It is time for one of the “cool mites” to be active. In this case, spruce spider mite, Oligonychus ununguis. During this time of year, spruce spider mite will be actively feeding on conifers such as arborvitae, Douglas fir, hemlock, juniper, spruce, and some pine species. Spruce spider mites use their piercing–sucking mouthparts to remove plant fluids and chlorophyll (green pigment). This leads to injured foliage appearing bronze or 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 (like right now!!!). It typically takes spruce spider mite 3 to 6 days to go from egg to nymph. These mobile or active stages feed mainly on needles, preferring older needles. There can be as many as three generations per year in Illinois.
Spruce spider mite presence can be verified by knocking the mites off branches onto a sheet of white paper, where they are easily seen. When spruce spider mites are crushed, they produce a green streak, in contrast to predatory mites, which leave a red streak.
The primary method of managing spruce spider mites involves implementing proper cultural practices, including watering, fertilizing, and mulching. Providing appropriate moisture and fertility requirements goes a long way toward reducing plant stress and avoiding high populations of spruce spider mite.
Pest-control materials may be used to manage established spruce spider mite populations. These include bifenthrin (Talstar), hexythiazox (Hexygon), summer oil, and insecticidal soap. These materials primarily work by contact activity, which means that it is critical to cover thoroughly all plant parts to obtain sufficient control of spruce spider mite. Hexygon is a miticide (= acaricide) that has activity only on mite eggs. It is important to consider that improper use of any of these pest-control materials can result in mite outbreaks because most of these materials are harmful to the natural enemies of the spruce spider mite. If “realistically” possible, use a hard stream of water to remove mites from plants before using any pest control material. This approach is less harmful to natural enemies. Exercise extreme caution when using summer oils on blue-needled conifers because the oil may cause discoloration. Be sure to read the label carefully before applying any pest-control material.