Effects of pesticide application and Neoseiulus barkeri on spider mite control using commercialized Neoseiulus californicus in a Japanese pear greenhouse

A predator-release device with a waterproof shelter protecting a slow-release sachet containing Neoseiulus californicus (McGregor) (Acari: Phytoseiidae) has become commercially available for spider mite control. In this study, we examined the population dynamics of spider mites and phytoseiid mites in a Japanese pear greenhouse equipped with predator-release devices during 2019–2021. The presence of commercialized N. californicus on pear leaves before noticeable spider mite occurrence was confirmed during each of the survey years. Our results demonstrated that the spider mite density remained low for the first two years, but a notable spider mite outbreak occurred in the final year, presumably because a fungicide highly toxic to N. californicus was used before the release of the commercialized N. californicus individuals. Results also demonstrated that Neoseiulus barkeri Hughes (Acari: Phytoseiidae), the second dominant phytoseiid mite species, increased its relative proportion in the summer and possibly affected spider mite occurrence through intraguild predation. These results suggest that avoiding harmful pesticides and elucidating interactions among phytoseiid mite species are crucially important for more effective utilization of commercialized N. californicus for spider mite control.