Spider mites are tiny sucking insects related to spiders and ticks. Depending on the species and season, they can be red, orange, yellow or green. They are difficult to see with the naked eye, but their presence can be detected by the damage they cause. Spider mites can infest many kinds of garden plants and trees and they can be a major pest of houseplants. Outdoors, most species are most active during the warmest months. They may attack indoor plants in any season.
Above: Spider mites, damage and webbing on an indoor gardenia. Top: Leaves dmaged by spider mites, showing pale flecks.
Spider mites feed by chewing on leaves and sucking out the plant’s sap. This action leaves small, pale spots. Plants eventually develop a bronze or grayish cast and/or a scorched appearance, and they may drop their leaves. Spider mites produce webs, but it can be difficult to use webbing as a diagnosis tool because it is easy to confuse with the webbing of true spiders.
Spider mites have several natural predators, including other kinds of mites, dark lady beetles, thrips and big-eyed bugs. Unfortunately their natural predators are often killed by insecticides such as carbaryl (Sevin). Their predators also dislike the conditions that spider mites relish: hot, dry weather.
The best defense against spider mites is to disrupt their favored conditions. Where spider mites have been a problem, keep plants watered during dry spells. Spray plants with a steady jet of water to remove any active spider mites; the force will likely also kill them. Spraying water on the plants also removes the dust that stands in the way of spider-mite predators. If you find webbing where spider mites are suspected and you determine it does not belong to a true spider, destroy the webbing to deter reproduction. Indoors, remove damaged leaves, sealing them in bags before throwing them out. Throw away heavily infested plants. Keep houseplants adequately watered and take measures to increase humidity, especially in winter, by misting the air around the plant and standing the plant on a tray of damp gravel. Periodically wipe the leaves with a damp cloth to remove dust and possible spider mites.
Petri dish image attribution: Forest & Kim Starr
Gardenia image source