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Spider Mites (Acari: Tetranychidae)
- Spider mites are probably the most important plant pest overall -
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Summary: Spider mites are tiny plant-feeding mites that cause significant injury in gardens and houseplants as well as agricultural crops world-wide. Gardeners can reduce damage using low toxicity insecticidal soaps.
Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)
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What are spider mites?
 Spider mites are small, plant-feeding mites that may severely damage plants. Mites pierce the leaf surface, disrupt leaf tissue and extract cell contents. Mite feeding makes a hole in the leaf's protective layer; leaves eventually dry out and turn brown because of water loss through these tiny holes.
Spider mites have eight legs in all stages except larvae. They develop from egg to adult in as little as two weeks during summer. There can be many generations per year and most spider mites spend winter months as adult females.
Spider mites have many natural enemies but other mites, predator mites, are the most important. Lady beetles, syrphid flies and lacewings also are good predators. Spider mites can be controlled in gardens and houseplants using the same methods that are used for aphids (see below).
Common misspellings and misnomers for spider mites: red mites, red spiders, spidermites.
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Spider mites on bean leaf. Large, dark mites (arrow) are adult females. This is the view through a good hand lens or magnifying glass. ©www.calyphoto.com.
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Line drawing of a spider mite ("head" toward left). Note "hairs" (setae) on legs and back. Adjust your monitor
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Least-toxic spider mite control
Use this method for aphid control as well.
Scout for spider mites during the growing season. Hold a piece of white paper under the leaves where you suspect an infestation. Tap on the branch hard enough to dislodge any mites, but not too hard! If spider mites are present they will appear as tiny, dark flecks, that are moving, on the white paper -- they are about the size of ground pepper. Lighter, and faster mites may be predators. Use your hand lens to get a closer look.
Pick a sunny, warm (about 75-85 °F) day. In the morning hose off the plants to remove as many mites and eggs as possible. This washing will remove dust, dirt and other debris that favor mites as well. Use enough water pressure to dislodge mites, but be careful not to damage the plant.
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Mix up a solution of insecticidal soap (1-2%). Apply spray to all leaf surfaces where mites may be hiding. Do the application in the morning because soap can burn foliage when air temperature is high. Before the soap solution dries rinse it off with a spray of water. This final step will remove residual soap that might burn tender leaf tissue. Repeat these steps at 1-2 weeks until you no longer find mites. This procedure will work for houseplants as well.
One final caution: soap solution should be almost clear with a slight milky color. Don't use the solution if it is milky white or solids form in the container as this is an indication that the soap has "gone bad" and may damage your plants.
For conifers (arbovitae, spruce, pine, fir, etc.) the procedure is a little different. Spruce spider mite (Oligonychus ununguis) is the most common spider mite on conifers. Unlike other spider mites, spruce spider mites lay eggs in the fall that won't hatch until the following spring. Other spider mites stop laying eggs by late summer. The spruce spider mite's red "winter egg" can be found on stems and needles. Winter eggs can be effectively controlled by late winter applications of horticultural or dormant oils. During summer, spruce spider mites can usually be controlled with washing and insecticidal soap as outlined above.
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Use a hand lens to check leaves for spider mites.
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Row covers protect plants from spider mites. They can also protect against heat and cold stress.
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