Although they're often lumped in with insects, there are a large group of plant pests that are not insects at all, but rather eight-legged arachnids closely related to spiders and ticks. These spider mites are extremely small, 1/60 to 1/25 inch in size, and are usually recognized by the fine silky webbing they spin on plants, and by the general damage they cause to plant foliage.
There are hundreds of different species categorized as spider mites that attack different types of indoor and outdoor plants. They come in a wide variety of colors, including red, yellow, green, and brown. Some even change colors throughout the year, while others are translucent. Spider mites most often cluster on the undersides of leaves, causing damage by sucking on the tissues. Some spider mites pose serious problems to the commercial fruit industry, and one mite, known as the spruce spider mite, causes severe damage to a variety of landscape conifers.
Recognizing Spider Mite Damage
Spider mites feed by sucking out plant juice from the leaves, which often leaves them looking stippled, yellow and dry, or covered with pale yellow spots or blotches. On conifers, spider mites often cause older, inner needles to turn yellow, then fall off. Spider mites may also suck chlorophyll, causing small white dots to appear on the leaves. Some spider mites inject toxins into the leaves, discoloring and distorting them. If you see any of these symptoms—especially in addition to some fine white webbing on the leaves—it's very likely you are dealing with spider mites.
These pests are so small that you won't usually be able to see them with the naked eye, but if you hold a white sheet of paper below the affected plant leaf, tap on the leaf with a pencil or your finger, you may then see mites crawling on the paper if you use a magnifying glass.
Outdoor plants most often affected are cucurbits (squash, pumpkins, cucumbers, etc.), beans, tomatoes, as well a wide range of landscape trees and shrubs. Many indoor houseplants can be affected by spider mites, especially those with thin, tender leaves. You are most likely to see spider mite damage in warm, dry conditions.
7 Ways to Get Rid of Spider Mites
Spider mite infestations are often signaled by leaves that grow yellow and dry-looking and by a plant that looks to be in general poor health. Fortunately, there are several methods for getting rid of spider mites.
Spray With Water
Most species of mites thrive in dry, warm weather, and a very effective control is to regularly spray plants with hard blasts of water, aiming especially at the undersides of leaves or needles. This treatment needs to be repeated regularly to keep the mites from regaining a foothold on the plants. Indoor houseplants can be placed in a kitchen sink or bathtub to get treated with the faucet sprayer.
Use Insecticidal Soaps or Oils
Insecticidal soaps that are formulated to kill insects and other pests are often quite effective against spider mites. Treatment will need to be repeated frequently until the mite problem is under control. Horticultural oil products made from highly refined plant oils, such as citrus, are also highly effective against mites since they smother the pests. Horticultural oils are less likely than chemical pesticides to kill beneficial insects, and are thus regarded as organic products.
For fruit trees, a mixture that combines horticultural soap with an oil is effective when applied every three to five days until the mites are killed.
A variety of predatory insects and mites are known to feed on spider mites. Several can be purchased at larger garden centers or from online retailers to apply to plants you want to protect or cure, including several predatory mites, Phytoseiulus persimilis, Amblyseius californicus, and Amblyseius andersoni. Helpful insects include big-eyed bugs (Geocoris spp.), ladybugs, lacewings (Chrysopa spp.), predatory thrips (Scolothrips spp.), and spined soldier bugs.
Because spider mites have such a large number of natural predators, many experts recommend a hands-off approach in outdoor settings, where diverse insect populations often keep mites under control. Paradoxically, it is often those landscapes where chemicals are used freely that will experience spider mite problems since helpful predatory insects are likely to be killed.
Use Neem Oil
Neem oil is a natural product derived from the neem tree. Neem oil is safe for humans and most animals, but it kills a variety of insects, pests, and mites. Neem oil contains azadirachtin, an active compound that interferes with the feeding, molting, mating, and egg-laying cycle of insects and mites.
While neem oil is regarded as a safe organic pesticide when it comes to humans and warm-blooded animals, it is decidedly toxic to fish and other aquatic life. Use it cautiously to prevent run-off from reaching streams and lakes where it can poison aquatic life.
Use Pyrethroid Pesticide
Pyrethroid pesticides are made from natural pyrethrins blended with other chemicals to improve the performance of the pyrethrin. Pyrethrin itself is a natural derivative of certain types of chrysanthemum flowers, and in pure form, it is considered a safe, organic pesticide. However, pyrethroid pesticides are not considered organic since they contain added chemicals, though they are still a safer alternative than some of the other purely synthetic chemical pesticides. Pyrethroid pesticides are most often recommended against spruce spider mites.
Spray With Chemical Pesticide
A variety of commercial chemical pesticides will kill spider mite infestations, including malathion, bifenthrin, cyfluthrin, and kelthane. These should be reserved for very serious infestations of important plants, and only where other methods have failed. Chemical pesticides are toxic to pets and humans, so they should be used with caution, especially on vegetables and fruits. Always read and follow label directions carefully.
Spider mites are known to quickly develop a tolerance for chemical pesticides, so it's best to rotate from chemical to chemical if you find it necessary to apply repeated treatments.
Call an Arborist or Tree Disease Specialist
Spruce or other evergreen trees under attack by the spruce spider mite can be very difficult to treat by DIYers, and a licensed professional may have access to stronger pesticides that are unavailable to homeowners. Such strong chemicals are a last resort and should be used only when less toxic methods prove futile, and only where the loss of an important tree is likely.
What Causes Spider Mites?
Spider mites are notorious for thriving in dry, hot weather—about 80 degrees is considered optimal—and most varieties of mites are most prevalent from July to September, when conditions are favorable. One notable exception is the spruce spider mite (Oligonychus ununguis), which is most active during cooler weather in spring and fall. The spruce spider mite is a serious pest of various pine trees, causing yellowing of needles and gradual defoliation that can kill the tree.
Indoor plants are most likely to get mites during the winter months, so check them regularly as the indoor air turns dry.
How to Prevent Spider Mites
Potted plants that spend summer months outdoors should be checked carefully before they are brought indoors for the winter. The dry winter months are especially favorable to spider mites, so it's best to try to inspect for mites before the plants move indoors. Similarly, purchased plants should be carefully checked before you bring them home from the garden center.
Keeping indoor air humidified may help prevent serious infestations, especially if you also spray your houseplants regularly to dislodge and wash away mites before they can gain a foothold.
All plants—both indoors and outdoors—will be less likely to experience spider mite infestations if they are blasted with water occasionally. Spider mites are very tiny pests that are easily washed away.
Do Spider Mites Bite?
Although spider mites have piercing mouthparts, they are such tiny creatures that biting through human skin is virtually impossible. Nor do they have any incentive to do so; these are not blood-feeding pests, but creatures that feed entirely on plant juices.
How Long do Spider Mites Live?
Spider mites typically live two to four weeks, but females can lay as many as 20 eggs in that time, each of which hatches and develops into a breeding adult after only about five days. Once infested with spider mites, a plant will stay under attack until treated.
Are Any Plants Immune to Spider Mites?
There are no plants that are utterly immune to all types of spider mites, but in general, those plants with thick, tough leaf structures are less likely to sustain serious damage. Some indoor plants that resist spider mites include the jade plant, rubber tree, snake plant, and zee-zee plant.
Some outdoor plants that are very commonly attacked by spider mites include broadleaf evergreens, elms, evergreens, fruit trees, honeylocust, maple, mountain ash, oaks, and roses.