One item every gardener should have in his or her arsenal is insecticidal soap. The product is great because it’ll kill almost any pest insect on contact, it’s safe to handle and economical. It also has minimal impact on beneficial insects and the environment.

Spider mite

Not only do insecticidal soaps work on pesky insects, they also kill spider mites.

Photo Credit: ©2003 Buglady Consulting

“Hey, soap is soap,” you might be thinking. But it really isn’t. In general, soap is a substance made by combining a fat with either an alkali, like sodium (as with hard soap), or potassium hydroxide (soft soap). While there are lots of fatty acids out there used to make soap, only certain ones have insecticidal properties.

So that said, and contrary to any rumors you may have heard, you can’t just use ordinary kitchen soap to control an insect problem in the garden. In fact, some household soaps can actually kill plants. (And these soaps are NOT organic!)

One fundamentally important property of horticultural soaps has to do with how many carbons are attached to them. Soaps with lots of carbons, for example, are long-chain fatty acids and have insecticidal properties. On the other hand, when carbons are removed from the soap, you end up with short-chain fatty acids, which tend to have herbicidal properties. (Herbicides are what you use to kill weeds.)

This is why it’s so important to use store-bought insecticidal soaps and not just any kitchen or bath soap for gardening situations. Do you know if your dish soap is a short- or long-chain fatty acid? Is it an insecticide or an herbicide? Do you know if it’ll burn your plant? These are very serious questions!

Dishwashing liquids and laundry detergents are designed to dissolve grease, not kill insects and mites. Products like Safer’s® Insecticidal Soap have been through more rigorous testing for efficacy and plant safety than household detergents and soaps. So we know how they’ll perform in the garden – and they’re clearly labeled for your horticultural use as insecticides, not for cleaning your dishes.

Other factors include the various dyes and perfumes that are added to kitchen soaps. These additives are often switched or changed to please the retail consumer – who knows how the additives will affect plant material? So even if you do find one that seemingly works, how long before the formula is “new and improved” or changed without warning?

With commercial insecticidal soaps you get a consistent formulation proved to work. But do keep in mind that phytotoxicity can be a concern, so it’s important to only use soaps specifically labeled as pesticides. Always use labeled rates and don’t spray plants that are under stress!

Insecticidal soaps can be a great tool in a pest management program. They kill a wide range of pests, are relatively inexpensive, are compatible with beneficial insects and don’t have resistance issues. In other words, when they’re used properly, these soaps can really help you clean up your garden of unwanted pests!