Rust mites are a damaging group of plant pests often overlooked because they are too tiny to see without magnification. These arachnids are not insects, but members of the Eriophyidae family, sometimes called eriophyid mites. Their microscopic size allows them to build up massive populations before detection, leading to significant cosmetic and sometimes yield-reducing damage in gardens and commercial orchards. An infestation can quickly cause widespread plant stress, making it important to identify the problem by its unique symptoms and implement a multi-faceted control strategy.
Recognizing Rust Mite Damage
Rust mites are typically less than 0.2 millimeters long, so identification relies on the specific damage they cause to plant tissue as they feed. They use piercing-sucking mouthparts to rupture the outer epidermal cells of leaves, stems, and fruit, causing the cells to collapse and die. This destruction of the plant’s surface cells results in a distinctive discoloration that signals their presence.
On leaves, feeding leads to a bronzed, russeted, or scorched appearance, often confused with nutrient deficiencies or fungal disease. Affected foliage may also develop a stippled or dusty texture, sometimes accompanied by leaf curl or distortion, particularly on new growth. For example, on citrus, the damage is known as “bronzing” on mature oranges, while on lemons, the rind can take on a silvery hue.
Damage to fruit is often the most noticeable and economically significant symptom, especially in crops like apples, pears, and citrus. Early-season feeding on developing fruit causes a condition called “russeting,” where the skin becomes rough, scarred, and brown. In severe cases, the fruit skin may develop a leathery texture or a rough, cracked surface known as “sharkskin,” which reduces the commercial value of the crop.
Non-Chemical and Cultural Control
Managing rust mite populations without synthetic chemicals requires a combination of cultural practices and natural treatments. Pruning and disposing of heavily infested plant material, particularly shoots and leaves with visible bronzing, immediately reduces the mite population. Bagging and removing this debris from the growing area prevents mites from migrating back to healthy plants.
Horticultural oils, such as highly refined mineral oil or neem oil, are effective non-chemical controls because they work by physically smothering the mites and their eggs. These oils must be applied thoroughly to contact the mites directly, requiring spraying both the tops and undersides of leaves and stems. For summer applications on active foliage, a lighter concentration, typically around 1% to 2%, is recommended to avoid phytotoxicity.
Insecticidal soaps (potassium salts of fatty acids) also kill mites on contact by disrupting their cell membranes. Like oils, soaps leave no toxic residue once dry, making them safe for beneficial insects and pollinators after application. Application should occur during cooler times of the day (early morning or evening) when temperatures are below 85°F to prevent leaf burn. Biological control involves releasing commercially available predatory mites that specifically target pest mites, such as Galendromus occidentalis or Neoseiulus cucumeris.
Effective Chemical Treatment Options
When cultural and non-chemical methods fail to control a severe infestation, targeted chemical treatments known as acaricides or miticides are necessary. Products must be specifically labeled for mites, as general insecticides are often ineffective or counterproductive, sometimes killing beneficial predatory mites. Acaricides have a specific mode of action against mites.
The primary strategy for chemical control is the rotation of different chemical classes to prevent the mites from developing resistance, a phenomenon mites are prone to due to their short life cycle and rapid reproduction. Effective active ingredients include abamectin, fenpyroximate, and spirodiclofen, which belongs to a different chemical group.
Sulfur, available as a dust or wettable powder, is an economical treatment approved for organic use and effective against rust mites, especially in vineyards and orchards. However, sulfur should never be applied within several weeks of a horticultural oil spray, as the combination can cause severe burn to the plant foliage. Always adhere to the product label for application rates and the pre-harvest interval.
Preventing Future Infestations
Long-term management relies on proactive monitoring and maintaining optimal plant health to increase resilience. Because the mites are invisible to the naked eye, regular inspection with a 10x or 20x hand lens is necessary to catch the start of an infestation before visual damage appears. Focus on checking the newest growth and the undersides of leaves, especially during warm, dry periods when mite populations thrive.
Maintaining good plant vigor through proper watering and balanced fertilization helps plants tolerate or outgrow damage from low mite populations. Stressed plants are more susceptible to pest attack, so reducing environmental stressors is a foundational preventative measure. For perennial crops, selecting mite-resistant or tolerant varieties for future plantings can reduce the annual risk of infestation.
Sanitation is also a preventative measure, as rust mites can move between plants on clothing, tools, or wind currents. Regularly cleaning and removing plant debris from the base of plants eliminates potential overwintering sites. Establishing a favorable environment for natural predators, such as predatory mite species, helps suppress pest populations.