Pests and harmful insects are undesirable for any growing operation — no matter if it’s a commercial one or a home garden. To control these pests, it’s necessary to find a safe and effective treatment. Do insecticidal soaps fall into that category? And if so, what crops are they effective on? First, we must define what insecticidal soap is.
What is Insecticidal Soap?
Insecticidal soaps are made from potassium salts of plant fatty acids and have been applied to certain trees and plants to control pests for over 100 years.  Insecticidal soaps work in direct contact with small insects that have soft bodies. Spray soaps suffocate insects such as mealybugs and spider mites by dehydrating the insects after affecting their cellular membranes. Other targets include aphids, thrips, and whiteflies. While it is commonly understood that insecticidal soap can kill these soft body insects on contact, the scientific literature is limited as to the effectiveness of insecticidal soap on insects with armored scales. 
Effective Crop Treatment with Insecticidal Soaps
Master gardeners and horticulturists at Clemson University explain that insecticidal soap is an “effective and low toxicity alternative to more toxic pesticides.”
They concur that spray soaps are mostly safe and effective for pest control; however, specific plants can be negatively affected by their application. Examples of plants that can be harmed by soap sprays include sweet pea, plum, cherries, and lilies.
One disadvantage of insecticidal soaps is that they must completely saturate the top and bottom of the plant’s leaves (and the insects) to be effective at killing pests. There is also a chance for phytotoxicity at temperatures above 90°F.
Although insecticidal soaps appear safe and effective for controlling pests on certain plants, no such studies have been done on cannabis plants.
Reference: Quesada CR, et al. Efficacy of horticultural oil and insecticidal soap against selected armored and soft scales. Horttechnology. 2017;(27)5:618–624. [journal impact factor = 0.734; times cited = 3 (SemanticScholar)]