The spruce spider mite (Oligonychus ununguis) is a common pest found on spruce trees. Adult mites are green-brown, have four pairs of legs and are scarcely visible to the unaided eye.
The Problem
These mites suck sap from the tree's needles, causing them to turn yellow, then brown and eventually drop off. Damage on a tree usually starts appearing on inner portion of the lower branches.
If an infestation persists, twigs and branches may die. During periods of drought, extremely severe attacks may kill the tree.
Spruce spider mites also spin a fine webbing that traps debris between the needles and makes the tree look dirty.
What You Can Do
Spruce spider mites survive best during hot, dry conditions. Heavy rain, high winds and high humidity usually control mites naturally.
Regularly washing a spruce tree down over the summer with a hard stream of water from the garden hose will control mite populations and remove unsightly webbing and trapped dirt.