Phil Mulder, Oklahoma State University
Spider mite infestation on underside of leaf. Photo by David Gent, USDA Agricultural Research Service, Bugwood.org.
Feeding from spider mites may cause early-season discoloration (i.e., yellowing and bronzing) of leaves and reduce photosynthesis, thereby altering fruit ripening. Species of spider mites that may cause problems in grapes vary throughout the United States. In the eastern half of the United States, two-spotted spider mite (Tetranychus urticae) and European red mite (Panonychus ulmi) are common mite pests. In the western United States, Pacific spider mite (Tetranychus pacificus), Willamette spider mite (Eotetranychus willamettei), and two-spotted spider mite are most common. It is important to monitor grapevines for the presence of mites and properly identify any mites found. Monitoring should begin in spring as leaves begin to expand; use a hand lens (10X - 14X magnificatino) to inspect fully expanded leaves and cane nodes for the presence of mites. Look for all life stages (adults, eggs, and nymphs). Try to identify which species of mites are present with assistance from your local Cooperative Extension Office. Predatory mites, which feed on mite and small insect pests, also may be present; thus, miticide applications could disrupt natural control of grape pests and lead to more serious pest outbreaks.
Two-spotted spider mite overwinters in weeds in and around the vineyard and migrates into grapes if weed populations are completely destroyed during the growing season. However, properly managed cover crops can reduce the impact of mites by providing habitat for natural enemies of mites and reducing dust in a vineyard. Besides the obvious leaf discoloration caused by mites, damage from two-spotted spider mites is also accompanied by heavy webbing across infested plants.
European red mite overwinters as tiny red eggs around cane nodes. In spring, they move to foliage where several generations can occur. Generally, webbing is not as common with European red mites; however, both mite species can occur together. Mite populations are generally highest in late July and August when hot, dry conditions are persistent and help concentrate their food source (i.e., the plant’s juices). ‘Concord’ and ‘Riesling’ are two grape cultivars that are relatively susceptible to these mites.
Pacific spider mite damage appears as yellow spots on leaves with a dead or necrotic area appearing as the damage progresses. Large densities of Pacific spider mite will result in leaf bronzing, burning, and extensive webbing on grape leaves. This mite can be found in early spring, but typically builds to large numbers in mid to late summer.
Willamette spider mite damages grape leaves and causes them to turn yellowish bronze in some grape varieties, and reddish in red grape varieties. Typically, there is no leaf burn associated with Willamette spider mite damage. This mite is often considered an early-season mite pest because it likes cooler temperatures.
Close-up of spider mites on underside of leaf. Photo by Natalie Hummel, Louisiana State University AgCenter, Bugwood.org.
To manage spider mites, consider integrating multiple methods of control such as biological control, practical cultural practices for your area, and chemicals. Many natural enemies will feed on spider mite pests, and some predatory arthropods are already present in the vineyard. Other species of natural enemies, such as predatory mites, can be purchased from commercial sources and released in a vineyard at a rate of about 1,000 per acre. Use of selective insecticides and miticides will also help preserve natural enemies.
Sound cultural practices include maintaining an appropriate cover crop for your area, reducing water stress on vines, and reducing dust in the vineyard. These practices will help ensure healthy grape vines that can withstand small amounts of stress from spider mites and other pests, and help maintain habitat for natural enemies.
When chemical control is warranted, choose a miticide that will preserve as many natural enemies as possible. Also, try to apply pesticides when natural enemies are least active in the crop (e.g., early morning), which minimizes chemical exposure to natural enemies. Please consult your local Cooperative Extension Office for miticides that are registered for your area and recommendations for proper use of these products. Please follow all label instructions to ensure optimal results from the chemical application.
Reviewed by Kris Godfrey, University of California
and Eric Rebek, Oklahoma State University