Production of winter eggs in Schizotetranychus brevisetosus (Acari: Tetranychidae) inhabiting evergreen Japanese blue oak.

The overwintering pattern of parasitic herbivorous arthropods is closely related to host phenology, because defoliation imposes strong selection pressures on various developmental stages. This relationship has been well studied in populations of spider mites (Acari: Tetranychidae) on deciduous hosts, but is little studied in populations on evergreen hosts, probably because their leaves are always available. However, spring defoliation may also influence the life cycle. We studied the overwintering pattern of Schizotetranychus brevisetosus (Acari: Tetranychidae), a specialist on evergreen oak, Quercus glauca, in Kochi, Japan. Only adult females and their eggs (winter eggs) survived the coldest months. We also observed a conspicuous seasonal change in egg colour and size: December winter eggs were 1.7× larger than September summer eggs, suggesting the winter eggs are diapausing. Adult females produced summer eggs until November and winter eggs from late November until they disappeared in March. The winter eggs hatched in early March. The immature stages developed in mid- to late March, when 39% of old leaves fell, some of which carried hundreds of immature individuals. Therefore, even in evergreen hosts, defoliation places potentially strong selection pressure on the immature stages. Despite this, S. brevisetosus appears not to change its egg hatching season to avoid the defoliation season, so it may have evolved a behavioural adaptation to escape falling leaves. The existence of several species with similar lifestyles suggests the importance of host phenology and predation pressure for evolution of the overwintering pattern.