Soapy water (a diluted dish soap solution containing surfactants) has some contact-killing effect on certain soft-bodied pests but is not a broad-spectrum insecticide.
The principle of action of soapy water is to disrupt the waxy protective cuticle of the insect, leading to dehydration, and to block the insect's spiracles (breathing pores), causing suffocation.
It is relatively effective against soft-bodied garden pests such as aphids, scale insects, and spider mites — this is a common method in organic gardening.
However, for household pests with hard exoskeletons, such as cockroaches, ants, and bed bugs, regular soapy water has very limited effectiveness — it cannot effectively penetrate their waxy layer.
Soapy water cannot replace professional insecticides or a comprehensive IPM strategy.
When using: too high a concentration can damage plant leaves (for gardening purposes), and the residue is no longer effective once it dries.