The spider mites, also called webspinning mites, are among the most common and widespread of all pests in the garden. Because spider mites reproduce rapidly in hot weather, our recent heat wave helped them to explode in many gardens. With hot weather and a plentiful food supply, such as your garden plants, mites can complete a generation in less than a week. This might explain why your squash, bean or marigold plants have turned yellow and are covered with a fine webbing.

Mites cause damage by sucking juices from leaves. A few mites are not a cause for concern, but high populations - levels high enough to show visible damage to leaves - can be damaging, especially on annual plants. Heavy feeding by mites usually results in defoliation of the plant; the damage is worse if the plant is water-stressed. Your fruit trees won’t be too seriously injured if they are attcked by mites at this point in the season, but next year’s crop may be affected if the trees are defoliated. On annual vegetable crops such as squash and cucumbers, a loss of leaves can have a serious effect on yield and lead to sunburning. Spider mites cause direct damage to crops like beans by attacking the pods. On ornamental plants, mites primarily affect appearance, but they can kill annual plants.

Mites are very tiny and difficult to see, so most gardeners notice the plant damage well before they even see the pests. To the naked eye, spider mites look like tiny moving dots; however, you should be able to see them with a 10X hand lens. Check the undersides of leaves for mites, their eggs and webbing. You can see them easier by shaking some infested leaves over a white sheet of paper. Once disturbed, they move around rapidly, making it easier to see them. Make sure you find active mites before you treat your plants, because sometimes the mites will be gone by the time you notice the damage. Plants often recover after mites have left.

Spider mites have many natural enemies which limit their numbers in many landscapes and gardens, especially when you limit your use of broad-spectrum pesticide sprays. Some of the most important natural enemies are predatory mites, which are about the same size as plant feeding mites, but have longer legs and are more active than the plant feeders. Other natural enemies of mites include the spider mite destroyer beetle, the larvae of certain flies and lacewing larvae.

Your cultural practices can have a great impact on spider mites. Dusty conditions often lead to mite outbreaks; washing the dust off trees and vines may help to prevent serious late season mite infestations. Water-stressed plants are less tolerant of spider mite damage, so be sure to provide your garden with adequate irrigation.

If you find it necessary to treat your plants for spider mite control, it’s best to use selective materials. Oil sprays are often effective in reducing mite populations, and can be used on many garden plants. Remember that the insecticide carbaryl (Sevin) can cause outbreaks of spider mites, so do not use the spray if your plants are already infested with mites. Sulfur dust or spray can be used to control mites on some vegetables, but will burn cucurbits. Be sure to carefully read and follow any pesticide label instructions.