Includes grape bud mite (Colomerus vitis)
Pest description and crop damage Grape bud mites are tiny (0.1 to 0.2 mm long), wormlike, and whitish yellow with two pairs of legs near the head. They can only be seen under magnification (45 X or higher on a microscope), and the damage they cause is usually the first indication of their presence. The grape bud mite resides within the bud and causes damage by feeding on the stem, leaf and flower primordia within the bud. In spring, damaged buds may be delayed and only weak shoots may grow, if any at all from damaged buds. Vines infested with bud mites may exhibit erratic budbreak, stunted shoots, malformed basal leaves, and have reduced number of flower clusters. Stunted shoots may have arrested development and fail to grow beyond a few inches. Damage is most visible on tissues rapidly growing during budbreak and until shoots are less than 6 inches in length. Symptoms observed from mite feeding should not be confused with symptoms of nutrient deficiency (boron or zinc), low reserve carbohydrates, herbicide damage, frost, or thrips damage. There are distinct differences between bud mite-related damage and other factors.
Biology and life history Grape bud mites overwinter as adults inside buds where they feed on bud tissues and may kill the tissues within the overwintering bud. When buds begin to swell in spring, mites migrate to newly developing tissues where they feed and lay eggs. Eggs hatch and develop into adults. Bud mites move to the newly forming buds in the axils of the leaf petiole where they reside until budbreak the following year. During the next growing season, once growth commences, mites within the inner scales are distributed along the shoot length as the shoot develops. Those mites in the outer scales remain at the base of the shoot.
Sampling and thresholds Eriophyid mites are difficult to identify in sampling because of their microscopic size and translucent color. Because they reside in the buds for the majority of the year, they can be difficult to find. Populations are assessed by examining buds on dormant canes in winter, using magnification of 45X or higher. A sample should consist of forty 1-year old shoots (canes) samples randomly from affected areas in the vineyard. Select only the basal portions of the cane with at least three nodes. Place canes in a plastic bag and keep refrigerated until inspection. To look for mites in buds, use a dissecting microscope, and gently pull the outer bud scales off and work from the outside in, searching for the mites. To date, there are no damage thresholds known. However, if high populations are present in dormant buds, damage may have already been done, and spring control will reduce populations of bud mites for future years. While bud mites are different from rust mites, they look similar under magnification.
Management-chemical control: HOME USE
Dormant-season oils used for other pests may control this pest.
Management-chemical control: COMMERCIAL USE
The emergence of these mite species as pests of Pacific Northwest vineyards may be related to a general reduction in sulfur use to manage disease in some areas of Oregon and Washington. Sulfur continues to be used by organic growers and in many other Oregon vineyards as an economical way to control powdery mildew. Sulfur is effective to control pest populations of rust and erineum (blister) mites but may cause a decline in predatory mites when used. Chemical management of pest mites should be based on properly timed applications of sulfur. The current recommendations for control of bud mites come from research conducted in Australia.
High-volume wettable sulfur (Microthiol Disperss, Kumulus DF, other brands) Check specific label for rate; apply at 100 gal water per acre. Make application at the woolly bud stage or during budbreak when temperature is above 60°F. A second follow-up spray is recommended approximately 7 to 10 days after the first, depending on weather conditions and growth rate, in order to target mites that were protected in unopened buds during the first spray application. High spray volume and timing are critical for targeting mites during migration and before they start producing eggs. Most brands are OMRI-approved for organic production.
Use sulfur sprays in newly established vineyards. Typically young vines (years 1 and 2) are not sprayed as regularly as producing vineyards, allowing mite populations to build during the growing season. It is best to apply sulfur during the growing season in young vineyards to control pest mite populations and powdery mildew.
For more information:
Grape Rust Mite http://www.extension.org/pages/33107/grape-rust-mite.