Phytoseiids as biological control agents of phytophagous mites in Washington apple orchards
Schmidt-Jeffris, Rebecca Ann
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The integrated mite management (IMM) program in Washington has depended on the biological control of spider mites provided by Galendromus occidentalis (Nesbitt). We explored methods of improving this program. A series of inundative releases of G. occidentalis was performed in commercial apple orchards. None of the releases increased G. occidentalis numbers or reduced pest spider mite populations. It was concluded that a cost prohibitive number of predators would be necessary to maintain pest mites below economic thresholds, emphasizing the importance of conservation biological control. Phytoseiids in apple orchards across Washington were sampled to determine diversity and elucidate factors affecting abundances. While the community was expected to be completely dominated by G. occidentalis, Amblydromella caudiglans (Schuester) was also highly abundant. G. occidentalis abundance was positively affected by conventional pesticide regimes and bifenazate use, whereas A. caudiglans was negatively affected by bifenazate and positively affected by herbicide strip weediness. This indicated that A. caudiglans was more susceptible to disruptive pesticide inputs than G. occidentalis. This was confirmed by an assay wherein several orchard pesticides caused higher mortality of A. caudiglans than G. occidentalis. The recent phase out of certain orchard pesticides may have allowed A. caudiglans to become more common. To understand the mite species complex in the absence of pesticides, an insecticide-free research orchard was monitored at regular intervals during two growing seasons. Generalist phytoseiids were more common than G. occidentalis throughout both seasons. These species were nearly dependent on Aculus schlechtendali (Nalepa) as a food source, as spider mites remained scarce. This emphasizes the role of spider mites as induced pests and suggests that in the absence of pesticide applications, a complement of generalist predators is capable of maintaining spider mite populations at very low densities. G. occidentalis was also found to be less affected by leaf surfaces than generalist phytoseiids examined in previous studies. These findings indicate that G. occidentalis is biological different from the generalist phytoseiids that may replace it as pesticide use changes. Therefore, IMM must constantly be re-evaluated in order to meet orchardists' needs.