Factors affecting biological control of apple mites by mixed populations of Metaseiulus occidentalis and Typhlodromus pyri

Metaseiulus occidentalis (Nesbitt) and Typhlodromus pyri Scheuten have complementary features/ traits that enable them to control effectively plant-feeding mites on apple. Populations of both predators gave as good or better biological control of the apple rust mite (Aculus schlechtendali Nalepa), European red mite (Panonychus ulmi Koch) and two-spotted spider mite (Tetranychus urticae (Koch)) than single-predator populations. With mixed predators, M. occidentalis provided better control of spider mites the first season after release, but T. pyri gave better control in the second season. Several factors affected the ability of predators to provide biological control: When prey were dense, M. occidentalis rapidly increased during the warm mid-summer, while T. pyri provided greater predation when it was cool at the start or end of the growing season. When few prey were present, searching by M. occidentalis was more confined on individual apple leaves, but it migrated between leaves and trees more often. Pollen feeding, cannibalism and interspecific predation were more common by T. pyri. In fall, oviposition by M. occidentalis stopped sooner and in the following spring, T. pyri reproduced before M. occidentalis. Research needs and management of mixed-predator populations are discussed.