Spider mites, in the family Tetranychidae, were originally an outdoor pest where they caused problems with crops. They have become a pest on houseplants as well as they are carried inside on clothing and pets. They live on the underside of leaves and the plants they infest may have webbing visible as well as yellow and red spots indicating a feeding area. They can be seen on the bottom sides of leaves with at least a 5x magnifying glass.
Spider mites thrive in dry, dusty conditions, such as in homes during winter when heaters are running. One way to try to get rid of them involves changing the environment to one they do not like. This can include spraying the underside of the leaves to dislodge them, wiping the underside of the leaves with a soapy sponge, spraying the entire plant to improve humidification and remove dust, increasing the humidity with humidity trays and more frequent watering. This is usually not enough to get rid of them, but does not hurt to try, especially if the infestation is small.
Chemicals are an option when treating houseplants to get rid of spider mites, though they tend to develop resistance to pesticides fairly readily. Insecticidal soap in a spray bottle is one of the safest methods, though it will only kill the mites and does not effect eggs or nymphs (young mites). This means that it will have to be used regularly every couple of days until the infestation is gone to be effective.
Do not use it when the plant is stressed or in a very hot environment, over 90 degrees Fahrenheit. This method is safe to use on fruit or herb plants that will be eaten. Permethrin concentate is a bit stronger than insecticidal soap but can still be used on edible plants as long as there are at least two weeks left till harvest. This is also available in a dust form.
When choosing pesticides for treatment, be sure they are specified for use on spider mites, known as miticides, as some pesticides have actually been shown to stimulate spider mite growth. For non-edible plants, Bifen or Talstar can also be used. They contain a synthetic pyrethrin that acts well against mites and is gentle on the plant.
Predator mites are the best biological control for spider mites as they are natural enemies. Other predators include bigeyed bugs, lacewings, minute pirate bugs, six-spotted thrips, the spider mite destroyer lady beetle and predatory midge fly larvae. The most important aspect of treating with other bugs is avoiding pesticides that will kill either or both the predator and prey.