Spider Mites

- Spider mites are the most important plant pest overall -

Summary: Spider mites are tiny plant-feeding mites that cause significant damage to gardens and houseplants, as well as agricultural crops world-wide. Gardeners can reduce damage from spider mites using low-toxicity insecticidal soap.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Spider mite identification

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 and 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 which have only six and they develop from egg to adult in as little as two weeks during summer. There can be many generations each year and most spider mites spend winter months as adult females hiding in plant debris at ground level or under bark scales on perennial plants.

Spider mites have many natural enemies but other mites, called predator mites, are the most important. Lady beetles, syrphid flies and lacewings also are good predators of spider mites. Spider mite control in gardens and houseplants is relatively simple using the same methods that are used for aphids.

Common misspellings and misnomers for spider mites: red mites, red spiders, spidermites.

spider mites on bean leaf

Spider mites on bean leaf. Large, dark mites (black arrow) are adult females. This is the view through a good hand lens or magnifying glass.

spider mite linedrawing

Line drawing of a spider mite ("head" toward left). Note "hairs" (setae) on legs and back.

Least-toxic spider mite/aphid control

Scout for pests often 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 table salt. Any lighter, and faster mites that you see may be predators. Use your hand lens to get a closer look.

If spider mites are found pick a sunny, warm (about 75-85 °F) day to apply control measures. 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.

Next, mix up a solution of insecticidal soap (see Using Insecticidal Soap For Spider Mites). 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 second 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 when mixed. 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.

Spruce spider mite

For landscape plants called conifers such as arbovitae, spruce, pine, and fir 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 but clusters of the spruce spider mite's red "winter egg" can be found on stems and needles during winter months.

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 for other spider mites (see How to Reduce Damage From Spruce Spider Mites).

Related Articles

How Do Spider Mites Damage Plants?

What are "Beneficials" or Natural Enemies?

Using a Hand Lens for Garden Pest Identification

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