Spider mite infestations can cause your Norway spruce trees’ needles to become mottled and discolored, turning them from yellow to bronze before they drop prematurely. Norway spruces can also suffer from twig dieback and even death from heavy infestations. Spruce spider mites (Oligonychus ununguis) are most active during the cooler spring and fall months, with females laying their eggs during winter on the bark and needles, and then hatching in April or May. The adult mites are extremely tiny, only ½ millimeter long, and are dark green with eight legs. Fortunately, it's not difficult to treat them.
Feed your Norway spruce trees with a low-nitrogen tree fertilizer according to the dosage instructions on the label to strengthen the tree’s ability to endure the damage from the infestation. Water your trees deeply to soak the soil down around the root area once each week when rainfall is less than 1 inch.
Spray the infested trees with a miticide, such as clofentazine, bifenazate or hexygon, or horticultural oil during the growing season. Also spray the trees with a pyrethroid miticide in early autumn or fall.
Spray the spruces with dormant horticultural oils in winter to kill the overwintering eggs and adult spider mites. Apply the dormant oil only if the infestation was heavy in the fall.
Plant flowers around and nearby the Norway spruce tree to attract natural predators of spider mites, which include lacewings, minute pirate bugs, lady beetles, predatory midges and rove beetles.