Soap spray can be used to control a range of crop pests. Soft soaps made from potassium compounds are preferable because they can add useful minerals to the soil. Avoid using strong powder detergent soaps because they contain caustic soda which can scorch plant leaves and, if used frequently, will reduce soil fertility. Liquid soap and soft soap solutions can also burn leaves if their concentration is too strong or if applied too frequently.
Among pests controlled by soap solutions are: aphids, crickets, armyworm, caterpillars, leafminers, mites, small beetles, slugs and snails, thrips and whiteflies (Elwell and Maas, 1995).
In general, soaps will kill aphids and small caterpillars at concentrations of 5-8 g per litre of water. Concentrations of 8 g per litre will kill beetles and larger caterpillars, while concentrations of 10 g per litre can kill plants and are sometimes used as herbicides. Soap solutions are effective only when wet, once dry, they lose their insecticidal properties (Rappaport, 1992).
Procedure & Preparation:
- Use utensils for the extract preparation that are not used for your food preparation and for drinking and cooking water containers. Clean properly all the utensils every time after using them.
- Do not have a direct contact with the crude extract while in the process of the preparation and during the application.
- Make sure that you place the extract out of reach of children and house pets when leaving it overnight.
- Harvest all the mature and ripe fruits before extract application.
- Always test the extract formulation on a few infected/ infested plants first before going into large scale spraying. when adding soap as an emulsifier, use a potash-based one.
- Wear protective clothing while applying the extract.
- Wash your hands after handling the extract.
How to Prepare Soap Spray
Use mild soap or potash-based soap.
1. Mix 1 tablespoon of dish washing soap or 3 tablespoons soap flakes (non-detergent) with 4 litres of water
2. Start with a lower concentration and make adjustments of the strength after testing on few infested plants.
3. Precaution: Soap spray may injure foliage. Always try on few infested leaves before going into full scale spraying. Soaps can burn leaves on sensitive plants, like kale crops and certain ornamentals. Several applications in short periods can aggravate drying of leaves. It may take 2 days for damage symptoms to appear.
Apply on the infested plants thoroughly, including the undersides of the leaves. Spray early in the morning or late afternoon.