Tetranychus urticae (common names include red spider mite and two-spotted spider mite) is one of many species of plant-feeding mites found across the world, and generally considered a pest. It is the most widely known member of the family Tetranychidae or spider mites. Its genome was fully sequenced in 2011, becoming the first one for an arachnid.
T. urticae is extremely small, barely visible with the naked eye as reddish or greenish spots on leaves and stems; the adult females measure about 0.4 mm long. The red spider mite, which can be seen in greenhouses and tropical and temperate zones, spins a fine web on and under leaves.
T. urticae is extremely polyphagous. It can feed on hundreds of plants, including most vegetables and food crops, including peppers, tomatoes, potatoes, beans, corn, and strawberries, and ornamentals such as roses. It is the most prevalent pest of Withania somnifera in India. 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 tiny pale spots or scars where the green epidermal cells have been destroyed. Although the individual lesions are very small, attack by hundreds or thousands of spider mites can cause thousands of lesions and thus can significantly reduce the photosynthetic capability of plants.
The egg of T. urticae is translucent and pearl-like. It hatches into a larva and then two nymph stages follow: a protonymph, and then a deutonymph, which may display quiescent stages. The adults are typically pale green in colour for most of the year, but later generations are red in colour; mated females survive the winter in diapause.
Other than certain aphids the two-spotted spider mite is the only animal that can synthesize carotenoids. Like aphids, the genes for carotene synthesis appear to have been got through horizontal gene transfer from a fungus.
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