Now that the frozen ground and a light dusting of snow has shut down the outdoor gardening season, it’s time for the reading and study season we all wait for. Preferably in a comfy chair with your lap warmed by a dog or cat. With that in mind, I want to spend several columns over the next few months on topics that will help you understand some more technical garden topics.
The first is on the difference between insecticidal soaps and dish soaps used as an alternative insecticide.
Insecticidal soaps are formulated specifically to deal with soft-bodied critters such as aphids, soft scale, psyllids, whiteflies, mealy bugs and spider mites. In general, two applications made five to seven days apart will reduce the population of target insects. Insecticidal soaps typically contain naturally occurring potassium salts of fatty acids that dry out the insects with little damage to plants if the directions are followed. The soap must be in direct contact to work properly.
Dish soaps like Dawn, Joy or Palmolive were never designed to be pesticides. They are detergents, not true soaps, and intended to quickly remove grease and oils from cookware and other household objects.
If you read the label, you won’t see any information about how to use it as a pesticide or on plants. You won’t find an EPA registration number that signifies the product has been tested for its efficacy as a pesticide. Without a proper set of directions for their use, they can do a lot of damage to plants by removing the protective waxy layers on the leaves that prevent dehydration. They are also nonselective, which means they will kill beneficial insects as well as the bad bugs.
Dish soaps are not considered to be organic because they are made of synthetic chemicals called surfactants while insecticidal soaps are made with naturally occurring oils and fats. Many brands of insecticidal soaps are registered to be used in organic gardening and farming and would be the better choice if you wanted to use organic practices in your garden.
What about castile soaps and the name brand Ivory Soap? Both have a long history as a folklore-based insecticide but neither has been formally tested by the EPA for risks to plants and humans when used in a garden setting. Additionally, many castile soaps contain fragrances and essential oils that can damage plants. These hard soaps need to be grated to dissolve them in water which is a lot of work.
Lastly, insecticidal soap labels are very clear and specific on how much to use and how often. This isn’t the case for dish and other soap mixes.
There are dozens of recipes on the internet about how to mix dish soap-based sprays. Some recipes call for a teaspoon to a quart of water while another might call for a quarter cup. Some call for the addition of vinegar and alcohol – all of which can severely damage the plant as well as kill the bugs.