The main species of spider mites infesting grapes in B.C. are the European red mite (ERM) (Panonychus ulmi) and the two-spotted spider mite (TSSM) (Tetranychus urticae). 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 AssailTM 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 AssailTM and related materials increases spider mite reproduction. Due to its persistence, increases in mite numbers can occur up to several months after application.
Mite outbreaks can also occur following applications of pyrethroid insecticides and other broad-spectrum insecticides.
Adults are small, around 0.5 mm in length, and best viewed with the aid of a magnifying lens. 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 hair-like projec-tion on the upper side.
TSSM vary in colour from pale yellow to greenish yellow to orange. The common pale form has a distinct dark dorsal spot on each side of the body. Its eggs are white.
MSM appear similar to TSSM but with more scattered, diffuse dark dorsal markings on the abdomen.
All species feed on the undersides of leaves. TSSM usually congregate in clusters and produce large quantities of silk webbing, while ERM produce less webbing and are more evenly distributed over the leaf surface.
ERM 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 and MSM 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.
There are multiple overlapping generations each season, and all stages can be found on grapes at any time during the summer months. Females are able to produce 200 or more eggs each. With an ability to grow quickly, spider mite populations can explode rapidly under favorable conditions.
Adults and nymphs feed by piercing individual leaf cells and removing the fluid contents. Healthy grapevines can tolerate moderate numbers of spider mites that cause chlorotic stippling 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, can also be a cosmetic problem for table grapes.
Spider mites rarely cause significant damage to grapes prior to mid-summer. Stippling and bronzing of leaves and the presence of webbing and cast skins is often the first indication of damage in summer.
No firm thresholds have been developed for spider mites on grapes in B.C. Firm thresholds have also not been established in the Pacific Northwest of the U.S., but it is suggested there that economic damage is unlikely at levels below 15-20 per leaf. An approximate guideline suggested from European research for ERM and TSSM is a threshold of 60-70% infested leaves in spring and 30 to 45% in summer.
A good hand lens or low-power dissecting microscope would help provide accurate counts of infestation levels. An alternate, fairly reliable method is to monitor damage to leaves and spray when a moderate 10% amount of bronzing has occurred to leaves. Because mite numbers can increase rapidly, it is necessary to monitor grapes frequently during hot weather.
When monitoring for mite damage, it is useful to consider previous infestation levels and the presence of predacious mites. 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 more closely.
More than five applications of sulphur in a growing season is thought to increase numbers of spider mites, as are dry and dusty conditions.
For more information about chemical control, see the Pesticides page.
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 provide pollen and alternate sources of prey. Proximity to uncultivated or unsprayed areas allows beneficial insects and predatory mites 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 B.C. 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 at high rates and pyrethroids such as permethrin.
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 four species feed on pest thrips and spider mites.
The western flower thrips is considered a pest of grapes, but it is also known to feed on the eggs of spider mites.
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 persistent 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 drive rows 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 drought stress, and these plants are also less able to tolerate damage. Overhead irrigation will help reduce mite infestations.
Grape-Spider Mite, on the Pacific Northwest Pest Management Handbooks website
