Stability and resiliency of biological control of the twospotted spider mite (Acari: Tetranychidae) in hop

Authors: ISKRA, ANNE - Oregon State University; WOODS, JOANNA - Oregon State University; Gent, David - Dave

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2019
Publication Date: 6/3/2019
Citation: Iskra, A.E., Woods, J.L., Gent, D.H. 2019. Stability and resiliency of biological control of the twospotted spider mite (Acari: Tetranychidae) in hop. Environmental Entomology. 48(4):894-902. https://doi.org/10.1093/ee/nvz071.
DOI: https://doi.org/10.1093/ee/nvz071

Interpretive Summary: The twospotted spider mite is a common pest that can be difficult to manage because of its broad host range and resistance to many commonly used pesticides. However, in natural environments a number of predatory arthropods suppress populations of spider mites at levels often well below those observed in commercial production. In this research, we conducted research over a 5-year period in a hop yard where biological control of twospotted spider mites was well established to understand the stability of biological control and how it responds to various disturbances. We used varying rates of nitrogen fertilization as a means to alter mite reproductive rates and also applied a broad-spectrum insecticide to remove predatory arthropods. Biological control generally was stable and resilient over a four-fold range of nitrogen fertilization rates, and in only one of five years did elevated nitrogen rates significantly affect populations of spider mites. In contrast, one application of a broad spectrum insecticide was sufficient to disrupt biological control and induce a severe outbreak of spider mites. The complex of natural enemies suppressed the outbreak during the same year in which the insecticide was applied, but only after populations of spider mites exceeded levels associated with economic damage. In the following year the system returned to an equilibrium state where spider mites were suppressed below economically damaging levels. Therefore, conservation biological control in hop appears stable and robust to factors such as nitrogen fertilization that increase reproductive rates of spider mites but sensitive to factors such as non-selective insecticides that are lethal to predatory arthropods. This research provides guidance to growers seeking ways to encourage greater levels of biological control.

Technical Abstract: The twospotted spider mite (Tetranychus urticae Koch) is a common pest in agricultural and ornamental crops. This pest can be controlled by resident predatory arthropods in certain situations. This research quantified the stability and resiliency of conservation biological control of the twospotted spider mite in hop over a five year period associated with nitrogen fertilization rate and use of a broad-spectrum insecticide. Biological control generally was stable and resilient over a four-fold range of nitrogen fertilization rates, and in only one of five years did elevated nitrogen rates significantly affect populations of spider mites. In contrast, one application of the insecticide bifenthrin was sufficient to disrupt biological control and induce a severe outbreak of spider mites. The complex of natural enemies suppressed the outbreak during the same year in which bifenthrin was applied, but only after populations of spider mites exceeded levels associated with economic damage. However, in the following year the system returned to an equilibrium state where spider mites were suppressed below economically damaging levels. Therefore, conservation biological control in hop appears stable and robust to factors such as nitrogen fertilization that increase reproductive rates of spider mites but sensitive to factors such as non-selective insecticides that are lethal to natural enemies. Conservation biological control appears resilient to a single use of a nonselective insecticide in the year following the application, but not within the year of application.