Spider mites (Family Tetranychidae) are common and destructive pests in agricultural settings and home gardens worldwide. They are arachnids, placing them in the same class as spiders, ticks, and scorpions. These minuscule pests, often less than 1/50th of an inch long, cause significant damage by feeding on plant tissue.
How Spider Mites Extract Plant Nutrients
Spider mites employ a specialized feeding method that relies on piercing and sucking. They possess needle-like mouthparts, known as stylets, which penetrate the leaf surface, typically entering between epidermal cells or through stomatal openings.
Once inside the leaf, the mite navigates its stylets to the mesophyll layer, which contains the spongy mesophyll and palisade parenchyma cells. These cells are packed with chloroplasts, containing chlorophyll. The mites puncture these cells and suck out the cellular contents, including chlorophyll and internal fluids. This feeding style is described as cell-content feeding. Mites may inject saliva into damaged cells to begin pre-oral digestion before consuming the liquefied contents.
A Broad Diet: Preferred Host Plants
Spider mites are generalist feeders; the common two-spotted spider mite (Tetranychus urticae) can infest over a thousand different plant species. This broad diet includes nearly every category of cultivated plant, making them a universal problem in horticulture and agriculture.
In vegetable production, spider mites commonly target crops like tomatoes, beans, corn, and members of the cucurbit family (squash, melons, and cucumbers). Heavy infestations can severely reduce fruit yield and plant vigor. Mites also prefer many horticultural and ornamental plants, frequently damaging roses, marigolds, impatiens, and annual flowers.
They also infest a variety of woody plants, including deciduous trees and conifers. Fruit trees such as cherry, plum, and pear are frequent targets, as are landscape shrubs like azaleas, holly, and rhododendron. Conifer species (spruce, fir, pine, and juniper) are often attacked by specific species, such as the spruce spider mite. Mites proliferate more rapidly on plants experiencing stress, such as drought or heat, or those with lush, high-nitrogen growth.
Identifying the Visual Signs of Feeding Damage
Cell-content feeding leaves behind recognizable visual damage on the host plant. The first and most common symptom is stippling, which appears as tiny, pale yellow or white speckles on the upper surface of the leaves. This speckled pattern results from the destroyed chlorophyll cells in the mesophyll layer that the mites have emptied.
As the mite population increases, individual stippling marks merge. This extensive damage causes the entire leaf to take on a yellowed or bronzed appearance. In severe cases, this discoloration can look like a rusty or brownish cast, particularly on woody plants. This heavy feeding leads to premature leaf drop.
Another sign of a severe infestation is fine, silky webbing. Spider mites produce this silk from specialized glands near their mouthparts to construct protective shelters and create bridges for movement. The webbing is finer than a typical spider’s web and is often noticeable between leaves, along stems, or over growing tips. The silk aids in dispersal and protects the mites from predators and topical treatments.