Species composition and seasonal occurrence of spider mites (Acari: Tetranychidae) and their predators in Japanese pear orchards with different agrochemical spraying programs.

Abstract

The species composition and seasonal occurrence of spider mites and their natural enemies were compared in three Japanese pear orchards with different control pressures, from 1996 to 1999. The orchards were an agrochemical-free orchard, a pesticide-free orchard, and a conventionally controlled orchard. The differences in the dominant species of spider mites and phytoseiid mites among these three orchards were observed. Among the spider mites, Amphitetranychus viennensis was dominant throughout the four years in the agrochemical-free orchard whereas Panonychus citri was dominant in the pesticide-free orchard. In the conventionally controlled orchard, the dominant spider mite species was Tetranychus urticae, except for in the year 1998. The dominant phytoseiid mites were Amblyseius orientalis and Amblyseius sojaensis in the agrochemical-free orchard, Typhlodromus vulgaris in the pesticide-free orchard, and Amblyseius californicus in the conventionally controlled orchard. The predacious insects of spider mites which were abundant in the three orchards were Stethorus japonicus, Oligota spp., Scolothrips takahashii, and Feltiella sp. Unlike the spider mites and phytoseiid mites, the dominant species of predacious insect varied greatly from year to year within each orchard. This suggests that the species composition of the predatory insects may be less affected by agrochemicals than that of spider mites or phytoseiid mites. Differences among the peak densities of spider mites among the orchards seemed to be due to differences in the dominant spider mite species rather than to differences in control pressure with chemicals. The peak density in the orchards where P. citri was dominant was higher than in those where A. viennensis or T. urticae was dominant, nevertheless the predacious insects occurred in all orchards. This suggests that the predacious insects would be less effective in suppressing P. citri than in suppressing T. urticae and A. viennensis.