Twospotted Spider Mite
Tetranychus urticae Koch, Tetranychidae, PROSTIGMATA


DESCRIPTION

Adult - The eight-legged adult is almost microscopic, being only 0.3 to 0.5 mm long. Oval in shape, the active summer female is yellowish to dark green with two or four dark dorsal spots. Smaller and more active than the female, the male has a narrower body and a more pointed abdomen.

Egg - The egg is spherical, minute and transparent when first deposited. It gradually assumes a yellowish-green color.

Larva - The six-legged larva, not much bigger than the egg, is colorless except for carmine eyes.

Nymph - The two nymphal stages are difficult to distinguish. Both are pale green, oval, and eight-legged. The pair of dark spots is visible at this point of development.


BIOLOGY

Distribution - Twospotted spider mites are widely distributed throughout the world. In the southeastern U.S., they are pests throughout the Coastal Plain area.

Host Plants - Twospotted spider mites have been found on over 180 host plants, including at least 100 cultivated species. Violets, chickweed, pokeweed, wild mustard, hairy vetch, red clover, Carolina geranium, and blackberry are common hosts from which infestations spread to crops nearby.

Damge - Mites pierce the epidermis and extract sap from the undersides of leaves. Infested foliage soon assumes a whitish or bronze appearance. Lightly infested leaves have pale blotches or spots showing through the leaf; heavily infested leaves turn completely pale and dry up. The undersurfaces of leaves usually are covered with silken webs over which the mites crawl. Heavily infested plants may have webs all over them. Close examination reveals adult mites on the leaves, but the larvae initiate damage.

Life History - Twospotted spider mites overwinter as females resistant to low temperatures. These females are red as opposed to the active summer forms which are yellowish-green. In greenhouses or during mild winters, some feeding and egg-laying activities may continue.

In warm weather, egg-laying activities increase; each female produces up to 19 eggs per day and up to 100 eggs in all. The number of eggs laid depends largely on temperature. After an incubation period of 3 to 19 days, six-legged larvae hatch from the eggs and feed. Immature mites go through a resting period between the larval and nymphal stages and again after each of the two nymphal instars. The mites may mature into adults in as few as 5 days or as many as 20, depending on temperature. Development is most rapid during hot, dry weather. Many generations are produced each year.


CONTROL

One form of cultural control is the destruction of weeds around the field in fall or early spring. This practice reduces the overwintering population. Destruction of weeds along fence rows or around the edges of fields during the growing season is not advisable. Such weed control forces mites to migrate into the field. The tomato variety Campbell 135 is highly resistant to twospotted spider mites.

Spider mites are distributed over a field in two ways: (1) migration of females from a heavily infested area to a lightly infested one, and (2) natural or mechanical transportation of mites (by wind, mammals, and man) from an infested area to an uninfested one. Consequently, known "hot spots" should be investigated last, not first when entering the field.

Several miticides provide effective chemical control. A second application is often advised 5 to 7 days after the first. For recommended miticides and rates, consult the current North Carolina Agricultural Chemicals Manual.

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