Mites are tiny arachnids, related to spiders and ticks, that exist in nearly every environment worldwide. The common term “grey mites” is non-scientific, often referring to species that appear pale white or grayish-white due to their translucent bodies and small size, typically less than one millimeter long. These microscopic pests are ubiquitous, and their presence usually indicates an underlying environmental issue. Identifying the specific species is necessary to determine the correct control strategy.
Identifying the Common Culprits
The most frequently encountered pests described as “grey mites” fall into two main groups: stored product mites and certain pale spider mites. The flour or grain mite, Acarus siro, is a primary example of a stored product mite, appearing pale grayish-white and soft-bodied with pinkish legs. In heavy infestations, these minute organisms can appear as a moving layer of dust over the infested material, though individual mites are difficult to see without magnification.
Pale varieties of plant-feeding spider mites, such as the two-spotted spider mite (Tetranychus urticae), can also be perceived as greyish, though their color is often translucent, pale yellow, or green. Unlike stored product mites, spider mites are plant pests usually found on the undersides of foliage. A distinguishing characteristic is the fine, silky webbing they produce, which is often the first visible sign of their presence.
Where Grey Mites Thrive
Stored product mites flourish in environments with elevated moisture levels and moderate temperatures. The flour mite, Acarus siro, prefers relative humidity between 65% and 80% and temperatures between 68°F and 77°F (20°C to 25°C). They feed on organic materials like cereal products, flour, grains, dried fruits, cheese, and spices, often consuming the germ of the grain. In households, these conditions are frequently met in kitchen pantries or food storage areas that lack adequate ventilation.
Conversely, plant-feeding spider mites thrive in hot, dry conditions and are problematic during the warmer months. These mites inhabit the foliage of host plants, using piercing mouthparts to extract cell contents and causing localized damage. Infestations often begin in the lower, older growth of a plant and spread upward as the population multiplies rapidly.
Impact and Health Concerns
The presence of stored product mites leads to significant economic loss by contaminating food, making it unusable for consumption. Heavily infested materials often develop an unpalatable, sickly-sweet or minty odor and have reduced nutritional quality. The physical presence of the mites, their waste products, and shed skins also create allergens that pose a health risk to humans and pets.
Inhalation or contact with these allergens can trigger asthma, allergic rhinitis, and various forms of contact dermatitis, such as “Baker’s itch.” Plant-feeding mites cause damage by feeding on leaf cells, which results in characteristic symptoms like stippling, where tiny yellow or white spots appear on the leaves. Severe infestations can cause the foliage to take on a bronzed appearance, leading to premature leaf drop and stunting the plant’s growth.
Effective Management Strategies
Management for stored product mites begins with a rigorous focus on sanitation and environmental control. All infested food products must be immediately discarded, and the affected storage area should be thoroughly cleaned and vacuumed to remove any remaining debris and mites. Reducing the relative humidity in the storage environment to below 65% is an effective preventative measure, as these mites are sensitive to moisture.
Temperature manipulation can also eliminate pests in non-perishable goods; a deep-freeze treatment of 0°F (-18°C) for several days achieves total mite mortality. For plant-infesting spider mites, non-chemical control involves frequent, forceful spraying of the foliage with water, focusing on the undersides of the leaves to physically dislodge the pests. This method is most effective in hot, dry weather and conserves natural mite predators.
If non-chemical methods are insufficient, treatments such as insecticidal soaps or horticultural oils, like neem oil, can be applied, as they work by smothering the mites. For severe infestations of stored product mites in commercial settings, specialized fumigation or the use of controlled atmospheres may be necessary. Any chemical application must fully cover the infested area, ensuring the product reaches the underside of the leaves where the mites congregate on plants.