Spider mites, particularly the two-spotted spider mite (Tetranychus urticae), are common agricultural and garden pests that damage plants by piercing cells and extracting their contents, leading to a characteristic stippled and discolored appearance on the leaves. These mites thrive in hot, dry conditions and can quickly spin protective webs, making them challenging to manage with traditional chemical sprays. Biological control offers a non-chemical, sustainable solution that utilizes natural enemies to suppress mite populations, providing long-term, dynamic control within the ecosystem.
Identifying the Key Predators
The most effective agents for controlling spider mites belong to the family of predatory mites, known as Phytoseiidae. The specialist Phytoseiulus persimilis is the most widely used predator for existing, high-density infestations. These mites are aggressive hunters that feed almost exclusively on various species of spider mites. Another common option is Neoseiulus californicus, a selective predator that can also survive on alternative food sources like pollen or other small pests when spider mite populations are low.
Beyond the specialized mites, several generalist insects also contribute to spider mite management. The Minute Pirate Bug (Orius spp.) is a versatile predator that feeds on mite eggs and nymphs, along with a wide range of soft-bodied pests. Green Lacewing larvae, often called “aphid lions,” are voracious generalists that consume spider mites in addition to other common garden nuisances. Furthermore, the specialized Spider Mite Destroyer beetle (Stethorus punctillium) is a tiny, black lady beetle that feeds specifically on spider mites, though it typically appears only after pest populations have reached high levels.
How Predatory Mites Control Infestations
Predatory mites, especially P. persimilis, are highly effective because their life cycle is significantly faster than that of their spider mite prey. A spider mite’s development from egg to adult can take as little as seven days in optimal hot conditions, but the predatory mite can complete its own life cycle in just six to twelve days. This allows the beneficial population to quickly outpace and suppress the pest population.
The specialist predatory mites actively search for all stages of the spider mite life cycle, including eggs, nymphs, and adults. An adult P. persimilis can consume between five and twenty pest mites or eggs daily, actively entering the protective silk webbing that spider mites create. This specialized “search and destroy” behavior helps prevent the formation of new, localized pest colonies.
Environmental conditions play a large part in the predatory mite’s success. Species like P. persimilis require a relative humidity above 60% for their eggs to hatch successfully. In contrast, spider mites thrive in low-humidity environments, which is why species like N. californicus or Galendromus occidentalis are better suited for drier or warmer climates. Understanding the environmental needs of the chosen predator is important for establishing a self-sustaining population that can maintain long-term pest suppression.
Successful Application of Biological Control
Handling and Release
Successful biological control begins with the correct sourcing and handling of the beneficial organisms. Predatory mites are typically shipped in small containers mixed with a carrier material like vermiculite or corn grit, often with a cold pack to keep them dormant during transit. Upon arrival, allow the containers to reach room temperature before the contents are gently sprinkled directly onto the infested plant foliage, ideally in the early evening to protect the mites from direct sunlight.
Timing and Ratios
The timing of the release is critical for maximum effect, with preventative or early-intervention releases being the most successful strategy. If the infestation is already severe, a preliminary application of a non-residual, soft chemical like insecticidal soap may be necessary to reduce the pest population to a manageable level. A general guideline suggests a predator-to-prey ratio of approximately 1:10 is needed to gain control, with releases focused on “hot spots” where spider mites are most concentrated.
Environmental Support
Environmental manipulation is necessary to support the beneficial mite population. Since many predatory mites require higher humidity, lightly misting the plants or wetting down greenhouse floors can increase the moisture levels needed for their egg survival. Avoid overhead watering immediately after release, as this can wash the beneficial mites off the foliage.
Chemical Compatibility
The most important management factor is the careful selection of any pest control products used alongside the predators. Broad-spectrum insecticides and miticides can be highly toxic and persistent, often requiring a waiting period of two weeks or more before releasing beneficial insects. Even common organic compounds like insecticidal soap or horticultural oil are lethal to predatory mites upon direct contact, but their low residual activity means they pose little risk once they have dried. Neem oil, which contains the active ingredient azadirachtin, can be moderately toxic to predators, so it is safer to wait approximately ten days after a spray before introducing new mites to the area.