Tactics for enhancing conservation biological control of twospotted spider mites, Tetranychus urticae Koch, in Pacific Northwest hop yards

Abstract

The twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a worldwide pest of numerous agronomic and horticultural plants. Conservation biological control of this pest can be unreliable due to asynchrony of natural enemies and certain horticultural practices that have a direct or indirect affect on natural enemies and the pest, such as sulfur fungicides applied for control of powdery mildew diseases. In this research, means to enhance the reliability of conservation biological control in hop through deployment of synthetic herbivore-induced plant volatiles and altered sulfur fungicide timing were investigated. Methyl salicylate (MeSA) is an herbivore-induced plant volatile (HIPV) that is attractive to several key predators of twospotted spider mite and hop aphid, Phorodon humuli (Schrank) (Homoptera: Aphididae). A two-year study was conducted to evaluate the recommended commercial use of synthetic MeSA in hop yards in Oregon. Slow-release MeSA dispensers were stapled to supporting poles in 0.5 ha plots and these plots were compared to a paired non-treated plot on each of three farms in 2008 and 2009. Across both years, there was a trend for reduced (range 40 to 91%) mean seasonal abundance of T. urticae in five of the six MeSA-baited plots. Stethorus spp., key spider mite predators, tended to be more numerous (range 13 to 51%) in MeSA-baited plots compared to control plots on a given farm. Mean seasonal densities of hop aphid and other natural enemies (e.g., Orius spp. and Anystis spp.) were similar between MeSA-treated and control plots. Variability among farms in suppression of twospotted spider mites and attraction of Stethorus spp. suggests that the use of MeSA to enhance CBC of spider mites in commercial hop yards may be influenced by site-specific factors related to the agroecology of individual farms or seasonal effects that require further investigation. The current study also suggests that CBC of hop aphid with MeSA in this environment may be unsatisfactory. Other studies were conducted during 2007 to 2009 in Oregon and Washington hop yards to evaluate the effect of timing of sulfur applications on twospotted spider mites and their predators. In both regions, applications of sulfur made relatively late in the growing season (mid-June to mid-July) were associated with the greatest exacerbation of twospotted spider mite outbreaks, particularly in the upper canopy of the crop. The severity of outbreaks was closely associated with sulfur applications made during a relatively narrow time period coincident with the early exponential phase of twospotted spider mite increase and rapid host growth. A nonlinear model relating mean cumulative mite days during the time of sulfur sprays to the percent increase in cumulative mite days (standardized to a non-treated plot) explained 58% of the variability observed in increased mite severity related to sulfur spray timing. Spatial analysis of twospotted spider mites counts and their eggs in the Oregon plots indicated patterns of motile stages of twospotted spider mites were similar among leaves treated with sulfur versus nontreated leaves; however, in two of three years eggs were less aggregated on leaves of sulfur-treated plants, pointing to enhanced dispersal. Apart from one experiment in Washington, relatively few predatory mites (Phytoseiidae) were observed during the course of these studies and sulfur-induced mite outbreaks generally occurred irrespective of predatory mite abundance. These studies indicate that sulfur applications induce outbreaks in hop through direct or indirect effects on twospotted spider mites, mostly independent of predatory mite abundance or toxicity to these predators. Avoidance of negative side effects of sulfur sprays was achieved by carefully timing applications to periods of low spider mite abundance and slower host development, which is generally early to mid-spring for hop. Together, these findings point to strategies to enhance the reliability and efficacy of conservation biological control of twospotted spider mites in hop and perhaps other systems.