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Spruce Spider Mites

Spider mites have been found on Colorado blue spruce at The Morton Arboretum in northeastern Illinois, which means they are active throughout the state. Spruce spider mites and other mites that feed on needled evergreens are most active in the early spring, usually ceasing their feeding by early May (in southern Illinois) to early June (in northern Illinois). They spend the summer as eggs that hatch in the fall for a short feeding time before winter.

Scout for spider mites by vigorously shaking or striking the foliage over a white piece of paper. Some of the mites will fall onto the paper where they can be seen as tiny, moving dots. Smashing them will produce greenish or yellow-orange streaks. Those making greenish streaks are usually spider mites that are feeding on the foliage; those making yellow-orange streaks are usually predaceous mites that are feeding on the spider mites. Numerous predaceous mites will reduce the need for chemical control.

Damage appears as light to brownish dots or stippling on the needles. The mites feed on cells of the leaf, removing and destroying the chlorophyll. This creates a light spot that dries and turns brown. From a distance, the foliage appears bronzish, dirty, or bleached. Heavily attacked foliage will turn brown, and severe attack can kill branches and trees. These mites attack spruce, juniper, pine, arborvitae, hemlock, and Douglas-fir.

Control spider mites with contact miticides such as insecticidal soap, summer spray oil, dicofol (Kelthane), and several pyrethroids. Do not use oil on bald cypress. Oil may cause damage on Savin junipers, spruces, and Douglas-fir and will take the blue color off of Colorado blue spruce. Be sure to achieve good coverage with any sprays, particularly insecticidal soap and summer spray oil. Two treatments a week apart will be necessary to obtain a high level of control. Thanks to Dave Shetlar, The Ohio State University, for information about oil-sensitive plants.

Author: Phil Nixon Fredric Miller


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