Population density and phenology of Tetranychus urticae Koch (Acari: Tetranychidae) is linked to sulfur-induced outbreaks of this pest

Authors: WOODS, J - Oregon State University; DREVES, A - Oregon State University; FISHER, G - Oregon State University; JAMES, D - Washington State University; WRIGHT, L - Washington State University; Gent, David - Dave

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2012
Publication Date: 6/30/2012
Citation: Woods, J.L., Dreves, A.M., Fisher, G.C., James, D.G., Wright, L.C., Gent, D.H. 2012. Population density and phenology of Tetranychus urticae Koch (Acari: Tetranychidae) is linked to sulfur-induced outbreaks of this pest. Environmental Entomology. 41:621-635.

Interpretive Summary: The twospotted spider mite is a worldwide pest of numerous agronomic and horticultural plants. Sulfur fungicides are known to induce outbreaks of this pest on several crops, although why this occurs is mostly unknown. We conducted three years of experiments in Oregon and Washington to evaluate how the timing of sulfur application influences spider mite outbreaks. We found that spider mite outbreaks were most severe when sulfur was applied later in the season, and the number of spider mites present when sulfur was applied was associated with the severity of the mite outbreak. Difference in the patterns of spider mite eggs among leaves were found between leaves treated with sulfur and nontreated leaves, indicating that sulfur affected mite dispersal. We found little evidence that sulfur significantly affected predatory mites. Collectively, these studies indicate sulfur induces mite outbreaks through direct or indirect effects on spider mites themselves, 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.

Technical Abstract: The twospotted spider mite, Tetranychus urticae Koch, is a worldwide pest of numerous agronomic and horticultural plants. Sulfur fungicides are known to induce outbreaks of this pest on several crops, although mechanisms associated with sulfur-induced mite outbreaks are largely unknown. Studies were conducted during 2007 to 2009 in Oregon and Washington hop yards to evaluate the effect of timing of sulfur applications on T. urticae and key 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 spider mite outbreaks, particularly in the upper canopy of the crop. The severity of mite outbreaks was closely associated with sulfur applications made during a relatively narrow time period coincident with the early exponential phase of 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 spider mites counts and their eggs in the Oregon plots indicated patterns of motile stages of 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 were observed during the course of these studies and sulfur-induced mite outbreaks generally occurred irrespective of predatory mite abundance. Collectively, these studies indicate sulfur induces mite outbreaks through direct or indirect effects on T. urticae, 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.