Diagnosis

This is a close-up of Red Spider Mites infesting a marigold plant. The cloudiness is caused by their webbing. The silvering of the marigold leaves is caused by the spider mites feeding on the leaf cells.

Spider mites are almost microscopic. Most are about the size of the period at the end of this sentence. Since the mite is too small to be seen without some sort of magnifying lens one way to check for their presence is to hold a sheet of white paper under a cluster of possibly infested leaves and strike the branch or stem. Closely examine the material that falls to the paper. Dust spots that move are probably spider mites.

Spider mites are not true insects as they have four pairs of legs and no antennae. They use a pair of needle-like stylets to rupture leaf cells when they feed. Cell sap is then sucked from the torn cells. This type of feeding causes a stippling on the foliage.

Severe spider mite damage. Where large populations exist, the injured areas coalesce and entire leaves become yellow, bronzed or brown. The leaves eventually bleach-out and dry, especially at the tips, and may drop. Continued feeding by the mites at this rate may eventually kill the plant.

Control

Spider mite infestations are most likely to begin in very hot and dry weather (they seem to dislike moist conditions so you can discourage them with a daily (morning) misting of your plants with the garden hose). But once you see the signs of spider mite infection environmentally safe control may be accomplished by hosing down the plants every couple of days with a forceful water spray. Insecticidal soap works well too. Broad spectrum insecticides should NOT be used as spider mites are rapidly becoming resistant to most chemical pesticides, which, by the way, also kill off their natural predators. As a result, the spider mite population often increases, rather than declines, following an application of insecticide.

Source: American Horticultural Society Pests and Diseases