Spider mites are not insects, but are more closely related to ticks and spiders. They are a common problem on both deciduous and evergreen trees and shrubs including spruce, pine, juniper, honeylocust, linden, elm and many others. In fact, under the right conditions spider mites can attack almost every plant in the landscape including flowers, vegetables and grass.
These tiny pests are less than 1 millimeter long and vary in color from yellow to red to green. They overwinter as eggs on the effected plants and produce many generations each year. Mites feed by sucking sap from the plant's leaves and stems and generally occur in the highest numbers on the undersides of leaves.
Warm, dry weather favors rapid mite development and reproduction, enabling them to complete a generation in as little as 5 to 7 days. Infestations are usually most severe in late summer and early fall. However, the Spruce spider mite, or "cool season" mite is most active during the cooler weather of early spring and late fall.
Symptoms of Infestation
Heavily infested leaves may be covered with very fine, irregular webs in which mites, eggs, and shed skins are suspended. Overall, it gives the impression of heavy dust particles on the plant. Except in the most severe cases, these webs are very small and do not stretch from leaf to leaf. Rather, they are almost invisible but can be seen only under very close inspection stringing across the leaf's midrib or underside of the leaf.
Damage symptoms progress from stippling to yellowing, wilting, browning, and eventually to death of the leaves or whole plant. In addition to crawling, spider mites can be spread by wind currents and by people or animals moving through the landscape. To check for spider mites, place a white piece of paper beneath the branch or leaves and tap several times. The mites will appear as very small, bits of dust that are crawling across the page!
Controlling spider mites is difficult because they reproduce so rapidly. One method to try involves spraying the plant with a strong jet of water once or twice a day to dislodge some of the insects and to create an environment that is cooler, more humid and less favorable for spider mite reproduction. Several days or even weeks of this treatment will be required to make a noticeable difference in spider mite populations.
Horticultural and dormant oils are also useful on plants with a history of spider mite problems, and can be sprayed on landscape plants during the summer, although some leaf discoloration or phytotoxicity may occur on some plants. Be sure to follow all label directions.
Dormant oil application, or horticultural oils used during the winter or very early spring prior to bud break, are useful in killing overwintering mite eggs and can help suppress mites populations. In cases of very severe infestations, use of a chemical miticide may be necessary.
Make sure your plant is listed on the label before spraying to prevent damaging the plant and follow the label directions for use of the product.