Spider Mites

By Pat Dickey, Fairfax Master Gardener
Spider mites are one of the most prolific feeders in our gardens, puncturing leaves and sucking sap from all types of plants – vegetables, ornamental trees and shrubs, annuals and perennials, fruit trees and brambles, and even houseplants. Even though we think of them as bugs or insects, they are neither of these. Spider mites are arachnids and relatives of spiders, daddy long legs and ticks. They are from the family Tetranychidae with 1,200 species.

two-spotted mites

Two-spotted spider mites

No larger than the period on a printed page, a spider mite can best be seen with a hand magnifier. Its life cycle begins when eggs are laid singly on the underside of leaves and occasionally on the top. The eggs develop into larvae, and those of the first instar are tiny and pale in color with spherical bodies and three pairs of legs. The older nymphs in the next two instars are red or green with four pairs of legs. Spider mite adults are eight-legged and oval with coloring of yellow, green, red, or brown. The body of an adult spider mite is one continuous oval, unlike the head, thorax and abdomen of an insect. Spider mites also lack wings, antennae and compound eyes. There are multiple generations each year, and their life span is 5 to 21 days.

The tell-tale sign that spider mites are feeding on your plants is the fine, silk webbing that they spin on the leaves and branches. Other signs of the presence of spider mites are the stippling of leaves from the punctures they make while feeding. These tiny white markings are from the pair of needle-like structures called stylets that the spider mite uses to pierce cells on the underside of leaves and draw out sap. As small areas of cells are killed, the upper surfaces of leaves begin to have a “sandblasted” appearance and become bronzed, bleached, yellow or gray.

Spider mites will populate in large numbers and can cause severe damage to plants, even death, if not treated. They feed on conifers and broad-leaved evergreens primarily in the cool weather of spring and fall. For control, a dormant oil application is recommended. In the summer, use insecticidal soap. Certain lady beetles, thrips and predacious mites offer some natural control for spider mites. It is not recommended that insecticides