Occurrence of Panonychus citri（Acari: Tetranychidae）and Natural Enemies in Citrus Fields under Conventional Pesticide Application in Shizuoka Prefecture

For 2 years we investigated the occurrence of Panonychus citri and its natural enemies in 30 citrus fields in eastern, central, and western Shizuoka Prefecture. Panonychus citri female abundance increased in fields with low frequencies of petroleum oil application or high frequencies of insecticide application. The main natural enemy species were Stethorus spp., Oligota spp., and Neoseiulus californicus. Stethorus spp. dominated in the east and increased in abundance in fields with higher densities of P. citri, but it was rarely observed in central or western Shizuoka Prefecture; notably, the application frequency of insecticides harmful to Stethorus spp. was lower in the east than in the other two areas. Neoseiulus californicus and Oligota spp. were observed in all three areas, and N. californicus abundance increased in fields with higher densities of P. citri. The density of N. californicus per P. citri was highest in the west, where Stethorus spp. were rarely found, and lowest in the east. Our findings suggest that the species composition of natural enemies was affected by insecticide application. Panonychus citri density was lower in the east than in the other two areas, suggesting that Stethorus spp. was a more effective predator than N. californicus at low prey densities.