Photoperiodic control of reproductive arrest in the oak-inhabiting spider mite Schizotetranychus brevisetosus (Acari: Tetranychidae).

Populations of Schizotetranychus brevisetosus Ehara (Acari: Tetranychidae), which live on the evergreen oak (Quercus glauca), survive the coldest months as either adult females or winter eggs. Adult females comprise the majority of the population in early November and oviposit from late November to early March. Most winter eggs hatch by late March, and adults of the next generation emerge in April. This species is considered an egg-diapausing species, but the environmental cues that regulate female reproductive arrest and resumption are mostly unknown. We investigated the photoperiodic responses of autumn reproductive arrest in 10 populations collected from different elevations in Shikoku, Japan. All populations showed long-day responses to critical daylength (CDL) around 12.2 h light (12.2L) at 20 °C, though there was no linear relationship between CDL and altitude. This result explains the steep decline in the proportion of summer eggs in November. Nonreproductive females developed under 10L at 20 °C commenced oviposition 14.3-20.6 days after transferring to 15L. This long pre-oviposition period explains the reduction in eggs before winter reproduction and suggests shallow adult diapause. Eggs thus obtained hatched in 12.9-15.3 days, similarly to summer eggs. Therefore, egg diapause in S. brevisetosus is much shallower than in species on deciduous hosts, which lay their winter eggs in early autumn to hatch at leaf flush in spring. The reproductive arrest and short hatching period may be an adaptation allowing egg-laying in midwinter, when predation pressure is low.