Spider mites damage a wide range of plants in gardens and landscapes. When signs of spider mites appear, gardeners may reach for an insecticide to control them. Unfortunately, some insecticides increase rather than decrease spider mite populations.
Landscape ornamentals I often see spider mite damage on include spruce, honey-locust and oak trees, potentilla shrubs, roses, New Zealand impatiens, and marigolds. Vegetables often infested with mites include tomatoes, snap beans and sweet corn.
The first signs of mites are usually a white stippling on leaves caused by mites inserting needle-like mouthparts to withdraw sap. Leaf yellowing is another symptom. Under heavy infestation, leaves will turn brown or bronze colored and fine webbing may be seen along the stems or between leaf veins.
When mites are suspected, check leaf undersides for spider mites and their webbing. Mites themselves are difficult to detect. They are just visible to the human eye. A hand lens is often needed to see them.
A simple way to check for the presence of mites is to gently tap a stem or branch over a white sheet of paper. If, after a few seconds, what appear to be specks of dust begin to move around on the paper, these are likely mites.
To be sure it isn’t a breeze blowing dust specks around, cut a small branch or stem with affected leaves and do the tap test indoors. I often slide my hand over the specks on the paper. If they smear green, yellow or red, they are likely a mite and not a dust speck.
If found, spider mites and their damage can be reduced with a strong spray of water directed at the plant and leaf undersides and repeated a few times each week to remove mites, webs and dust. For smaller plants, this is an effective control measure.
If insecticides are used, gardeners need to be selective about which products they use as some insecticides will increase mites. This may be due to insecticides killing natural enemies of mites; however, insecticides like carbaryl (Sevin), some organophosphates and some pyrethroids have been shown to stimulate mite reproduction.
If insecticide treatment for mites is needed, it is recommended to use an insecticidal soap or insecticidal oil. Petroleum-based horticultural oils or neem oils are also acceptable. Oils and soaps must contact mites to kill them, so thorough coverage, especially on leaf undersides, is needed.
If chemical insecticides are used, avoid the use of products containing carbaryl (Sevin). Look for products containing acephate (i.e. Orthene) or an actual miticide. Read and follow all label directions for the use of these products on plants.
Do not use chemical insecticides, soaps or oils on water-stressed plants or when temperatures exceed 90 degrees Fahrenheit as plants maybe damaged. Repeat applications are also needed to control new spider mites hatching from eggs.