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Spruce Spider Mite

October 2, 2002

As temperatures start to decline over many regions of Illinois, spruce spider mite will start its second cycle of activity. Spruce spider mite, Oligonychus ununguis, is a cool-season mite, in contrast to the twospotted 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). As a result, injured foliage 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. The 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 may be 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. The mobile or active stages feed mainly on needles, preferring older ones. There can be up to three generations per year in parts of Illinois.

The presence of spruce spider mite 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 implementing proper cultural practices (including watering, fertility, and mulching to minimize stress) and using pest-control materials.

Pest-control materials recommended for managing spruce spider mite include bifenthrin (Talstar), dicofol (Kelthane), dimethoate (Cygon), hexythiazox (Hexygon), summer oil, and insecticidal soap. These materials primarily work by contact activity, which means it is important to cover thoroughly all plant parts where spruce spider mite may be located. Hexygon is an ovicide/miticide with activity on mite eggs. Improper use of any of these materials can lead to mite outbreaks because most of these pest-control materials can kill natural enemies of the spruce spider mite. 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 because they may cause discoloration.

Author: Raymond A. Cloyd


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