PERSISTENCE OF TYPHLODROMUS-PYRI AND METASEIULUS-OCCIDENTALIS (ACARI, PHYTOSEIIDAE) ON APPLE AFTER INOCULATIVE RELEASE AND COMPETITION WITH ZETZELLIA-MALI (ACARI, STIGMAEIDAE)

Biological control of European red mite, Panonychus ulmi (Koch), and apple rust mite, Aculus schlechtendali (Nalepa), varied in 1991 in 20 prey-predator treatment plots after single and mixed releases of Metaseiulus occidentalis (Nesbitt) and Typhlodromus pyri Scheuten were made in 1990. Control of spider mites in 1991 was better in mixed-species than in single-species release plots. M. occidentalis was the dominant phytoseiid in mixed-species release plots in 1990, but T. pyri was more common in 1991. In fall, eggs of M. occidentalis disappeared before eggs of T. pyri did. In spring, eggs of T. pyri appeared first. Oviposition by M. occidentalis may have been slowed by cool weather and limited use of pollen as food. With few prey present, competition favored T. pyri, and M. occidentalis eventually disappeared from mixed-, but not from single-species plots. M. occidentalis was more affected by Zetzellia mali (Ewing) than was T. pyri, but T. pyri had more impact on populations of this stigmaeid mite. Competition by Z. mali with the phytoseiids may cause negative and positive effects on biological control. The two phytoseiids have complementary traits that allow them to control pest mites together, but their management may be complicated by interspecific competition.