Spider mites attack both garden plants and indoor houseplants. A severe infestation can completely defoliate a plant. Low humidity indoors is the type of environment that spider mites thrive in. They seem to favor ivy and the Norfolk Island pine. Spider mites can be transmitted through infected soil or even enter your home through the screens of open windows. Houseplants that are summered outdoors can become infected and transmit spider mites to other indoor plants. Inspect such plants carefully before bringing them indoors in autumn.
The spider mite that most commonly attacks houseplants is the twospotted spider mite (Tetranychus urticae), from the Tetranychidae family. It is related to common spiders, daddy-longlegs and scorpions. Depending on the species and the season, they can be red, brown, yellow or green. They create webs, which may be confused with webs of larger spiders that do not feed on plants. They lay eggs near the veins of the leaves and the shells remain after the spiders hatch.
Symptoms of spider mites include the leaves becoming flecked or discolored (bronzing). In severe infestations the leaves may become extremely discolored, causing the plant to appear gray or bronze. Leaves and needles drop prematurely. Infected areas show light colored flecks and make the plant look speckled. Small webs form at the leaf junctures, with an appearance that is similar to webs of larger spiders.
Moisture management is crucial, as dry conditions are conducive to their development, while their natural enemies require more humid conditions to flourish. Plants that are drought-stressed may experience changes in their chemistry that more readily attract spider mites.
Provide your houseplants with adequate moisture to reduce moisture stress and make them less attractive to spider mites. To remove and kill a substantial number of spider mites, hose down your houseplants with a forceful stream of water with the hose or put them in the shower and rinse them off.
General insecticides often kill their natural enemies, so use a pesticide specifically formulated for the control of spider mites, called miticides or acaricides. Horticultural oil that is sprayed on is the most effective spider mite control for houseplants.
Following the instructions of the manufacturer of the horticultural oil, treat all susceptible plants at the same time, removing and destroying leaves that are heavily infested. Re-apply every 10 to 14 days until symptoms disappear.
Quantities of predatory insects are available from larger seed catalog companies, including lady beetles (Stethorus sp.), known as "spider mite destroyers," predatory mites (Galendromus occidentalis or Phytoseiulus persimilis), minute pirate bugs (Orius sp.), and big-eyed bugs (Geocoris sp.). Follow the instructions provided by the vendor as to the proper and most effective use of these predatory insects in reducing populations of spider mites.