As fall approaches, spruce spider mite eggs that have been sitting dormant all summer will begin to hatch. Active mites are likely to be found on spruces, pines, junipers, and other needled evergreens in the fall. These mites typically cause more damage in the spring, resulting in tiny, light-colored spots (stippling) that eventually turn brownish. From a distance, foliage appears to be bronze to gray in color. As summer approaches, these mites lay eggs that lay dormant over the summer, and no active mites are found on the trees. This late-spring cessation of feeding typically occurs at about shoot elongation and candle formation, so new growth does not show injury. Mites at this time of year feed on both first-year as well as previous years’ needles, resulting in the same type of damage as that cause by spring feeding populations.
Scout for the mites by sharply striking branches over a piece of white paper. Many of the mites will be knocked off the branch onto the paper. Slow-moving greenish to gray mites that streak green when smashed are plant-feeding mites. Faster-moving reddish mites that streak red when smashed are predatory mites that feed on the spruce spider mites. Based on the relative abundance of spruce spider mites, as well as relative numbers of predatory mites, a control decision can be made. Numerous predatory mites in comparison to plant-feeding mites can probably control the situation without pesticide use.
If pesticides are needed, bifenthrin (Talstar), insecticidal soap, and summer spray oil should provide control. Christmas tree growers have the added option of using dimethoate (Cygon).