Mites are tiny, eight-legged arthropods often found in horticultural environments, belonging to the same class as spiders and ticks. Two main types frequently concern growers: plant-feeding mites (pests) and predatory mites (beneficial). The differences between these groups are often confused, especially when fine silk threads are discovered on plants. Understanding the true nature of the webbing is the first step in effective pest management.
Webbing Behavior in Mites
Predatory mites, such as species used for pest control like Phytoseiulus persimilis, generally do not construct noticeable silken webs. These beneficial mites are active hunters, characterized by their speed and mobility as they search for prey across plant surfaces. They may produce minimal strands of silk, but these are typically used only for temporary anchoring or brief navigational support.
In stark contrast, pest mites, such as the two-spotted spider mite (Tetranychus urticae), produce copious amounts of highly visible webbing. This extensive silk production gives the pest species the common name “spider mites.” The presence of dense webbing is a clear indication of a significant pest mite infestation, not predatory mite activity.
Why Pest Mites Create Webs
The dense silk structures created by pest mites serve several functions that aid their survival and multiplication. The primary purpose is physical protection, forming a shield against larger predators and environmental disturbances. The webbing acts as a physical barrier that most natural enemies cannot easily penetrate.
The webs also help create a favorable microclimate by trapping humidity close to the leaf surface, which benefits the mites’ development. Furthermore, the silk provides a network of pathways, allowing the mites to move efficiently between feeding sites. When populations become extremely high, the mites use these silk strands for aerial dispersal, drifting to new host plants.
Identifying Predatory Mites and Pest Mites
Distinguishing between beneficial and harmful mites requires close observation, often with the aid of a hand lens. Predatory mites typically have a pear-shaped body and possess longer legs relative to their size. Their movement is a key indicator; they are significantly faster than their plant-feeding relatives, actively running and searching across the leaf surface.
Pest mites, like the two-spotted spider mite, tend to have a more oval body shape and move sluggishly. Pest mites are often green, yellow, or red, and the two-spotted species features a pair of distinct dark spots. Predatory mites, by comparison, are frequently translucent, pale tan, or take on a reddish-orange hue after consuming prey.
The Role of Predatory Mites in Pest Management
Predatory mites are widely used as biological control agents in integrated pest management programs, especially in greenhouses and high-value crops. Specialized species like Phytoseiulus persimilis are effective against spider mites, feeding voraciously on all life stages, including eggs, nymphs, and adults. An adult P. persimilis can consume between five and twenty prey items per day.
Other commercially available species, such as Neoseiulus cucumeris or Amblyseius swirskii, are generalist predators that target a broader range of pests. These species prey on pest mite eggs, as well as the immature stages of thrips and whiteflies. Their introduction provides a sustainable method of limiting pest populations without relying on broad-spectrum chemical treatments.