Population dynamics of Panonychus osmanthi (Acari: Tetranychidae) on two Osmanthus species.

Panonychus osmanthi is a non-diapausing species of spider mite that superficially resembles P. citri. It infests Osmanthus species, which are evergreen roadside and garden trees. The population dynamics of P. osmanthi were studied on Osmanthus aurantiacus and O. x fortunei during a three-year period. Seasonal changes in P. osmanthi populations were fundamentally the same in each year, although their density differed greatly from year to year. The P. osmanthi population was bimodal, with one peak in spring (May-June) and another in winter (November-January). Populations abruptly declined after the spring peak. Predators showed a delayed density-dependent response to changes in spider mites from spring to summer, whereas in autumn and winter, predators were few because they had entered diapause. To determine the effect of predators on the rapid decline of spider mites just after the spring peak, the predators were removed by treating the trees with a synthetic pyrethroid. As a result, spider mite density did not decline after the spring peak and remained at a high level during the June-August period when spider mite density is usually very low. This suggests that predators play an important role in the drastic decline of P. osmanthi density just after the spring peak.