Once spider mites are present, natural mechanisms tend to be the most consistently effective in controlling an infestation. These include isolating the affected plants or keeping the garden environment under conditions that are antagonistic to spider mites. Using biological predators is also a viable option, such as the Phytoseiulus persimilis, Phytoseiulus longpipes, and Metaseiulus occidentalis. Chemical controls are available and have worked for some but tend to have mixed results because many spider mites have easily built up immunities to many miticides on the market.
Spider mites, especially the two-spotted spider mites, tend to be extremely small and invisible to the naked eye. Most gardeners will need to inspect their plants personally under a magnifying glass to confirm the mites existence before attempting any pest-control tactics. Typical signs of infestation include dried out or curling leaves and small, brownish dots all over the plant. These are formed when the mites suck the juices out of the plants, causing them to dry out and leaving tiny, yellow or brown dots where they have pierced it. White webbing on the underside of leaves is another sign of infestation and is the byproduct of the silk threads the spider mites use to move around the plant.
Isolating the infested plants is the first action any gardener should take. Spider mites tend to be extremely adept at transportation, using silk threads and air currents to migrate from one plant to another. Keeping infested plants quarantined will not only curb their migration, but will reduce air circulation between the plants. This will reduce the level of moisture in the air, aiding in controlling the mite population since it tends to thrive in drier climates.
Other measures should also be taken to keep the moisture levels high around the affected plants. Humidifiers and regular misting are effective methods. Keeping plants over water or growing moss in the space between plants can also be efficient.
Cold water tends to be antagonistic to spider mites. Splashing the affected plants with a stream of extremely cold water every day may not only discourage them from populating that area, but may also drown many, if not most, of them. Water should be sprayed all over the plant, especially on the undersides of leaves, and the plants being sprayed should be separated from the rest to discourage the mites from migrating to them for refuge.
Affected plants should also be kept in the shade and out of the sun or arid weather if possible. The mites tend to thrive better with the evaporative nature of an arid climate, so keeping up an environment of constant shade and humidity will discourage infestation. It will also help the plants themselves, which are most likely weakened from losing many of their juices.
Some gardeners swear by natural predators as an effective means of controlling a spider mite infestation. Metaseiulus occidentalis are one species gaining popularity as a predatory mite for these purposes. They thrive in temperatures anywhere between 45 and 90 degrees Fahrenheit (7.2 to 32.2 Celsius).
The Phytoseiulus persimilis is known for being aggressive and fast-acting. They thrive in temperatures of 70 to 90 degrees Fahrenheit (21 to 32.2 Celsius) and can also be cannibalistic. This will often lead to these predatory mites exterminating themselves after they have exterminated their gardener's infestation.
Phytoseiulus longpipes are in the same family as Phytoseiulus persimilis, originating in Africa. They are able to do well in extremely warm temperatures and are reported to be able to control a low infestation within several weeks. After their food supply has run out, Phytoseiulus longpipes will die.
Using insecticides to control spider mite infestation can also be unreliable because of the many immunities the mites have built up to different brands of miticide. Some anti-transpirants for plants can be indirectly effective in warding off an infestation by helping the plants to retain moisture. This will eventually cause the mites to die out without harming the plants themselves.
Many gardeners also swear by a simple alcohol solution as a means of killing mites. Some use a 1:1 ratio of alcohol to water, while others use 1:3, sometimes adding a few drops of dishwasher detergent. The alcohol solution should be sprayed thoroughly all over the plant, especially on the underside of leaves. It is most effective to apply it in the early morning or late in the day, since evaporation rates are lower. Application can be repeated as often as every 15 to 20 minutes.