Spruce spider mites can build up during cooler weather in late summer. The following is more information.
Spruce spider mites feed on fir, arborvitae, spruce, and other conifers. This mite feeds on older foliage first and damaged foliage has a yellow to bronze coloration. Spruce spider mites begin to feed about 2301- 3957 [3094 peak] GDD base 50 and usually continue until a hard frost. The mite is dark green to dark red after initial feeding and overwinters as a reddish colored egg. The
damage found during the summer on plants is the result of heavy feeding by this mite the previous fall and spring.
Spruce spider mite infestation with webbing. Photo from the USDA Forest Service - Northeastern Area Archive, USDA Forest Service, Bugwood.org
Information from Brian Kunkel, Ornamental IPM Specialist, UD