Spider mites can cause extensive leaf damage
Spider mites are tiny (1/32") plant-feeding mites (see drawing left) that are important pests in both home gardens and commercial agriculture. Even a minor spider mite infestation can do significant damage to plant productivity. Injury is the result of damage to the plant's protective outer layers, the epidermis and cuticle, that occurs as the mites feed. Research has shown that injury to these layers causes excessive and uncontrolled water loss and eventually injured plants dry out and die (see photo of injured leaf below).
Take a look at the photo of the two leaves. The photo was taken during an experiment to determine the effects of spider mite feeding on peppermint leaves. The leaf on the right was infested with spider mites, the leaf on the left was kept spider mite-free with a barrier of sticky material (not visible in this photo). After about 15 days the mites were removed and a variety of measurements were made to determine the health of "injured" vs. "uninjured" leaves.
The leaf that was fed on was dry and brittle compared to the uninjured leaf. When water loss was measured injured leaves (even dry ones) had far higher rates of water loss and this loss was uncontrollable. Healthy leaves are able to conserve water, especially at night. There were many other physiological differences that all related the leaf's ability to control water. Spider mite feeding disrupted the ability to control water loss by disrupting the leaf's protective layers.
Use the link below for additional information about the original experiments and references to the scientific literature.