The spruce spider mite (Oligonychus ununguis) is a cool
weather mite. Hot dry weather causes this "insect relative"
(an adult mite has eight legs so it's not a true insect which has
six legs) to cease feeding and to disappear till fall. Spruce spider
mite can cause severe feeding damage to evergreens when they build
to a high population. Spider mites are a piercing sucking type insect
relative. They feed on chlorophyll in leaf cells. The feeding empties
the cell of the green chlorophyll. Thus a tiny cream to yellowish
spot occurs. The feeding damage is called stippling. Severe stippling/feeding
damage can cause the needle or scale (leaves) of the evergreen to
die. Spruce spider mites often produce a single silk thread (similar
to a regular spider).
Often an additional population of predatory mites may overlap and feed on the spruce spider mites. When benificials are numerous, no insecticide or miticide should be used. These pesticides will kill the good mites easier than the harmful mites. In addition, the good mites tend to eat more of the bad mites than the pesticides often kill. Predatory mites move faster than spider mites. If both types are present at the same time, you can separate the two by their speed of movement. However, if only one of the two is present, it becomes more difficult to determine if the mite is beneficial or the harmful spruce mites. Predatory mites tend to be reddish and when squashed, leave a red stain. The spruce mite tends to be dark green to dark brown. When squashed they leave a greenish stain.
Check for mites by holding a piece of white paper under a branch and tapping the branch against the paper. If small dust/ like particles are seen moving around, check the moving speed or their color when squashed. If only a small number of spruce mites are found, you may not need to do anything. If, however, a high population of only spruce spider mites is found, a miticide control is usually recommended. If there are numerous beneficial mites present at the same time as the spruce mites, a chemical control is not suggested.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Philip L. Nixon, Extension Specialist-Entomology, Department of Natural Resources and Environmental Sciences University of Illinois at Urbana-Champaign.