It prefers the cooler parts of the plant and is found mostly in the shady parts of the vine. In certain areas and during certain years, populations can persist throughout the growing season. Willamette 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. Willamette mite is seldom a pest in the San Joaquin Valley, especially on Thompson seedless. It is only occasionally found on grapes in California and rarely causes damage.
Damage is worse along the shoulder and tops of the vine canopies.
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.