Spider Mite Damage To Trees

By: Ruth Vonderohe, UNL Extension Educator

Mites are difficult to detect. The first signs are yellowing and a whitish flecking on leaves. Check leaf undersides for spider mites or their webbing. A hand lens is needed to see them; or branches can be tapped over a white sheet of paper to check for active mites. Spider mites can be reduced with a strong spray of water directed at leaf undersides and repeated on a regular basis to remove mites, webs and dust. If insecticides are used, the right insecticides must be applied or mites population can increase.

Insecticides Increasing Spider Mites: Spider mites frequently become a greater problem after application of insecticides. Such outbreaks can be a result of the insecticide killing natural enemies of mites or certain insecticides stimulating mite reproduction. For example, spider mites exposed to carbaryl (Sevin) in the laboratory have been shown to reproduce faster than untreated populations. Carbaryl, some organophosphates, and some pyrethroids apparently favor spider mites by increasing the level of nitrogen in leaves. Insecticides applied during hot weather appear to have the greatest effect on mites, causing dramatic outbreaks within a few days.

If insecticide treatment for mites is needed, use selective materials, preferably insecticidal soap or insecticidal oil. Petroleum-based horticultural oils or neem oils are both acceptable. Do not use soaps or oils on water-stressed plants or when temperatures exceed 90°F. These materials may be phytotoxic to some plants, so check labels and/or test them out on a portion of foliage several days before applying a full treatment. Oils and soaps must contact mites to kill them so excellent coverage, especially on the undersides of leaves, is essential and repeat applications may be required.

Source: Spider Mites, University of California