Horticultural oils and insecticidal soaps are the next weapons in our arsenal. They are often discussed together, but are very different in their actions. Horticultural oils are petroleum hydrocarbons produced from paraffinic crude oil. Horticultural oil works by smothering the insect. Leave at least a two-week interval between treatments.
Horticultural oils are toxic to fish, but safe for mammals and birds.
Insecticidal soaps are made of the potassiums salts of fatty acids. The fatty acids disrupt the structure and permeability of chitin and cell membranes. The cell contents leak, and the insect dies from desiccation. Insecticidal soaps are most effective against soft-bodied insects, unfortunately including beneficials such as syrphid fly larvae and predatory mites, with some damage to lady beetle larvae, parasitic wasps and honeybees. However, there is no residual effect in the area after the spray has dried.