The best soap to make insecticidal soap is pure liquid castile soap from olive oil. Olive oil has 18-carbon oleic acid chains that kill bugs on contact without hurting your plants. It mixes clean in water, sprays well, and breaks down fast.
Your choice of castile soap for insecticidal soap matters more than most growers know. Not all castile soaps use the same oil base. I tested three types over a month: olive oil, coconut oil, and palm oil. The olive oil version killed aphids in minutes with zero leaf damage on every plant. The coconut soap also killed bugs fast but left faint brown spots on my pepper leaves after the third spray. Palm oil fell in the middle.
Kansas State University research shows why olive oil soap wins this test. Fatty acid chain length decides if a soap kills bugs or kills plants. Oleic acid from olive oil has 18 carbon atoms in its chain. That's long enough to break through insect cell walls. Shorter chains with 9 carbons or fewer act like weed killers instead. Coconut oil has a mix of short and long chains. This is why it works but carries more risk of leaf burn than olive oil castile soap.
Dr. Bronner's Pure Castile Liquid Soap is the most popular pick among home gardeners. The unscented "Baby" version is your safest bet. It skips all added scents and oils. The peppermint kind is worth a try too since some growers say the scent helps push pests away. Kirk's Castile Soap is another good choice and costs a bit less per bottle.
Stay away from any soap that lists added scents, dyes, moisturizers, or germ killers on the label. These extras raise the chance of leaf burn. They can leave residue that blocks leaf pores too. Even "natural" scents from plant oils can cause issues on tender species. The fewer things on the label, the safer the soap is for your garden use.
To mix your olive oil soap pest spray, add 2.5 tablespoons of pure castile soap to one gallon of distilled water. Hard tap water reacts with soap and cuts its pest-killing power. Shake it up, pour it into a spray bottle, and test on a few leaves first. This simple mix costs about 40 cents per gallon and works just as well as the $10 store brands. When I first made the switch from store-bought to homemade, I couldn't tell any difference in how well it killed aphids on my roses.
Read the full article: Insecticidal Soap for Garden Pests
