But it's hard to miss the damage they cause. Spider mites suck the juices from plant leaves and flowers, causing them to look yellow, stippled or bleached out. New growth can be deformed and eventually leaves dry up and drop off. Spider mite populations can build very quickly and, before you know it, you have a serious problem and some sad-looking plants. To confirm you have spider mites, tap some of the infested leaves over a white piece of paper. You should be able to see the little monsters running for cover. Fine webbing on the underside of leaves is also a sign of spider mites.
Spider mites attack many plants, but summer's flowers, like roses, impatiens, marigolds and others, seem to be particular favorites, as are azaleas. They love, hot dry weather and drought-stressed, dusty plants. So good care and an occasional spray with a hose to clean foliage can help prevent them.
Before spider mites get a foothold, you'll need to take a more determined action. To prevent and control spider mites, apply Bayer Advanced Garden(tm) 2-in-1 Systemic Rose & Flower Care. One application not only feeds the plant but protects against insects (and spider mites) for up to six weeks.
The above info from Gardening Gazette