Hemlocks are attacked by tiny, yellow rust mites that cause needles to turn brown and drop prematurely. Dwarf Alberta spruce and many pines are attacked by cool-season mites, such as the spruce spider mite and white pine tip dwarf mite. Spray plants thoroughly with horticultural oil when you see fifty or more mites and no predators. Spraying your trees and shrubs with horticultural oil to kill overwintering pests in early spring may not benefit them in the long run. If you are practicing proper monitoring techniques, cover sprays won't be necessary because you can distinguish between plants with pest problems and those without.
Horticultural oil is most effective when pests have hatched and are vulnerable.
Treating the dormant pest may kill beneficial insects and can injure the plant. Use fertilizer wisely this coming season. Overfertilization can cause symptoms easily mistaken for nutrient deficiency. An overfertilized plant will often burn, discolor, wilt, and may even invite hungry pests. If a plant is infested with mites, scales, aphids, or adelgids, applying fertilizer may make the problem worse.