How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

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 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. 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. Because they are so similar in appearance, it is difficult to discern between the Pacific and Willamette spider mites 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 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) 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.

DAMAGE (View photos of mites and damage.)

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.

MANAGEMENT

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

Biological Control (View photos of natural enemies.)
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. This mite is 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. Other predators, including sixspotted thrips (Scolothrips sexmaculatus), can also be important. To preserve these natural enemies, avoid using disruptive materials, especially carbaryl, dimethoate, dicofol, and methomyl.

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

Monitoring and Treatment Decisions
Monitor for webspinning spider mites as part of dormant and budbreak spur monitoring as described in the DELAYED-DORMANT AND BUD BREAK SAMPLING and record observations on a monitoring form (100KB, 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 (100KB, PDF).

PDF: You need a PDF reader, such as Acrobat Reader version 5 or later, to view or print this PDF. If no reader is installed on your computer, you can download a free copy of Adobe Acrobat Reader.

PUBLICATION

UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448
Insects and Mites
W. J. Bentley, UC IPM Program, Kearney Agricultural Research Center, Parlier
L. G. Varela, UC IPM Program, Sonoma County
F. G. Zalom, Entomology, UC Davis
R. J. Smith, UC Cooperative Extension, Sonoma County
A. H. Purcell, Environmental Science, Policy and Management, UC Berkeley
P. A. Phillips, UC IPM Program, Ventura County
D. R. Haviland, UC IPM Program, Kern County
K. M. Daane, Kearney Agricultural Research Center, Parlier
M. C. Battany, UC Cooperative Extension, San Luis Obispo County
Acknowledgment for contributions to the insects and mites section:
J. Granett, Entomology, UC Davis

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