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Spider Mite Damage

- Severe leaf injury from tiny holes -

Summary: Even a few spider mites can cause considerable damage to the leaves on which they feed. Research has shown that spider mites damage the water proof surface layers of leaves and this leads to most of the injury and productivity loss.

Jack DeAngelis, PhD

OSU Ext. Entomologist (ret.)

Why so much damage?

Some researchers estimate that spider mites (see What are Spider Mites?) account for as much as a 5% loss in total agricultural productivity worldwide. Gardeners also know that these pests can cause severe injury in a very short time. Until fairly recently, however, it was not known exactly how spider mites cause so much damage.

Spider mites feed by piercing the leaf surface with stylet-like chelicerae and extracting leaf cells and fluid. This manner of feeding causes tiny holes to be punched into the otherwise protective, waterproof leaf surface.

Leaves that have been fed on by spider mites are usually dry and brittle and the usual green color is lost (right). Injured leaves are shed more quickly and eventually the whole plant may die. Even a minor spider mite infestation can have a significant impact on a plant's health.

spider mite injured - paired leaf experiment

Paired-leaf experiment on peppermint. Twenty adult spider mites were confined to the leaf on right. The leaf on the left remained mite-free. After fifteen days the leaves were compared, injured vs. uninjured. Injured leaves were dehydrated, and continued to lose significantly more water than did uninjured leaves. Injured leaves were also unable to carry out normal physiological processes. Injured leaves were shed at a greater rate, too.

Leaves normally control water loss through a system of stomates, or valves, that can be opened and closed. When the stomates are closed the surface of a leaf is highly resistant to water loss. Spider mite feeding disrupts this system by creating holes that allow water to escape. This uncontrolled water loss eventually dehydrates the leaf (see photo above).

Ironically, the water stress caused by spider mite feeding actually makes the leaves a better food source because stressed leaves have higher levels of sugars and soluble nitrogen - both of which are needed by spider mites.

The articles listed below are from research done during the early 1980's on the effects of spider mite feeding on host plants.

Technical research articles

DeAngelis, J. D., R. E. Berry, and G. W. Krantz. 1983. Evidence for spider mite (Acari: Tetranychidae) injury-induced water deficits and osmotic adjustment in peppermint. Environ. Entomol. 12:336-339.

DeAngelis, J. D., R. E. Berry, and G. W. Krantz. 1983. Photosynthesis, leaf conductance, and leaf chlorophyll content in spider mite (Acari: Tetranychidae) - injured peppermint leaves. Environ. Entomol. 12: 345-348.

DeAngelis, J. D., A. B. Marin, R. E. Berry, and G. W. Krantz. 1983. Effects of spider mite (Acari: Tetranychidae) injury on essential oil metabolism in peppermint. Environ. Entomol. 12: 522-527.

DeAngelis, J. D., K. C. Larson, R. E. Berry, and G. W. Krantz. 1982. Effects of spider mite injury on transpiration and leaf water status in peppermint. Environ. Entomol. 11: 975-978.

Spider mite and egg (electron micrograph magnified 130x). The three "buttons" in background are peppermint leaf glands, the large sphere is a spider mite egg, the female spider mite is at the center of this micrograph.

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Jack DeAngelis, Ph.D.

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