Insecticidal Soap is defined as any of the potassium fatty acid soaps used to control many plant pests and diseases. This product is typically sprayed on plants in the same manner as other insecticides. It works only on direct contact with the pests. The fatty acids disrupt the structure and permeability of the insect cell membranes and the insect quickly dies. Soaps of this kind also work well on fungus infections such as botritis and leaf spot as well as on common houseplant pests such as spider miteLearn how to identify and treat Spider Mites on plants. This sucking plant pest causes yellow blotchy leaves with a red haze and a gritty feel to them. and mealy bugLearn how to identify and treat Mealy Bugs, a houseplant pest that leaves sticky, white, cottony residue on houseplants..