Spruce spider mites, Oligonychus ununguis, are important pests of evergreens and present unique management challenges. This pest is most active in the cool spring and fall when it feeds on the needles of conifers causing a bronze or rust coloration to the needles, and potential leaf drop. Their populations build up quickly and by the time damage is visible in the summer, it may be too late for a treatment.
Spruce spider mites present unique management challenges. Their populations build up 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. If a spruce spider mite problem was noted the previous fall, or if there is high population of overwintering eggs, soil applications of Lepitect at egg hatch in the spring are very successful. Otherwise, monitor and treat with a foliar spray of Lucid or Bifenamite when the mites are active in the spring and fall.
Spider mite populations may increase in conifers treated for other pests with Xytect or other imidacloprid formulations. Be sure to monitor for spider mite flare ups in these instances and be ready to respond with treatments for spider mites if necessary.
Other Treatment Practices
- Use a low nitrogen fertilizer
- Adequate water is a key factor in maintaining healthy trees. A slow, deep watering event once every few weeks during dry conditions will help maintain soil moisture levels and minimize the stress that invites spider mites.
- Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 3 inches of wood chips or shredded bark under the drip line can have a very beneficial effect by holding in moisture and promoting healthy fibrous roots.
- Trees planted too close together are more susceptible.
- Place flowering plants in vicinity as they provide nectary sites for predators.
- Cambistat can be also be used to promote tree health and reduce the incidence and severity of stress mediated diseases and insect problems on trees growing in urban areas.
Lepitect applications provided excellent control of spruce spider mite in trials at Ohio State University. (Should be timed 30 days prior to insect activity).
One application of Lucid has provided control of spruce spider mite for up to 43 days after treatment. Horticultural oil will turn the colored evergreens such as Colorado Blue Spruce and Meyeri juniper green, subsequent growth will be the correct color.
Control is most effective when populations are identified early. Multiple applications of miticides may be necessary. Lucid controls adults and nymphs. Bifenamite controls all stages of mite development and are ideally applied before severe damage is done to the plant. Lepitect will control adults and nymphs, and will provide 30 days residual control.
Proper identification and treatment timing are important when managing mites. Using the prescribed miticides is very effective at controlling mites. Using conventional insecticides can create mite flare ups and should be avoided.
Signs of Damage
- Spider mites will often cause stippling damage on leaves as they feed
- Heavy infestations can often cause a bronzing or rust coloration on conifers
- Damaged needles may drop prematurely and twig and branch dieback may occur
- Fine webbing can often be seen between branch joints, twigs, and petioles
- Spider mite adults and egg masses can be seen with a hand lens, often on the underside of leaves
- Shake a branch over a white piece of paper
- Spruce spider mites are slow moving and green when squished
- Fast moving red dots are predatory mites
Photo: Petr Kapitola
State Phytosanitary Administration, Bugwood.org
Photo: USDA Forest Service – Region 4 – Intermountain Archive
USDA Forest Service, Bugwood.org
Trees At Risk
- Colorado spruce (Picea pungens)
- Norway spruce (Picea abies)
- White spruce (Picea glauca)
- White pine (Pinus strobus)
- Arborvitae (Thuja occidentalis)
- Cedars (Cedurs sp.)
- Douglas fir (Pseudotsuga menziessii)
- Hemlock (Tsuga sp.)
- Junipers (Juniperus sp.)
- Larch (Larix sp.)
- 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 off in summer.
- Spruce spider mites are active in the cool seasons of the spring and fall when the daytime temperatures are 60-70°. 192-726 GDD.
- Activity eases in summer with the arrival of 90° and resumes with cooler weather in fall. 2375-2806GDD.
- When temperatures cool to 60-70° during the day, feeding resumes and continues through late fall. Female adults lay eggs on the bark of small branches until a hard frost occurs.
- The eggs overwinter on the bark and needles.
- Several generations per year.
Penn State Extension
University of Wisconsin Extension
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.