Ministry of Agriculture

Grape Insect and Mite Pests

Spider Mites

(European Red Mite, Panonychus ulmi (Koch))
(Two-spotted Spider Mite, Tetranychus urticae-Koch)

Prepared by D. Thomas Lowery, Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, Summerland.
Photos courtesy of M. Weis, Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, Summerland.

The main species of spider mites infesting grapes in British Columbia are the European red mite (ERM) and the two-spotted spider mite (TSSM). They differ somewhat in biology, but feeding damage and management strategies are largely the same. Spider mites should be considered secondary or induced pests of grapes; their numbers increase following the use of broad-spectrum insecticides that reduce numbers of spider mite predators.  Although it is somewhat selective in action and preserves populations of some predators, the insecticide Assail is toxic to predacious thrips that are effective predators of mites in B.C. vineyards. Studies in Washington State have also shown that the active ingredient in Assail and related materials increases spider mite reproduction. Due to its persistence, increases in mite numbers can occur up to several months after application.

Identification

Adults of both species are small, around 0.5 mm in length, and just barely visible to the naked eye. They are nearly spherical in shape and have eight legs and short piercing mouthparts. The ERM is dark red in colour with long pale hairs (setae) projecting from small pale circular areas on the dorsal surface. The oval eggs are bright orange with a distinctive hairlike projection on the upper side. The TSSM varies in colour from pale yellow to greenish yellow to bright orange; the common pale form has a distinct dark dorsal spot on each side of the body. Its eggs are white. Both species feed on the undersides of leaves. TSSM usually congregate in clusters and produces large quantities of silk webbing, while ERM produce less webbing and are more evenly distributed over the leaf surface.

two-spotted spider mite European red mite
Adult two-spotted spider mite Adult European red mite

Life Cycle and Damage

ERM, our most common species, pass the winter as eggs laid on vine canes and trunks. Eggs hatch in spring and the young microscopic nymphs begin to feed on young foliage. TSSM overwinter as fertilized females under the bark or in other sheltered areas around the base of vines. When warmer weather arrives in spring the females begin to feed and deposit eggs. Both species can produce around six to eight overlapping generations each season, and all stages can be found on grapes at any time during the summer months. Females of both species are capable of producing 200 or more eggs each and development can be rapid during warm weather. Spider mite populations can, therefore, explode rapidly under favourable conditions.

Both adults and nymphs feed by piercing individual leaf cells and removing the fluid contents. Healthy grapevines can tolerate moderate numbers of spider mites, which will cause chlorotic spots on the leaves. Heavy feeding results in brown leaves that fall prematurely; reducing photosynthetic activity and vine vigour. Heavy feeding damage can delay ripening of fruit. The large amount of webbing produced particularly by the TSSM is also a cosmetic problem for table grapes.

Monitoring and Spray Thresholds

Spider mites rarely cause significant damage to grapes prior to mid-summer. No firm thresholds have been developed for spider mites on grapes in British Columbia, but approximate guidelines can be suggested from research conducted in other countries. In Europe, a threshold for ERM on grapes was determined to be 60 to 70% infested leaves in spring, while in summer leaves with 1 or more mites should not exceed 30 to 45% of total leaves on a shoot. Because a similar threshold has been suggested for TSSM in Switzerland, numbers of both species can be combined into a single spider mite count. A good hand lens or low-power dissecting microscope would help provide accurate counts of infestation levels. A fairly reliable method is to monitor damage to leaves and spray when a moderate amount of ‘bronzing’ has occurred, around 10% defoliation. Because mite numbers can increase very rapidly it is necessary to monitor grapes frequently during hot weather.

When monitoring for mite damage it is useful to consider previous infestation levels. Vineyards with low chemical inputs that have not experienced severe spider mite outbreaks in the past are unlikely to require treatment, while those that have been treated with broad-spectrum insecticides should be observed closely.

Biological Control

Spider mites were of minor importance prior to the widespread use of synthetic pesticides. Due largely to the harmful effects of pesticides on populations of predators, spider mites are now the most important pests of grapes in many regions of the world. The first course of action for the management of spider mites is the preservation and enhancement of beneficial species. Pesticides should only be applied when necessary and only to parts of the vineyard where pest populations are sufficiently high to warrant control. Whenever possible select materials that are least damaging to non-target species. Numbers of spiders and beneficial insects are usually higher in vineyards with permanent, mixed groundcover that provides pollen and alternate sources of prey. Proximity to uncultivated or unsprayed areas allows beneficial insects to re-colonize vineyards following spray treatments.

Several species of predatory mites (e.g. Metaseiulus, Typhlodromus and Amblyseius species) feed on spider mites and their eggs. Most beneficial mites are a pale opaque colour but they can appear light orange if they have been feeding on ERM. They are oblong in shape and slightly smaller than adult spider mites. The long-legged orange whirligig mite (Anystis agilis) is uncommon on grapes in British Columbia Like most other predatory mites, this large species is more active than its prey. Predatory mites are very sensitive to a range of pesticides, including sulphur fungicides and pyrethroids such as permethrin (Pounce™).

A number of spiders and predatory insects feed on spider mites or their eggs. Many species, such as the minute pirate bug (Orius tristicolor), are generalist predators that do not specialize on mites, while others such as the aptly named spider mite destroyer, Stethorus picipes, are very effective predators that feed almost exclusively on mites. Stethorus picipes is a small, dark species of ladybeetle with a slightly hairy appearance. The elongate bodies of the larvae are also nearly black in colour with a body covered with numerous hairs. At least four species of predatory thrips can be found in Okanagan Valley vineyards; adults of three of these are black, while the fourth, the six-spotted thrips, Scolothrips sexmaculatus, is pale with dark spots on the wings. All 4 species feed on spider mites, and healthy numbers of these predators are usually associated with low populations of their prey. The western flower thrips is considered a pest of grapes, but it is also known to feed on the eggs of spider mites.

Cultural Control

Vineyards can be managed in ways that help alleviate mite problems. Hot, dry and dusty conditions favour the buildup of spider mite populations. Where mites are a problem, roadways can be treated with oils or other materials to reduce dust. Vineyards that are continuously cultivated will produce more dust than those with permanent ground covers. Moreover, a permanent mixed groundcover will support greater numbers of beneficial insects and predaceous mites. Compared with bare soil, planted aisles will decrease air temperatures and raise humidity levels somewhat, providing less favourable conditions for spider mites. Spider mite populations can become elevated on vines suffering severe drought stress, and these plants are also less able to tolerate damage. Overhead irrigation will help reduce mite infestations.

Chemical Control

A number of miticides (e.g. Acramite™, Kelthane™ , Envidor™ and Pyramite™) are registered on grapes for mite control. Insecticidal soap applied for the control of leafhoppers will help suppress mites.  Refer to Pest Control Products Recommended for Use on Grapes in British Columbia and rotate materials with different modes of action (chemical group) to help prevent development of mite resistance.

September 2010

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