Spider mites are a common pest of spruce, pine, fir, hemlock, juniper and arborvitae. They will first cause yellow spots on the needles. Heavy infestations can often cause a rust coloration. Damaged needles may drop prematurely, and twig and branch dieback may occur. Fine webbing can often be seen on needles. Spider mite adults (0.5mm oval shaped) can be seen with a hand lens. Adults are dark green to black with salmon-colored legs and spines. Brown eggs can be found in bud scales and at the base of needles. The eggs hatch in early spring and young mites begin feeding on old foliage. Spruce spider mites will not feed on the current year’s growth until after it hardens during the summer. Female adults lay eggs on the bark of small branches until a hard frost occurs. The eggs overwinter on the bark and needles. Depending on the weather, there will be multiple generations per year.
Spruce spider mites present unique management challenges. Populations build quickly and by the time damage is visible in the summer, it may be too late for a treatment. Monitoring is important for successful management of this pest. Properly timed soil injections of pesticides can provide excellent control of spider mites. In some cases, foliar spraying may be a good option. The size of the host must be considered when choosing the time and method of treatment.
Contact Plant Health Solutions for proper diagnosis and to develop a treatment plan.