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Spider Mites in Vineyards, Fall 2002

Spider Mites in Vineyards, Fall 2002

Anne Connelly and Jim Fisher

OSU Viticulture Extension &
USDA-ARS Entomology

Next spider mite update March 2003

Topics:

1) sampling and evaluation

2) cultural and biological controls

The three major spider mite pests on wine grapes are Willamette mite, Eotetranychus willamettei (McGregor), Twospotted mite, Tetranychus urticae (McGregor), and Pacific mite, Tetranychus pacificus. Descriptions and visuals of these pests are available (1,2). Pacific and Willamette mites deposit their eggs singly on the underside of leaves and when populations are high on the upper portion of the leaves. A fine papilla hair distinguishes the Willamette mite egg. At the larval stage mites have six legs and dark food spots can be seen on the Pacific and Willamette mite. At the next two stages the eight-legged mites have darker food spots.

The adult stage of spider mites provides an opportunity to distinguish between the three species. Pacific females at first seem devoid of food spots but as feeding commences two large diffuse spots appear on the back (dorsum) in the forward position and two smaller spots in the rear. The rear spots distinguish them from Twospotted. Adult female Willamette mites are pale with several small dark spots along the side of their body and behind their eyes. Pacific and Willamette adult males are the size of mature females and have pointed abdomens. Mixed populations will often be seen together. A 45X power is needed to distinguish between species. If that is not available collect specimens in 70% alcohol for proper identification by Entomologists. Injury levels in California have been established. No changes in grape productivity have been seen in populations of less than 30 Willamette mites per leaf. On rare occasion Willamette mites can cause leaf damage on young shoots in the spring but plants often outgrow the injury. Willamette mite produces considerably less webbing on plants. Twospotted mites produces more webbing and can actually web leaves together.

Spider mite activity has increased this season in the North Willamette, particularly in the hills west of Hwy 99 from McMinnville to Dundee. Research in this country on spider mite biology and control in wine grapes has largely been conducted in California (1,2) and to a lesser extent in Washington (3). A survey of Oregon vineyards was conducted in 44 vineyards in 1998 and 1999 in five valleys in western Oregon (4). Tetranychus urticae, the Twospotted spider mite, was the dominant pest and Typhlodormus pyri was the dominant predator mite. Eleven out of the 44 vineyards had excellent control of the spider mite with the predator T. pyri, 27 had good control, and six sites had poor control. Sites adjacent to riparian habitat had fewer spider mites but similar levels of predator mites. T. pyri does not have a high dispersal rate and does not immigrate rapidly and throughout a vineyard.

The resting stage, also called diapause, can come from the onset of fall temperatures or from heavy feeding. Mites will move into the bark and leaf litter. Once leaves have fallen and mites have gone into diapause, it is best to wait till next year for control measures.

Look for a discussion on this topic at the Oregon Horticulture Society Meetings / Grape Section in Portland on January 28, 2003.

References Cited

  1. Grape Pest Management, 2nd edition. 1992. University of California, Division of Agriculture and Natural Resources, No.3343: 180-192
  2. UC Pest Management for Webspinning Spider Mites.
    http://www.ipm.ucdavis.edu/pmg/r302400111.html
  3. Spider Mites- Secondary pests of Washington State Wine Grapes. 2001.Walsh, D.
    http://www.grapesociety.org/2000meeting proceedings/mitecontrol.html
  4. Prischmann, D.A., Croft, B.A., and Luh, H.K. 2002. Biological Control of Spider Mites on Grape by Phytoseiid Mites (Acari: Tetranychidae, Phytoseiidae): Emphasis on Regional Aspects. J. Econ. Entomol. 95-2: 340-347