2014 Volume 56 Pages 29-36
Experiments were carried out by using four small glass houses. A predatory mite, Phytoseiulus persimilis Athias-Henriot had been observed to control the population of the two-spotted spider mite, Tetranychus urticae Koch successfully on egg-plants since the ratio of the number of P. persimilis female adults per plant to that of T. urticae was more than 0.1. Spraying of propylene glycol monolaurate, which killed T. urticae selectively with minimal effects on P. persimilis,increased the ratio of the predators drastically, which had been released once and increased on egg-plants before spraying the acaricide. Some plants, where the ratio of the predators was less than 0.1, remained although the average ratio of the predators was more than 0.1. However, thenumber of plants, where the ratio of the predators was less than 0.1, tended to decrease gradually with the passing of the days. Positive correlations between the densities of the both species were observed after the ratio of the predators had been more than 0.1 for two weeks. These results suggested that P. persimilis tended to disperse depending on their own density and density of T. urticae when the ratio continued being more than 0.1.