Webspinning spider mites, the most damaging spider mite, can cause significant cellular damage to grape vines. It poses a threat at much lower densities than other mites. Although found on vines in early-mid season, it doesn’t cause damage until mid-season through harvest.

The Pacific spider mite, Tetranychus pacificus is the primary pest mite species in the San Joaquin Valley and may also be the primary pest mite in certain North Coast grape-growing areas. Adult Pacific spider mite females vary from slightly amber to greenish in color. Later in the season as they go into diapause or under high population densities adult females can turn orange to reddish.

 

 

When they emerge adult Pacific spider mites have few, if any, food spots. However, one they start feeding, two large diffuse spots usually appear forward and two smaller spots appear on the rear portion of the abdomen. Pacific spider mite prefers the warmer upper canopy of the vine. Although it can cause damage early in the season, Pacific spider mite generally prefers the hotter, dryer part of the season.

Because they are so similar in appearance, it is difficult to discern between the Pacific and Willamette spider mite, Eotetranychus willamettei, unless they are side-by-side. The Pacific mite is larger in size than the Willamette mite. Pacific spider mite forelegs are reddish in color and those of Willamette spider mite are translucent to pale yellow.

The pale yellow Willamette spider mite is often considered an early-season mite. It prefers the cooler parts of the plant and is found mostly in the shady parts of the vine. In certain areas, like The North Coast, and during certain years, populations can persist throughout the growing season. Willamette spider mite is primarily a problem in the Salinas Valley and Sierra foothill production areas where it can cause economic damage to varieties such as Zinfandel.

In the North Coast it can cause damage in early spring when shoot growth is delayed or later in the season in vines with small canopies. Willamette spider mite is seldom a pest in the San Joaquin Valley, especially on Thompson Seedless.

The twospotted spider mite, Tetranychus urticae, is almost identical in appearance to the Pacific spider mite except it rarely has spots on the rear of the body. It is only occasionally found on grapes in California and rarely causes damage.

 

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University of California Cooperative Extension farm advisors offer the following tips and techniques for identifying and managing this costly pest.

They recommend checking for webspinning spider mites and predatory mites weekly during rapid shoot growth on the first emerging leaves.  Beginning at bloom divide the vineyard into quadrants. In each quadrant, randomly select five vines each at least a few vines in from the end of the row. Look for mites and mite predators weekly on each of the 20 vines using a 10-14X lens.