**Eriophyid Mites** (family Eriophyidae) are a unique and highly specialized group of microscopic plant-feeding mites, often called **gall mites** or **rust mites**. They are some of the smallest arthropods, possessing only two pairs of legs (most mites have four). Their feeding induces bizarre and abnormal growth on their host plants, ranging from velvety layers (**erineum**) to small, felt-like patches (**rusting**), or large, woody swellings (**galls**). They are economically significant pests of numerous trees, fruits, and ornamental plants.
Taxonomy and Classification
Eriophyid Mites belong to the class Arachnida, subclass Acari (mites and ticks). They are distinct from other plant-feeding mites (like spider mites) due to their vermiform (worm-like) body shape and the presence of only two pairs of legs, both situated at the anterior end of the body. They undergo simple metamorphosis. Each species is generally highly host-specific.
Physical Description
Eriophyid Mites are too small to be seen without magnification (often less than $0.2$ mm long). They are translucent or pale white/yellow. Their body is elongated and cylindrical, resembling a tiny worm. Due to their size, their presence is confirmed entirely by the unusual damage symptoms they cause on the host plant.
The damage takes three common forms:
- **Galls (Eriophyid Galls):** Tiny, hard, pimple-like growths on the upper leaf surface (e.g., maple bladder gall). The mites live inside these galls.
- **Erineum Patches:** Velvet-like or felt-like patches of fine hairs on the leaf surface, often brightly colored red or yellow (e.g., walnut leaf curl).
- **Rusting/Bronze:** Discoloration and bronzing of the foliage, especially prominent on fruits like apples or pears (rust mites).
Distribution and Habitat
Eriophyid Mites are found globally. Their habitat is restricted to the specific tissues of their host plants. They overwinter as adults (called deutogynes) in protected places such as bud scales, bark crevices, or in the developing tissues of the terminal buds. They emerge in the spring to feed on new, tender growth.
Behavior and Life Cycle
Eriophyid Mites have a rapid life cycle, sometimes completing a generation in less than two weeks, leading to dozens of generations in a season. They travel short distances via air currents or by clinging to insects or birds (**phoresy**). Their feeding involves injecting saliva into plant cells, which triggers the host plant to produce the abnormal growth (gall, felt) that serves as the mite’s shelter and food source.
The appearance of galls and discoloration is the plant’s reaction to the feeding, not the mites themselves. Since the galls form around the mites, chemical treatments are ineffective once the galls have hardened.
Feeding and Damage
The damage is highly variable:
- **Aesthetic:** Galls on ornamental trees like maple or ash are visually unsightly but typically do not harm the overall health of a large tree.
- **Economic:** Rust mites on fruit trees (apples, pears, citrus) cause discoloration and scaring of the fruit, reducing market value. Certain eriophyids also vector plant viruses, such as **wheat streak mosaic virus**.
- **Physiological:** Severe infestations can lead to leaf drop, stunted growth, and death of terminal buds in vulnerable plants.
Management and Prevention
Control focuses on early season intervention, as the mites are protected later in the year.
- **Dormant Oil:** The most effective control is applying a **dormant oil spray** in late winter or early spring before bud break, which kills the overwintering adults (deutogynes) before they can begin feeding.
- **Acaricides:** Specific miticides or sulfur sprays can be applied during bud break and early leaf expansion to target the mites before gall formation begins.
- **Pruning:** Removing and destroying affected galls or leaves can reduce local populations, especially on small plants or shrubs.
Conservation and Research
Research on Eriophyid Mites focuses on the molecular mechanisms by which they induce gall formation and the development of resistant host plant varieties, particularly for economically critical crops like wheat and fruit trees.