The Two-spotted Spider Mite

The twospotted spider mite, Tetranychus urticae is a serious pest of a variety of agricultural crops and ornamental plants. This mite has been recorded to feed on more than 180 different plant species in both greenhouse and outdoor environments. Despite popular belief, spider mites are not insects; they are members of the Order Acarina, which includes spiders, ticks and mites. Whereas insects have three pairs of legs and three body regions (head, thorax, abdomen), spider mites have 4 pairs of legs and one body region.

Two-spot feeds by using mouthparts modified to pierce plant cells. The contents of the plant cells are sucked up by the mite. This behavior leads to the characteristic speckled appearance of leaves attacked by T. urticae. Because the chloroplasts in leaves are gradually destroyed as the population of feeding mites increases, photosynthesis declines, stomata close, and transpiration decreases - leading to reduced production in the case of agricultural crops. Large colonies of T. urticae produce a very fine webbing around the leaves where they feed and toward the tops of plants where they tend to congregrate.

Populations of twospotted mite tend to explode during periods of low humidity and high temperatures. High temperatures (up to 100^oF) decrease the lifecycle from three weeks to a mere 7 days. Low humidity allows the mites to more easily remove waste products from their bodies via evaporation, thus feeding and reproduction are enhanced.

Tetranychus urticae can be recognized by the presence of two large dark green spots on the dorsal part of the abdomen. Significant variation exists in this trait, however, and it may not be a reliable character for identifying T. urticae. For example, the pacific spider mite has dark green spots, but the spots are usually smaller and more numerous and extend to the posterior area of the abdomen. Two-spot mite coloration varies from light green, dark green, brown, black, to orange. Much of this variation depends on the host plant and other environmental factors.

Predacious mites, such as Phytoseiulus persimilis, can be used to control two spotted mite. Under certain conditions this phytoseiid mite can completely eradicate twospotted mite from a greenhouse.