Two-spotted spider mites in the home garden and landscape
The two-spotted spider mite, Tetranychus urticae (Family: Tetranychidae) has been reported infesting over 200 species of plants. This mite can be found on deciduous trees, evergreens, bedding plants and annual garden plants. Some of the more common ornamental plants that can become infested include arborvitae, azalea, spruce, and rose. Common bedding plants that are hosts include lantana, marigolds, New Guinea impatiens, salvia, and, viola. Garden vegetables at risk include cucumbers, snap beans, peas, tomatoes, and lettuce. They can also be found on blackberry, blueberry and strawberry. Spider mite infestations are particularly common during hot, dry summer weather.
Frank Peairs, Colorado State University, Bugwood.org
Two-spotted spider mites
Mites are not insects but are arachnids, a related group of arthropods. Other types of arachnids include spiders, ticks, daddy-longlegs and scorpions. All arachnids, including mites, have two main body parts and eight legs.
Two-spotted spider mites are minute (1/50th inch long), yellow-orange in color, and have two dark spots, one on each side of the body. They are difficult to see without the aid of a 10x hand lens. When a heavy infestation occurs webbing will also be present.
Jeff Hahn, University of Minnesota
Webbing caused by spider mites
These mites overwinter as eggs on vegetation. Larvae hatch and complete development in 1-2 weeks, depending on the temperature. Under high temperatures (>90°F) colonies can explode and increase exponentially in less than two weeks.
Once two-spotted spider mites hatch, they establish colonies on the undersides of leaves and produce webbing over infested leave surfaces. This webbing earns them the name "spider" mites.
Whitney Cranshaw, Colorado State University, Bugwood.org
Spider mite damage to snap bean
Spider mites injure leaves with piercing-sucking mouthparts. This injury produces tiny white or yellow spots or "stippling". The leaves lose photosynthetic surface as feeding continues. As colonies grow and feeding intensifies, entire leaves progress from grayish green to yellow, brown or coppery, and eventually drop off. These symptoms may be confused with drought stress. On azalea, in addition to stippling on the foliage, there may be distortion of flowers and foliage. On garden vegetables and bedding plants, severe infestations may cause plants to die.
Examine plants for stippling and/or webbing. Look closely on the underside of discolored leaves for the presence of spider mites (you'll need a hand lens). You can also hold a white piece of paper or cardboard underneath potentially infested leaves; shake the leaves and look for spider mites that have fallen. It is important to check garden plants every 3-5 days, especially under drought conditions.
Spider mites thrive on plants under stress; therefore it is important to provide proper growing conditions. The best way to do this is through proper watering. Most plants should receive about one inch of water a week. It is also important to conserve moisture through proper mulching. Select drought tolerant plants for locations that are particularly hot and dry. For more information, see Protect your landscape during times of drought. While it is important to fertilize plants when there is a nutrient deficiency, it is important not to do so during drought as this can add further stress to plants. Also, be careful not to overwater as this can lead to root rot.
A water hose can be used to spray infested leaves to dislodge some of the spider mites. This can also wash away their protective webbing.
Several insects, such as certain species of lady beetles (e.g. Stethorus sp.) and predatory mites (e.g., Phytoseiulus persimilis) naturally help keep spider mite populations in check. However, when the weather is favorable for spider mites and their numbers dramatically increase, it is difficult for these natural enemies to keep spider mites populations in check.
These natural enemies can be killed and spider mite problems can flare up when certain insecticides are used, like carbaryl (Sevin) and imidacloprid. Always use insecticides carefully and judiciously and do not make unnecessary applications.
There are few miticides (acaricides) or insecticides that are available for use in home gardens and landscapes for spider mite management.
Insecticidal soap and horticultural oil are reasonably effective insecticides. They are considered to be low impact products and have little, if any, impact on people, animals, and nontarget insects. It is important to get thorough coverage with these materials; target the underside of leaves as well as the top. These products will only kill mites that the pesticide directly contacts and they do not have any residual activity. Because of this, applications may need to be repeated.
Bifenthrin is a long lasting insecticide that is also effective against mites. It has broad activity and is effective against a variety of insects so use it carefully and judiciously.
Since most infestations occur when it is hot and dry it is important when spraying for mites to water the plants thoroughly before spraying, and to spray in the early morning or early evening. This will help to reduce the risk of further stressing the plant and causing injury.
CAUTION: Read all insecticide label directions very carefully before buying and again before using to ensure proper application. Be sure that the label specifies it can be used on the specific plant you wish to treat. The label is the final authority on how you may legally use any pesticide. Whenever using any pesticide, including low impact, natural or organic insecticides, follow all label directions including information on transporting, storing, mixing, applying and disposing of pesticides.