated from ground cover vegetation onto the pear trees in spring. Thereafter, mite densities remained at low levels throughout the summer, and a second peak occurred in autumn. In years in which there was a single population peak in autumn, few or no mites appeared on pear leaves during the period of the preceding spring to summer, suggesting that the autumn mites were immigrants from other places, such as neighboring pear orchards. At 18°C and 10 h of light, the incidence of diapause varied from 15.6 to 67.7% among the four populations. In the filed, the proportion of diapausing females reached 50% between mid-September and early October. Although diapausing females had overwintered in the pear bark crevices and vinyl binding ropes, the number of females gradually decreased as the season progressed. This decrease may be because some females came out of diapause in response to occasional warm days in winter and migrated onto the ground cover vegetation and/or died from starvation during subsequent cold and dry periods.