BIOLOGICAL-CONTROL OF APPLE MITES - IMPACT OF ZETZELLIA-MALI (ACARI, STIGMAEIDAE) ON TYPHLODROMUS-PYRI AND METASEIULUS-OCCIDENTALIS (ACARI, PHYTOSEIIDAE)

Zetzellia mali (Ewing) feeds on and is fed upon by Typhlodromus pyri Scheuten and Metaseiulus occidentalis (Nesbitt), especially when phytophagous mites are scarce on apple foliage. These mites interact indirectly through competition for prey. Probably because of both indirect and direct effects, T. pyri was less dense and M. occidentalis was much less dense in plots with Z. mali than in plots where Z. mali was suppressed by endosulfan. M. occidentalis also had less effect on Z. mali than T. pyri. Biological control of Panonychus ulmi (Koch) and Aculus schlechtendali Nalepa was effective where Z. mali occurred alone at high levels or with either or both phytoseiids. Plots that received mixed T. pyri-M. occidentalis had effective control of pest mites with or without Z. mali. Z. mali displaced M. occidentalis and controlled pest mites in plots with M. occidentalis only. In plots with T. pyri only, dominance of T. pyri or Z. mali was mixed-most often near equal levels occurred, but sometimes one or the other dominated. In mixed plots T. pyri displaced M. occidentalis in 1991, but interactions between T. pyri and Z. mali in 1992 were like those in plots with T. pyri only. Releases of 25-50 M. occidentalis and 200-300 P. ulmi into trees with many Z. mali did not allow for establishment of M. occidentalis, but establishment readily occurred where few Z. mali were present. Management of M. occidentalis, T. pyri, Z. mali, and pest mites is discussed in relation to long-term biological control.