SPIDER MITES
Mites are common pests in landscapes and gardens and can be found feeding on many fruit trees, vines, berries, vegetables, and ornamental plants. Although related to insects, mites are not insects but members of the arachnid class along with spiders and ticks. The spider mites, also called webspinning mites, are the most common mite pests and among the most ubiquitous of all pests in the garden and farm.
The webspinning spider mites include Pacific spider mite, twospotted spider mite, strawberry spider mite, and several other species. Most common ones are closely related species in the Tetranychus genus and they cannot be reliably distinguished in the field. However, there is little need to do so since their damage, biology, and management are virtually the same.
IDENTIFICATION
To the naked eye, spider mites look like tiny moving dots; however, you can see them easily with a 10X hand lens. Adult females, the largest forms, are less than 1/20 inch long. Spider mites live in colonies, mostly on the under surface of leaves; a single colony may contain hundreds of individuals. The names "spider mite" and "webspinning mite" come from the silk webbing most species produce on infested leaves. The presence of webbing is an easy way to distinguish them from all other types of mites.
Adults have 8 legs and an oval body, with 2 red eyespots near the head end of the body. Females usually have a large dark blotch on each side of the body and numerous bristles covering the legs and body. Immatures resemble adults, except the newly hatched larvae, which have only 6 legs. Eggs are spherical and translucent, like tiny droplets, becoming cream colored before hatching.
LIFE CYCLE
In some parts of California, spider mites may feed and reproduce all year on plants that retain their green leaves through the winter. In colder areas and on deciduous trees that drop their leaves, webspinning mites overwinter as red or orange mated females under rough bark scales and in ground litter and trash. They begin feeding and laying eggs when warm weather returns in spring.
Spider mites reproduce rapidly in hot weather and commonly become numerous in June through September. If temperature and food supplies are favorable, a generation can be completed in less than a week. Spider mites are generally favored by hot, dusty conditions and are usually found first on trees or plants adjacent to dusty roadways or at margins of gardens. Plants under water stress are also highly susceptible. As foliage quality declines on heavily infested plants, female mites catch wind currents and disperse to other plants. High mite populations may undergo a rapid decline in late summer when predators overtake them, host plant conditions become unfavorable, following rain, or the weather begins turning cooler.
DAMAGE
Mites cause damage by sucking cell contents from leaves. A small number of mites is not usually reason for concern, but very high populations--levels high enough to show visible damage to leaves--can be damaging, especially on herbaceous plants. At first, the damage shows up as a stippling of light dots on the leaves; sometimes the leaves take on a bronze color. As feeding continues, the leaves turn yellow and drop off. Often leaves, twigs, and fruit are covered with large amounts of webbing. Damage is usually worse when compounded by water stress.
Loss of leaves will not cause yield losses in fruit trees during the year of infestation unless it occurs in spring or very early summer, but it may impact next year's crop. On annual vegetable crops, such as squash, melons, and watermelons, loss of leaves can have a significant impact on yield and lead to sunburning. On crops such as sugar peas and beans, where pods are attacked, spider mites can cause direct damage. On ornamentals, mites are primarily an aesthetic concern, but can kill plants if populations become very high on annual plants.
MANAGEMENT
Monitoring
Mites are tiny and difficult to detect. Usually plant damage--stippling or yellowing of leaves--will be noticed before you spot the mites themselves. Check the undersides of leaves for mites, their eggs, and webbing; you will need a hand lens to identify them. To observe them more closely, shake a few off the leaf surface onto a white sheet of paper. Once disturbed, they will move around rapidly. Be sure mites are present before you treat. Sometimes the mites will be gone by the time you notice the damage; plants will often recover after mites have left.
Biological Control
Spider mites have many natural enemies, which limit their numbers in many landscapes and gardens, especially when undisturbed by pesticide sprays. Some of the most important are the predatory mites, including the western predatory mite. Predatory mites are about the same size as plant feeding mites but have longer legs and are more active than the plant feeding mites. Various insects are also important predators--the sixspotted thrips, the larvae and adults of the spider mite destroyer lady beetle, the larvae of certain flies and various general predators such as minute pirate bugs, bigeyed bugs, and lacewing larvae. Purchase and release of predatory mites can be useful to establish populations in large plantings or orchards, but the best results are obtained by creating favorable conditions for naturally occurring predators--for instance, by avoiding dusty conditions and pesticide sprays.
Cultural Control
Cultural practices can have a significant impact on spider mites. Dusty conditions often lead to mite outbreaks. Oil orchard roads or water pathways at regular intervals. Maintain ground covers in orchards and limit traffic, especially in hot spots where mites are frequently a problem. Water-stressed trees and plants are less tolerant of spider mite damage. Be sure to provide adequate irrigation. Mid-season washing of trees and vines with water to remove dust may help prevent serious late season mite infestations.
In gardens and on small fruit trees, regular, forceful spraying of plants with water will often reduce spider mite numbers adequately. Be sure to get good coverage, especially on the underside of leaves. If more control is required, use an insecticidal soap in your spray (as described below), but test the soap out on one or two plants to be sure it is not damaging to plants.
Chemicals
Spider mites frequently become a problem after application of insecticides. Such outbreaks are commonly a result of the insecticide killing off the natural enemies of the mites, but also occur when certain insecticides stimulate mite reproduction. For example, spider mites exposed to carbaryl (Sevin) in the laboratory have been shown to reproduce faster than untreated populations. Carbaryl, some organophosphates, and some pyrethroids apparently also favor spider mites by increasing the level of nitrogen in leaves. Insecticides applied during hot weather usually appear to have the greatest effect on mites, causing dramatic outbreaks within a few days.
If a treatment for mites is necessary, use selective materials, preferably insecticidal soap or insecticidal oil. Do not use soaps or oils on water-stressed plants or when temperatures exceed 90°F. These materials may be phytotoxic to some plants, so check labels and/or test out on a portion of the foliage several days before applying a full treatment. Sulfur dust or spray can be used on some vegetables, but will burn cucurbits. Do not use sulfur dust if temperatures exceed 90°F and do not apply sulfur within 30 days of an oil spray. Sulfur dusts are skin irritants and eye and respiratory hazards. Always wear appropriate protective clothing.