This article is written by Dr. Wilson Wong on how to use sulfur soap to kill mites. He is also the creator of GREEN CULTURE Singapore group in Facebook.
The picture shows the sulfur soap product most commonly available in Singapore.
Sulfur, in its elemental form (as a molecule S8) is better known as a fungicide where it is used to manage fungal diseases such as powdery mildew, rust, scab, brown rot, and so on. It is often sold as wettable sulfur, which is micronized sulfur powder with a dispersant and sticking agent added. Lime sulfur is another product that has the same properties, made from reacting lime with sulfur. Both products are not readily available for sale in Singapore.
What is less well known is that sulfur can be used to control spider mites and broad mites. Both are common horticultural pests. For control of mites, the size of the sulfur particle is important. Sulfur flakes that are used for deterring snakes in the garden are too big to be of use. Due to the lack of wettable sulfur and lime sulfur in Singapore, the next best product that can be used is sulfur soap.
Sulfur soap contains micronized sulfur (often up to 10%) in a soap base (a typical ingredient list of a sulfur soap includes soap noodle (Sodium palmate, palm kernelate), sodium chloride, water, fragrance, sulfur, coconut fatty acid). As one can see, it is made of saponified vegetable fats that are used to make soap. What is beneficial is that the soap base can be used against small sucking insects like aphids, mealy bugs, scales and so on, which kill by suffocation, mites included.
To use make a spray of sulfur soap for pest control, grate the soap bar into fine flakes and dissolve a level teaspoon of these flakes in small amount of hot water. When totally dissolved, top the solution up to 800 mL or 1 L with tap water. Shake vigorously before spraying. This solution can be kept for up to a week. Ensure all the soap bits are dissolved so as to not clog your sprayer.
Sulfur is known to be of low toxicity, and poses very little if any risk to human and animal health. It is harmless to bees. Note that sulfur is toxic to all mites, including predatory mites, so caution is needed in its use to not upset the ecosystem. Sulfur is allowed in organic agriculture. The rest of the ingredients in the sulfur soap bar are quite harmless – saponified vegetable fats.
Always test a small portion of your plant before spraying so as to ensure the dose is suitable for use on your plant without causing phytotoxicity. Make a more dilute solution if required. Avoid breathing in the mist or letting it get into your eyes.
Before consuming your plants, give the harvested parts a good soak and rinse. For heavily infested plants, prune away badly infested parts. Repeated applications on alternate days may be necessary. Spray during the cooler part of the day.
Sulfur soap seems to be suitable for use on plants that had previously been sprayed with oil-based pesticides like neem oil and summer oil. This is probably due to the soap component washing away the oils from plants. Wettable sulfur on its own is not suitable for use on oil-sprayed plants without having to wait for at least several weeks.
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