They are very small and not seen easily with the naked eye. They have piercing mouthparts that they use to suck plant sap. Their feeding results in speckling (formation of tiny yellow spots) on needles. Some needles may turn brown and drop off. With heavy infestations, fine webbing may be seen on the plant. Several seasons of heavy mite feeding may kill a juniper. Although most spider mites increase in numbers during hot, dry weather, spruce spider mites are cool-weather mites. Their population peaks during spring and fall, but drop dramatically during the heat of summer when predators feed on them.
Control: Naturally occurring enemies of mites include various predator mites, ladybird beetles (ladybugs) and other insects. These predators will usually suppress mite populations. Since insecticide use kills beneficial predators as well as mites, insecticides should be avoided unless absolutely necessary. Misuse of insecticides can result in increased problems with mites by causing the death of natural predators of the mite. Miticides, labeled specifically for mite control, are less harmful to beneficial insects. Mites can be removed with a strong spray of water, if applied on a regular basis.
To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If dozens of mites are seen per whack, serious damage can result. Continue to check population numbers at seven to ten day intervals. Pesticides labeled for homeowner use against spruce spider mite include insecticidal soaps, dimethoate (Cygon 2EC), acephate + hexakis (Isotox Insect Killer Formula VI). As with any pesticide, read and follow all label directions and precautions before using.
NOTE: Control of diseases and insects on large trees is usually not feasible, since adequate coverage of the foliage with a pesticide cannot be achieved.