Tetranychus urticae (an animal with over 60 common names, including twospotted spider mite) is one of many species of plant-feeding mites found in dry environments, and generally considered a pest. It is the most widely known member of the family Tetranychidae or spider mites.
T. urticae is extremely small, barely visible with the naked eye as reddish or greenish spots on leaves and stems; the adults measure about 0.5 mm. The red spider mite, which can be seen in greenhouses and tropical and temperate zones spinning a fine web on and under leaves. The red spider mite is extremely polyphagous; it can feed on hundreds of plants, including most vegetables and food crops including peppers, tomatoes, potatoes, beans, corn, strawberries; and ornamentals such as roses. It lays its eggs on the leaves, and it poses a threat to host plants by sucking cell contents from the leaves cell by cell, leaving very tiny, pale spots or scars where the green epidermal cells have been destroyed. Although the individual lesions are very small, commensurate with the small size of the mites, the frequently-observed attack of hundreds or thousands of spider mites can cause thousands of lesions and thus can significantly reduce the photosynthetic capability of plants, greatly reducing their production of nutrients, sometimes even killing the plants. Although this way of feeding could spread plant viruses, this is considered of secondary importance.
During the summer, T. urticae has a greenish brown appearance with two darker spots, but as winter approaches it gains a strong red color. Some populations are permanently greenish or reddish, and these are considered different species of Tetranychus by some authorities.