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Mixing water and soap and spraying plants is a homemade method that can help combat some garden pests, especially aphids. This mixture works on insects that feed on sap and can reduce their presence on leaves and stems without having to turn directly to conventional insecticides.
The effectiveness of this method has also been the subject of scientific research. A study published in the scientific journal Insects analyzed the effect of different soaps against banana aphids, a pest that affects crops. Its results shed more light on the potential of these products to control insects and the conditions that should be considered when using them.
How soap works on aphids and other insects
Soaps used to control pests can work through direct contact with the insects. Their action affects the outer structures that protect them and can alter their cell membranes, causing death. For that reason, the spray must reach the specimens on the leaves and stems.
Aphids are one of the targets of this type of preparation. These small insects feed on plant sap, weaken growth, and can contribute to other problems, such as the spread of plant diseases. A mix of water and soap can help reduce their populations, although its effectiveness depends on the product used and how it is applied.
What the scientific study found about water and soap
The study, titled Evaluation of the Effectiveness of Plant Extracts and Soaps for Controlling Banana Aphid (Pentalonia nigronervosa, Hemiptera: Aphididae) Under Laboratory and Greenhouse Conditions, was published Dec. 23, 2025, in Insects.
The research team, led by Geofrey Ogwal, tested three types of soap - bath soap, laundry bar soap and liquid soap - along with different plant-based preparations made with garlic, chili pepper, neem and mint. The tests were carried out in both the laboratory and on banana plants grown under controlled conditions.
The results showed that the preparations caused mortality of between 28.6% and 100% of the aphids within 96 hours of application, compared with 3.3% in the group treated only with water.
In addition, some combinations of plant extracts and soap achieved more than 70% mortality in laboratory tests.
In tests on plants, certain mixtures cut populations to fewer than 20 aphids per plant, compared with more than 200 in the control group after eight weeks. In that context, these results support the potential of soaps to control this pest.
How to use this mixture without harming plants
Household detergents and soaps with perfumes, degreasers, or other additives can cause damage to plant tissue. To avoid problems, it is worth keeping a few precautions in mind:
- Choose a suitable product: It is better to use insecticidal soap formulated specifically for plants instead of conventional detergents.
- Check tolerance: First, test the mixture on a small part of the plant and watch for spots or damage.
- Apply directly to the insects: Spray the areas where aphids are concentrated, including stems and the undersides of leaves.
- Avoid the hottest hours: Apply the product when there is no strong sun to reduce the risk of leaf burn.
- Follow the manufacturer’s instructions: Use the recommended concentration and do not increase the amount of soap on your own.