On the Seasonal Fluctuation of Population of Fruit Tree Red Spider Mite in Sapporo (Acarina:Tetranychidae)

In the orchards around Sapporo, the population of Panonychus ulmi was exceedingly dominant as compared with other four species, Tetranychus viennensis, Tetranychus telarius, Bryobia praetiosa and Bryobia rubrioculus, in the year of 1957. The seasonal prevalence of P. ulmi was showed in Fig. 1. The seasonal change of age distribution of this species on the apple leaf takes place generally as follows: Larva association→Larva-Nymph association→Adult-Egg association→Egg association→(Nymph association)→Egg association→(Egg-Larva-Nymph association)→Egg-Nymph-Adult association→Adult association. The apparent sex-ratio of P. ulmi shows that it has higher ratio in the female or balanced ratio in both sexes. It is variable during the season.