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What causes plants to look dirty and lose their green color during the heat of the summer? One culprit is spider mites.

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Another droughty year with intense heat - another bad year for spider mites. Or a good year for mites, depending on your point of view. Spider mites are prolific arthropods, related to ticks and spiders, that attack most types of plants and suck the water and life out of them. Spider mites are small (1/60 inch) and make the leaves appear dirty due to their copious webbing and adhering dust. They feed primarily on leaves, but can also feed on fruits. Symptoms are at first white stippling, turning to bronzing or silvering as mite feeding increases, and finally resulting in complete browning or necrosis of foliage when damage is severe. Shake suspected mite infested leaves over a white sheet of paper and if the small dots that fall to the paper walk around, they are spider mites. So what do you do when the weather is hot and dry and mites are increasing in the late summer?

Good plant care that includes adequate water, fertilization and reduction of stress. Water stressed plants attract mites. Water adequately and if possible, sprinkler-irrigate to occasionally to wet the leaves, unless this may cause other problems. A stiff spray down with a hose can wash off some mites and the moist environment deters their population growth. Some plants are more likely to attract mites, including some weeds. Field bindweed, morning glory, mallow, and knotweed are attractive weed hosts. Avoid ornamental plants that have chronic problems with spider mites. Utah is a haven for spider mites, and they will flourish in hot years.
Predatory mites that prey on spider mites occur naturally. They are more prevalent on perennial plants, such as fruit trees, caneberries and some ornamentals where they can over winter and find spider mite prey year after year. Predaceous mites can be purchased and released, but generally the native species perform better in home yards and gardens. Avoid insecticides that are nonselective to help preserve the predators. Examples of insecticides that can kill predaceous mites include carbaryl (Sevin), malathion, pyrethroid insecticides, such as Asana, Pounce, Tempo and Talstar, and miticides, such as Kelthane and Vendex.

Soap and oil can help control spider mites by disrupting their cuticle (skeleton on the outside) and suffocation. Insecticidal soap and horticultural oil (1 to 1.5%) should be applied at dusk for best effects. If applied in the heat of the day, the products will dry too quickly giving reduced effect and can burn plant leaves. Soap and oil applications should be made two or three times, 5-7 days apart for better kill of mites. Chemicals that specifically target mites, such as Kelthane and Vendex, should only be used in cases where mite populations are high and severe plant damage is likely. They are effective in killing spider mites and most beneficial arthropods. Following use of a miticide, resurgence of spider mite populations is common, requiring additional treatments. And beneficial arthropods may be diminished for several years.

Posted on 14 Sep 2006

Diane Alston
Hort-Entomologist Specialist

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