Spider Mites small arachnids that are difficult to see with the unaided eye. They are a common pest on many plants in yards and gardens. Spider mites have four pairs of legs, no antennae, and a single, oval body region. Their colors range from red and brown to yellow and green depending on species and seasonal changes. Many spider mites produce webbing, particularly when they occur in high populations. The webbing provides some protection from natural enemies and environmental fluctuations.
The most important spider mite pest is the two-spotted spider mite, Tetranychus urticae, which attacks many vegetables, fruits, flowers, and houseplants. Evergreens host other mites, Oligonychus ununguis, Oligonychus subnuus, and Platytetranychus libocedri. Honeylocust are almost invariably infested with the honeylocust spider mite, Platytetranychus multidigituli. Another complex of mites is associated with turfgrass, including clover mite and banks grass mite. These are discussed in Clover and Other Mites of Turfgrass.
- Spider mites are common plant pests which cause flecking, discoloration (bronzing) and scorching of leaves. Heavily infested plants may be stunted, discolored or killed.
- Natural enemies include small lady beetles, predatory mites, minute pirate bugs, big-eyed bugs and predatory thrips.
- One reason that spider mites become a problem is insecticides that kill their natural predators.
- Irrigation and moisture management can be important cultural controls for spider mites.
Two-spotted spider mite injury and webbing on indoor-grown medical marijuana. Image creidt: Whitney Cranshaw, Colorado State University, Bugwood.org.
Spider mite destroyer (Stethorus punctum picipes) Image creidt: F.C. Schweissing, Bugwood.org
Two-spotted spider mite and eggs. Image credit: Bruce Watt, University of Maine, Bugwood.org
Two-spotted spider mite stifling injury to bean leaves. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Spider mites develop from eggs, which are usually laid near the veins of leaves during the growing season. Most spider mite eggs are spherical and extremely large in proportion to the size the of the mother. After egg hatch, the old egg shells remain and can be useful in diagnosing spider mite problems.
Spider mite activity usually peaks during the warmer months. They can develop rapidly during this time, becoming full-grown in as little as a week after eggs hatch. After mating, mature females may produce a dozen eggs daily for a couple of weeks, leading to an extremely rapid population increase. During winter, most mites change color, often turning more red or orange. Outdoors, the two-spotted spider mite and honeylocust mite survive winter as adults hidden in protected areas such as bark cracks, bud scales, or under debris in the garden. Other mites survive the cool season in the egg stage. The spruce spider mite and most of the mites that can damage turf grass are most active during cooler periods of the growing season (spring and fall). These cool-season mites may cease development and produce dormant eggs to survive hot summer weather.
Dry conditions greatly favor all spider mites, an important reason why they are so important in the more arid areas of the country. They feed more under dry conditions, as the lower humidity allows them to evaporate excess water they excrete. At the same time, most of their natural enemies require more humid conditions and are stressed by arid conditions. Furthermore, plants stressed by drought can produce changes in their chemistry that make them more nutritious to spider mites.
Injury is caused as spider mites feed, bruising cells with their small, whiplike mouthparts and ingesting the sap. Injured areas appear marked with many small, light flecks, giving the plant a somewhat speckled appearance. Following severe infestations, leaves become discolored and bronzed and may drop pre-maturely. Spider mites frequently kill plants or cause serious stress to them.
Adequate watering of plants during dry conditions can limit the importance of drought stress on spider mite outbreaks. Periodic hosing of plants with a forceful jet of water can physically remove and kill many mites, as well as remove the dust that collects on foliage and interferes with mite predators. Spider mite outbreaks can occur due to the use of insecticides that kill their natural enemies.
Various insects and predatory mites provide a high level of natural control, including Spider mite destroyers (Stethorus spp.), minute pirate bugs, big-eyed bugs (Geocoris spp.), and predatory thrips. Many mites in the family Phytoseiidae are predators of spider mites. Some of these are produced in commercial insectaries for release as biological controls indoors or in greenhouses, although effective use has been demonstrated outdoors in Colorado
Chemical control usually involves pesticides that are specifically developed for spider mite control (miticides or acaricides). Few insecticides are effective for spider mites and many even aggregate problems.
Godfrey, L. 2011. Pests in Gardens and Landscapes. University of California – IPM. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn7405.html
Utah State University. (n.d.). Twospotted Spider Mite. Utah State University – Extension. Available https://extension.usu.edu/vegetableguide/cucumber-melon-pumpkin-squash/spider-mites#:~:text=Use%20a%20strong%20stream%20of,foliage%20encourage%20spider%20mite%20reproduction.