Soaps and oils seem to disrupt cell membranes and must have direct contact with soft-bodied insects to be effective. Soaps are not very effective against insects with heavy cuticles or those that are highly mobile. Most adult beneficial insects are not harmed by soap sprays, but the immature stages can be since they often have soft bodies and have limited mobility.
Soaps can also aid in the passage of insecticides through the waxy insect cuticle.
Some plants can be damaged by soap sprays because they break down the waxy protective outer layer on the plant surface. Dishwashing detergents and laundry products may damage plants. Commercial products deliver a more consistant result. Some researchers believe that bug sprays may work if a disease is present in the insect, which is spread through the spray to other insects.