Spider mites are arachnids, eight-legged creatures related to spiders. They are extremely small and are difficult to see without a hand lens but can be detected by the damage they cause (white or yellow specking on the upper surface of the leaves and, eventually, distorted leaves).

Healthy plants, kept reasonably moist, are less sensitive to mite infection. If mites are detected, then a strong spray of water will remove most of them. Be sure to spray the underside of the leaves and repeat as necessary.

Insecticidal soaps and horticultural oils are effective and are only minimally harmful to beneficial insects outdoors. Again, spray both side of leaves. Broad-spectrum insecticides should be avoided. 

Courtesy of NYBG Plant Information 

Also, see our updated Gardenia Research Guides available on this linked page.