Webspinning Spider Mites

The Pacific spider mite is the primary pest mite species in the San Joaquin Valley and may also be the primary pest mite in certain coastal 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. Upon emergence adult Pacific spider mites are almost void of food spots. As feeding begins usually two large diffuse spots 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. 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 Willamette spider mite is pale yellow. It 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 (e.g., North Coast), during certain years or conditions, and on certain varieties, 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.

Damage specific to each species can be helpful in identification. Pacific spider mite damage begins as yellow spots. As damage progresses, dead (necrotic) areas appear on the leaves. High populations can render the leaves unfunctional with leaf burning and bronzing and copious amounts of webbing. Damage is worse along the shoulder and tops of the vine canopies. Willamette spider mite feeding in mid- or late season causes foliage to turn yellowish bronze, but usually no burn occurs unless vines are weak. In red varieties, infested leaves may turn reddish.

Manage webspinning spider mites in a vineyard by integrating biological, cultural and chemical controls.

Many natural enemies help to control pest mite populations. The western predatory mite, Galendromus (=Metaseiulus) occidentalis, is commonly present in vineyards and can be quite effective in reducing all stages of spider mite populations. In Sonoma and Napa vineyards, the most abundant predatory mite is Typhlodromus pyri; when present in spring, it can prevent the establishment of phytophagous mites. Predatory mites are translucent to light amber, pear shaped, and quite active. The effectiveness of this predator depends upon its ability to increase its population size as the season progresses. Disruptive sprays applied early will reduce the survival of this beneficial mite. Naturally occurring predator mites will survive sulfur sprays and dusts, but released ones may not survive dusting sulfur unless they have sulfur resistance. Predator mites, including insecticide-resistant ones, are available commercially to augment populations in the field. The western predatory mite, Galendromus (= Metaseiulus) occidentalis and Neoseiulus californicus are both available for release from commercial insectaries. G. occidentalis is used most commonly in hot inland valleys and N. californicus is more adapted to very cool coastal areas. The optimal timing for releases is in the spring when spider mite populations begin to appear.

Other predators, including sixspotted thrips (Scolothrips sexmaculatus), can also be important.

To preserve these natural enemies, avoid using disruptive materials.

Apply water or other materials formulated to reduce dust on roads in the vineyard. If possible, maintain resident vegetation or other cover in the vineyard middles to further reduce dust. Irrigate in a manner that will avoid stressing vines. Although overhead watering has been shown to reduce mite problems, it can also increase some disease problems.

Organically acceptable methods include biological and cultural control methods as well as oil or soap sprays.

Monitor for webspinning spider mites as part of dormant and budbreak spur monitoring as described in the DELAYED-DORMANT AND BUDBREAK SAMPLING (wine/raisin grapes or table grapes) and record observations on a monitoring form (example form—PDF). During rapid shoot growth, look for spider mites and predatory mites weekly on the first emerging leaves. During bloom, follow the guidelines for MONITORING INSECTS AND SPIDER MITES. When monitoring mites, note the presence of mite predators. The table below can be used in determining the treatment guidelines for various combinations of Pacific mite injury levels and predator-prey distributions in Thompson Seedless raisin vineyards. After bloom, record your observations on the insect and mite monitoring form (PDF).

• Delayed/dormant budbreak monitoring form (PDF)
• Insect and spider mite monitoring form (PDF)
• Photos of natural enemies of spider mites
• Photos of spider mites and their damage