A very simple method to control aphids (plant lice) or thrips is by spraying a soap solution. Soap solutions are very effective against these smaller insects, but will also kill bigger insects.
A soap solution can be made by mixing 30 gram of soap in 5 liters of water. It is recommended to test this method first with a few plants to make sure that the plants do not get damaged. Inspect the plants after two days to verify if the soap solution burned the leaves.
Soap (body soap, washing powder, liquid detergents) is relatively safe for bigger animals, birds and people. Solutions of around 0.5 – 0.8% are generally effective against insects, while concentrations above 1% can damage the plants, which means it could be used as a herbicide.
Soap solutions are only effective when wet. Once dry, they lose their insecticidal properties. This means that soap solutions have to be sprayed directly towards the pest insects. This is rather an advantage in the context of Integrated Pest Management. It means that the control operation is targeted at the pest only and will not have a long-lasting effect on other insects such as natural enemies.
Other uses of soap
Soap is also used in insect traps. For example light traps often consist of a light source placed above an open container with water. When insects fall in the water they get killed, but bigger insects could still escape. Adding a bit of soap to the water will reduce the surface tension, which helps the capture of the insects.
Gardeners troubled by scale insects can use white oil, which can be prepared by mixing 500ml of vegetable oil with half a cup of dish washing liquid. Blend the mixture thoroughly. Then dilute it at the rate of one tablespoon of the concentrate to one liter of water before spraying. Store the concentrate in a cool place.