Effects of Predacious Insects on the Population Dynamics of Three Spider Mite Species (Acari

The effects of predacious insects on the population dynamics of three spider mite species, Amphitetranychus viennensis, Tetranychus urticae, and Panonychus citri, which are serious pests infesting the Japanese pear, were compared by experimentally introducing each spider mite species onto Japanese pear trees in an orchard. A. viennensis and T. urticae immediately decreased in density after the introduction, because the density of predacious insects, Scolothrips takahashii and Feltiella sp., increased steeply. In contrast, P. citri increased rapidly, reaching a maximum number of 37.9 adult females per leaf 15 days after introduction, because of the lack of immediate response of the predators, and thereafter decreased abruptly with the occurrence of voracious predacious insects, Oligota spp. and Stethorus japonicus. Another experiment in which the predators were excluded by bagging shoots was also carried out, with the result that the densities of all the spider mite species increased. These results confirmed that predacious insects suppressed the density of spider mites in different fashions: against A. viennensis and T. urticae, even when they were at low densities, the predacious insects responded swiftly, whereas against P. citri, predacious insects occurred only when the spider mite density became high.