Tetranychus urticae, commonly known as the two-spotted spider mite, is a tiny but destructive pest that affects a vast range of plants. This mite, despite its common name, is not a true spider but belongs to the subclass Acari. It poses a significant threat to various plants, from common houseplants to agricultural crops, by feeding on plant cells and causing substantial damage. Its widespread distribution and rapid reproduction contribute to its status as a pervasive pest.
Identification and Life Cycle
Adult two-spotted spider mites are small, typically measuring about 0.4 to 0.5 mm in length, making them barely visible to the naked eye. They possess eight legs, distinguishing them from insects, which have six. Their color varies, appearing greenish-yellow, pale green, or translucent, often with two distinct dark spots on their backs. These spots are accumulations of waste products; newly molted mites may not display them. Overwintering females frequently appear orange to reddish-orange.
The life cycle of Tetranychus urticae includes several stages: egg, larva, two nymphal stages (protonymph and deutonymph), and the adult. Eggs are microscopic, spherical, and initially transparent, gradually becoming yellowish. They are laid individually on the undersides of leaves, sometimes within fine silk webbing.
Larvae hatch with six legs, while subsequent nymphal stages develop eight legs. The entire life cycle can be completed in as little as 5 to 7 days under optimal warm and dry conditions, such as temperatures around 80°F (27°C). This rapid development allows populations to multiply quickly, leading to swift infestations.
Recognizing Plant Damage
The earliest indications of a Tetranychus urticae infestation appear as fine, pale stippling or tiny yellow dots on the upper surfaces of leaves. This damage occurs as mites use their piercing mouthparts to extract contents from individual plant cells, including chlorophyll. As feeding continues, these damaged cells appear whitish or bronzed, progressively causing leaves to turn yellow, bronze, or even brown.
Affected leaves can become dry and brittle, leading to premature leaf drop. In severe infestations, the most definitive sign of Tetranychus urticae is the production of fine, silken webbing. This webbing can extensively cover leaves and stems, providing a protective environment for the mite colony and aiding dispersal to new plant parts or nearby plants. Heavy populations can severely defoliate plants, significantly reducing photosynthetic capability and potentially causing plant death.
Conditions Favoring Infestations
Tetranychus urticae thrives in hot, dry, and dusty environments. These mites develop and reproduce rapidly when temperatures are high, ideally around 80°F (27°C). Low humidity accelerates their reproductive cycle and population growth, making indoor plants or greenhouse environments susceptible if humidity is not managed.
Plant stress makes plants more vulnerable to infestations. Underwatering, for example, weakens plants, making them easier targets for mite feeding and establishment. Broad-spectrum insecticides are a factor contributing to sudden mite outbreaks. These chemicals eliminate natural predators of Tetranychus urticae, such as beneficial mites and insects, removing natural population controls and allowing mite numbers to increase rapidly without opposition.
Control and Management Methods
Controlling Tetranychus urticae infestations involves a range of methods, starting with less aggressive approaches. Cultural and mechanical controls offer initial lines of defense. Spraying plants with water can dislodge mites and their webbing, while also increasing local humidity, which is unfavorable for the mites. Regularly wiping leaves with a damp cloth can remove mites and their eggs. For heavily infested areas, pruning and disposing of affected leaves or stems can reduce mite populations.
Biological control utilizes natural enemies to manage mite populations as part of integrated pest management (IPM). Predatory mites, such as Phytoseiulus persimilis, are effective predators that feed on all life stages of spider mites. Other beneficial insects like ladybugs, particularly Stethorus species, and lacewing larvae also prey on spider mites. Introducing these natural predators can suppress mite populations, especially in greenhouse settings.
Chemical control options include softer choices like horticultural oils and insecticidal soaps. These products work by direct contact, suffocating mites or disrupting their cell membranes, and require thorough coverage of all plant surfaces, especially undersides of leaves. Repeat applications are often necessary due to their lack of residual activity. Stronger chemical miticides (acaricides) are available for severe infestations, but Tetranychus urticae is known for its ability to rapidly develop resistance to these pesticides. To maintain effectiveness and prevent resistance, rotating different chemical classes with varied modes of action is recommended, alongside monitoring for mite susceptibility.