Spider Mites (Tetranychus urticae)

Spider mites are members of the Acari (mite) family Tetranychidae, which includes about 1600 species. They generally live on the under sides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed.

Spider mites are less than 1 mm in size and vary in color. They lay small, spherical, initially transparent eggs and many species spin silk webbing to help protect the colony from predators; they get the ’spider’ part of their common name from this webbing. Hot, dry conditions are often associated with population build-up of spider mites.

The best known member of the group is Tetranychus urticae (the glasshouse red spider mite, or two-spotted spider mite), which is common in tropical and warm temperate zones, and in glasshouses. Other species which can be important pests of commercial plants include Panonychus ulmi (fruit tree red spider mite) and Panonychus citri (citrus red mite).

Spider mites, like hymenopterans and some homopterous insects, are arrhenotochous: females are diploid and males are haploid. When mated, females somehow avoid the fecundation of some eggs to produce males. Fertilized eggs produce diploid females. Unmated, unfertilized females still lay eggs, that originate exclusively haploid males.

Tetranychus urticae (an animal with over 60 common names, including red spider mite and two-spotted 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 (peppers, tomatoes, potatoes, beans, corn, strawberries) and ornamentals: roses, etc. 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 pemanently greenish or reddish, and these are considered different species of Tetranychus by some authorities.

Its natural predator, Phytoseiulus persimilis, commonly used as a biological control method, is one of many predaceous mites which prey exclusively or mainly on spider mites.