Spider mites are tiny arachnids, related to spiders and ticks, and are common, destructive pests in gardens and greenhouses. They feed on plant cells, leading to characteristic stippling and webbing on leaves. Elemental sulfur is a viable control option and remains an effective treatment against spider mites.
Sulfur as an Acaricide
Elemental sulfur is classified as an acaricide, meaning it targets mites, and controls spider mites through a dual-action mechanism. Primarily, it functions as a contact poison, disrupting the mite’s physical processes when the residue is touched. The residue deposited on the plant surface is toxic to the mites’ cuticle and internal systems. The secondary mechanism is a fumigant effect, which increases significantly as temperatures rise. Sulfur slowly vaporizes into sulfur dioxide gas, which the mites inhale. This gas interferes with the cellular respiration of the mite, disrupting its energy production. This fumigant action allows sulfur to reach mites in protected areas, such as under webbing or deep within foliage.
Proper Application and Environmental Conditions
Sulfur is available as a fine dust or as a wettable powder mixed with water for spraying. Successful application requires complete coverage, as sulfur must physically contact the mite or the surface it walks on.
Mites often congregate on the undersides of leaves, making thorough coverage necessary. Follow the manufacturer’s concentration guidelines when mixing; an overly diluted spray is ineffective, while an over-concentrated mix can damage the plant. Applications are repeated at seven to ten-day intervals to eliminate newly hatched mites.
The most restrictive factor for sulfur application is temperature, due to the risk of phytotoxicity. Growers must avoid using sulfur when temperatures are expected to exceed 85°F (about 30°C) within 24 hours. High temperatures increase sulfur vaporization, intensifying the fumigant effect and causing severe leaf scorch.
Sulfur should never be applied to plants treated with horticultural oils (neem or mineral oil) within the last two to four weeks. The combination of residues can cause significant phytotoxicity.
Specific Limitations and Non-Target Effects
While sulfur effectively kills active spider mite stages (nymphs and adults), it generally does not have a strong ovicidal effect on mite eggs. This lack of complete egg mortality necessitates repeated applications to control subsequent generations as they hatch.
Another limitation is the potential for spider mite populations to develop resistance over time. However, sulfur’s multi-site mode of action makes this less common than with some synthetic pesticides.
A major concern is sulfur’s broad-spectrum toxicity against beneficial organisms, particularly predatory mites (Phytoseiidae species) that naturally control spider mite populations. Sulfur can decimate these natural enemies, leading to a rapid resurgence of pest mites later in the season.
Certain plant species, including cucurbits (melons and squash) and some grape varieties, are highly sensitive to sulfur. These plants can suffer severe phytotoxicity even at recommended rates and temperatures.
Non-Sulfur Control Options
Where high temperatures or plant sensitivity prohibit sulfur use, several alternative options exist. Highly refined horticultural oils, including mineral oil-based products and neem oil, work by physically smothering mites and their eggs on contact. These oils have no residual activity once dry, making them safer for beneficial insects that arrive after the application.
Insecticidal soaps, which are salts of fatty acids, are another contact option that disrupts the mite’s cell membranes. Like oils, soaps require direct application to the pest and are non-residual.
A more sustainable and selective approach is biological control, involving the release of natural predators like the predatory mite Phytoseiulus persimilis. This specialist predator actively seeks out and consumes spider mites, offering long-term control without the risk of plant damage or broad-spectrum toxicity.