Spider mites are common pests that target a wide range of plants, from houseplants to agricultural crops. Despite their name, they are arachnids, related to spiders and ticks, not insects. They feed exclusively on plant matter, causing significant harm that often goes unnoticed until the damage is extensive. This article describes how spider mites damage plants and details the observable symptoms resulting from their feeding activity.
Identifying the Pest and Infestation
Spider mites are diminutive, measuring less than 0.5 millimeters, making them difficult to spot without close inspection. Their color varies widely depending on the species and host plant, appearing red, green, yellow, or nearly transparent. They congregate and feed primarily on the undersides of leaves. A simple way to check for their presence is the paper shake test: tap a suspect leaf sharply over white paper. If mites are present, tiny specks will fall and move slowly. A heavy infestation is confirmed by distinctive, fine silk webbing spun across the undersides of leaves and stems, which protects the colony and aids movement.
The Mechanism of Damage
Spider mites possess specialized, needle-like mouthparts, called chelicerae, which they use for piercing and sucking plant tissue. The mite inserts a stylet into the leaf, navigating between epidermal cells or through a stoma, to access the underlying mesophyll cells rich in chlorophyll. The mite injects saliva to pre-digest the cell contents before extracting the liquid. This feeding action destroys individual plant cells by removing chlorophyll and internal fluids. Since chlorophyll captures light energy, the destruction of these cells directly impairs the plant’s capacity for photosynthesis, limiting its ability to produce energy and sustain growth.
Visual Symptoms on Plants
The initial and most characteristic sign of spider mite feeding is stippling, which appears as numerous tiny, pale, or yellowish dots scattered across the leaf surface. These spots correspond to the exact locations where a mite pierced a cell and removed the chlorophyll. As the mite population grows, stippling marks merge, causing the leaves to take on a dull, bronzed, or grayish-brown appearance, often called bronzing. This color change results from extensive chlorophyll loss and indicates a high-density infestation. In severe cases, sustained feeding leads to widespread chlorosis (complete yellowing of the foliage), leaf curling, distortion, and premature leaf drop. If left unchecked, this destruction of photosynthetic tissue can stunt growth and ultimately lead to the plant’s death.
Factors Influencing Damage Severity
The rate and extent of damage are significantly influenced by environmental and host-plant factors. Spider mites thrive and reproduce most rapidly under high heat and low humidity; temperatures exceeding 85°F accelerate their life cycle, allowing a new generation to emerge in as little as five to seven days. Low humidity favors population growth partly because it inhibits natural fungal pathogens. Plants already experiencing environmental stress, particularly drought, are more susceptible to severe injury, as the combined stress compounds the damage. Spider mites feed on over 1,100 plant species, but symptom severity varies greatly depending on the host. Some plants exhibit higher tolerance, while others, such as roses, tomatoes, and certain ornamentals, are highly susceptible.