Final Report: This is year 3 of a 3 year project for the Washington Grape & Wine Research Program
Date: December 15, 2015

Project Title: Grapevine Mites: Current Status in Eastern Washington Vineyards
Principal Investigator(s): David G. James
Organization: Washington State University, Prosser

Collaborator(s): Robert Branscum
Organization: Sustainable Practices Lab, Dept. of Corrections, Washington State Penitentiary, Walla Walla

Summary of Project Results:

A survey of the mite fauna in eastern Washington vineyards over a three year period (2013-15) indicated that spider mites occurred in about 50% of vineyards but the majority of these (>75%) had non-damaging populations. The few vineyards that had damaging populations may have created mite outbreak situations by regularly using neonicotinoid insecticides. Willamette Spider Mite is present in some central Washington vineyards and was responsible for more than half of the damaging mite populations observed in this study. Pacific Spider Mite may be present in Washington vineyards. A new eriophyid mite, Blister Mite, was recorded in commercial vineyards in central Washington for the first time. Rust mites occurred at low, non-damaging levels during this survey. A diverse fauna of predatory mites (at least seven-eight species of phytoseiids and tydeids) occurred in about 50% of vineyards and is likely in part responsible for low spider mite populations.

I Project Summary: 1. What is the current issue and why does it need to be researched?

During the past decade mites appear to have decreased in importance as wine grape pests in eastern Washington. Spider mites, principally twospotted mite (Tetranychus urticae) and McDaniel’s mite (Tetranychus mcdanieli) were significant pests in Washington vineyards in the late nineties and early this century. The decline in their pest status appears to be associated with the reduced use of broad-spectrum insecticides (e.g. Carbaryl, Lorsban) documented for Washington vineyards (Prischmann et al. 2005, Ferguson et al. 2007). Spider mite outbreaks in vineyards are now uncommon, populations likely controlled by stronger communities of natural enemies associated with reduced pesticide inputs (Prischmann and James 2003). In 2005 the reduction in spider mite problems was replaced by outbreaks of other mites, specifically eriophyid or rust and bud mites (Walton et al. 2007). These had likely been suppressed by the former pesticide regime. Fortunately, their impact was short-lived with strategic applications of sulfur pre bud-burst, sufficient to maintain populations below damaging levels. Currently, spider, rust and bud mites can be considered occasional, minor pests but the potential for local problems still exists if broad-spectrum sprays are used or natural enemy populations decline. The threat of new grapevine mite pests is also real. Additional eriophyid mite species (blister mites, leaf curl mites) have recently been detected in some Yakima Valley vineyards. The distribution of these mites in eastern Washington vineyards and their potential to cause economic damage is unknown. An additional spider mite species (Willamette mite, Eotetranychus willamettei) was discovered in 2012 with high densities causing leaf damage, warranting miticide applications, in an Oregon vineyard near Walla Walla. This detection was considered unusual since this species is considered to prefer more humid, cooler regions like the Willamette Valley where it is endemic. The possibility exists that the Walla Walla version of Willamette mite is better adapted to hot, dry conditions, posing an increased threat to eastern Washington grapes. Whether Willamette mites are present in other eastern Washington vineyards is unknown. The current status of grapevine mites in eastern Washington vineyards is imperfectly known. Grower preparedness and sustainability of current status will benefit from a better understanding of the species present and their relative abundance. An assessment of the incidence and abundance of predatory mites is also important so we can assess the role that biological control is playing in maintaining sub-economic levels of mites. The threat posed by ‘new’ mite species also needs to be evaluated.

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Pest & Disease //