Leaves infested by the two-spotted spider mite, Tetranychus urticae (Koch), first appear to have areas of white stippling. Later, the leaves may bronze, dry, and eventually fall off. This is the result of mites piercing plant cells to remove nutrients and chlorophyll. In heavy infestations, the undersides of the leaves also will have silken threads spun across the surface. Mites are more prevalent during hot, dry periods, and injury is exacerbated if soil moisture is low. Heavy mite populations also can predispose plants to winter injury.
The adult female is tiny (1/50 inch long), greenish, and has two black spots on her back. Females overwinter in the folds of old leaves on the ground, in cracks and crevices of the posts and canes, and so forth. In the spring, lower leaves usually are infested first, but the mites move up the cane as the season progresses. Several overlapping generations hatch during the summer, and tremendous populations can build in a short time if conditions are right.
Insecticides applied for other pests can contribute to mite problems. Chemicals that kill beneficial predators of mites often do not kill the mites themselves. Fields should be inspected weekly and treated with a horticultural oil or safer soap if a sharp population increase is noted or if leaf spotting appears. Populations isolated within a field can be spot treated.