Insecticidal Soap

Insecticidal soaps are an easy DIY method of pest control in the garden.

People have been using soap sprays for a long time to protect their plants, but the science behind using soap has only begun to demonstrate just how insecticidal soaps work. Current research has shown that spraying soapy water on insects kills them off by:

• removing their protective waxy coating (cuticle), causing them to dry up and die
• the physical spray mechanism knocks many soft-bodied insects, such as aphids, off the plant
• some insects are immobilized by soapy water, making it easier to wash them off plants.

Before jumping on the insecticidal soap bandwagon, however, you need to understand that not all soaps are created equally and that many soaps are actually detergents that can kill your plants.

Homemade insecticidal soaps

True insecticidal soaps contain potassium salts of fatty acids and are designed specifically for use on plants. These fatty acids are commonly found in fish oil, lard, and olive, palm, coconut and other plant oils. These fatty acids are mixed with potassium hydroxide, which is strongly alkaline, to create soap, much the way fatty acids are mixed with sodium hydroxide to make lye. While potassium salts and sodium salts will both kill insects, sodium salts are toxic to plants. [The same problem occurs when people use baking soda (sodium bicarbonate), rather than potassium bicarbonate, on plants to fight fungal disease. Baking soda is phytotoxic, whereas potassium bicarbonate is not.]

Not all household liquid soaps are safe for use on plants. In fact, I couldn’t find a list of any that are truly safe. Laundry soaps and dry dishwashing detergents are also too harsh to be used on your garden plants. Also, many liquid dishwashing soaps contain bleach, fragrances, and colors, and other chemicals that can harm or kill your plants. As tempting as it may be to grab your bottle of dish soap from the kitchen sink, this is not a good idea. [I challenge you to take a close look at the ingredients list on your dishwashing soap and look up any words you don’t know.]

Effectiveness of insecticidal soap

Instead of burning up your plants with detergent, go to the store and buy a bottle of insecticidal soap. It is less expensive that many other pesticides, plus it is less damaging to the environment and other living things. Insecticidal soap that has been properly formulated and applied will kill many common pests, including:

• ants
• aphids
• Asian citrus psyllid
• bagrada bugs
• earwigs
• eriophyid mites
• leaf-footed bugs
• mealybugs​

• psyllids
• rose slugs
• soft scale
• spider mites
• spittlebugs
• thrips
• whiteflies

Unfortunately, insecticidal soap can also kill off the larval forms of many beneficial insects, such as ladybugs and lacewings.

How to use insecticidal soap

Insecticidal soap only works when it comes in direct contact with and completely covers an insect pest. Use these tips to safely use insecticidal soap:

• do not use on ferns, Euphorbias, sweet peas, azaleas, bleeding hearts, plums, cherries, seedlings, or new transplants
• follow package directions EXACTLY
• apply in early morning or evening
• avoid applying when temperatures are above 90° F
• if you are unsure about your plant’s sensitivity, test a small area first

Insecticidal soaps have little or no residual effects, so treatments must be repeated regularly until the desired level of control is reached.

So, insecticidal soap isn’t the Quick Fix you might have thought it was before reading this post, but it is effective when used properly.