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Spruce Spider Mite

May 2, 2006
It is during this time of year that spruce spider mite (Oligonychus ununguis) feeds on conifers, including arborvitae, Douglas fir, hemlock, juniper, spruce, and several species of pines. Spruce spider mite has piercing–sucking mouthparts, used to remove plant fluids residing in the chlorophyll (green pigment), resulting in injured foliage appearing bronze or brown.

Spruce spider mite adults are oval-shaped and about 1/60-inch in length. They may be black or tan, whereas the nymphs vary in color from light gray to green. Eggs, which are the overwintering stage of this mite species, are round and brown. They are typically laid underneath bud scales or in the axils of needles from September through November. The eggs hatch into nymphs during spring…like right now! Spruce spider mite takes 3 to 6 days to go from egg to nymph. The active nymphs feed primarily on needles, preferring older needles. There are typically as many as three generations per year in Illinois.

How can you tell if spruce spider mites are present? Well, you can knock the mites off branches onto a sheet of white paper, where they will be easy to see. If you crush or smash spruce spider mites, they leave a green streak, in contrast to the red streak when predatory mites are crushed.

To successfully manage spruce spider mite, it is important to implement proper cultural practices, including irrigation, fertility, and mulching, which will avoid stressing susceptible host plants. This will help minimize having to deal with high populations of spruce spider mite. There are a number of pest control materials that are recommended for controlling spruce spider mite, such as bifenthrin (Talstar), hexythiazox (Hexygon), insecticidal soap, and summer oil. All these pest control materials have contact activity only, which means it is extremely important to cover all plant parts thoroughly to obtain sufficient control of spruce spider mite populations. The miticide (= acarcide) Hexygon is active primarily on mite eggs, having minimal affect on the nymphal and adult stages. It is important to note that improper use of any of the above-mentioned pest control materials may result in a spruce spider mite outbreak, as most of these products are harmful to the natural enemies of spruce spider mite. If feasible, applying a hard water spray removes mites from plants (however, remember that water is not registered by the EPA as a pesticide), thus negating the need to use any pest control materials. In addition, this approach is less harmful to natural enemies. Be sure to exercise caution when using summer oils on blue-needled conifers because the oil may cause discoloration. As always, be sure to read the label carefully prior to applying any product to control spruce spider mite.