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Spruce Spider Mite

September 12, 2001

As temperatures decline over the state, spruce spider mite starts its second cycle of activity. Spruce spider mite, Oligonychus ununguis, is a cool-season mite, in contrast to two-spotted spider mite, Tetranychus urticae, a warm-season mite. Spruce spider mite feeds mainly on conifers such as arborvitae, Douglas fir, hemlock, juniper, spruce, and some pines. They use their piercing–sucking mouthparts to remove plant fluids and chlorophyll (green pigment). Injured foliage generally appears bronze to brownish.

Adult mites are oval-shaped and about 1/60 inch long. They are black or tan, whereas the nymphs are light gray-green in color. The round, brown eggs are laid under bud scales or in the axils of needles. Female mites lay the overwintering eggs on plants from September through November. These eggs hatch into nymphs during spring. Spruce spider mite has two major periods of activity: The first is from April through mid-May, although they are usually present into June in northern Illinois; the second is from late September through mid-October. It generally takes spruce spider mite 3 to 6 days to go from egg to nymph. All motile or active stages feed mainly on needles, preferring older ones. There can be three generations per year in parts of Illinois.

Spruce spider mite presence can be verified by knocking them off branches onto a white sheet of paper, where they are easily seen. They produce a green streak when crushed, whereas red streaks indicate predatory mites.

Managing spruce spider mite involves proper cultural practices (such as watering, fertility, and mulching) to minimize stress, and using pest-control materials. Pest-control materials that may be used to manage spruce spider mite include bifenthrin (Talstar), dicofol (Kelthane), dimethoate (Cygon), hexythiazox (Hexygon), summer oil, or insecticidal soap. Because these materials work only by contact activity, it is important to get thorough coverage. Hexygon is an ovicide/miticide with primary activity on mite eggs. Improper use of any of these materials can lead to mite outbreaks because most of these pest-control materials may kill the mite’s natural enemies. If feasible, use a hard stream of water to remove mites from plants, as this approach is less harmful to natural enemies. Be careful when using summer oils on blue-needled conifers, as they may cause discoloration.

Author: Raymond A. Cloyd


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