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| Photo: Courtesy Standford University |
The Master Gardener Volunteer Program is thankful to the University of Florida/Institute of Food and Agricultural Sciences (IFAS) Extension faculty Dr. Adam Dale and Dr. Matthew Borden for their publication “Managing Plant Pests with Soaps.” For further information, please reference this publication and contact your county Extension office or the UF/IFAS Pesticide Information Office if you have any questions.
Today more people than ever recognize the importance of using pesticides wisely, protecting pollinators, and managing gardens with low-impact strategies (like integrated pest management). Soaps come up often in conversation on these topics, especially in ones about low-impact or organic pest management.
In English, we use the word
"soap" to refer to a number of products: hand soap, dish soap,
castile soap, laundry soap, and insecticidal soap, to name a few. They may
share a name, but these "soap" products are actually very different.
Still, because people are familiar with soaps' use in the kitchen, laundry, and
bathroom, many feel safe using them in the garden, too.
To protect consumers and the environment,
soaps designed for use as pesticides are evaluated by the Environmental
Protection Agency (EPA). They are registered as insecticidal soaps and their
labels contain directions for use, precautionary statements, and instructions
for storage and disposal. They also list relevant environmental, physical, and
chemical hazard warnings, such as "hazardous to aquatic
invertebrates." As tempting as DIY recipes can be, ingredient and
concentration inconsistencies make these concoctions an unknown danger to
yourself, your plants, and the environment. Below are some common misunderstandings
about soap and detergent use in home gardens.
Misunderstanding #1: Dish soap is
a natural, safe alternative to pesticides.
Research shows: Dish "soaps" are not true soaps; they are detergents, synthetically produced and chemically designed to be powerful cleaners.
Cleaning products (including detergents
like Dawn®, Joy®, and Palmolive®) have labels that explain how they may safely
be used. As with all other chemicals, the label is the law. If the instructions
do not include garden or pesticide use, then the product was not intended to be
used in that manner. It may even be dangerous to do so.
Understandably, companies package cleaning products in ways that help their consumers feel the product will be safe and effective. Advertising for these products often includes words like "gentle" or "natural," along with images of fields and flowers. Make no mistake — no matter how attractively packaged, detergents are not appropriate pest control for organic or conventional gardening.
Misunderstanding #2: Soaps are
mild; they don't hurt plants.
Research shows: As discussed above, most
dish soaps are actually detergents. To do their job well, they must be powerful
enough to strip oils and other lipids from man-made surfaces.
Plant leaves are covered with a layer of
waxy lipids. This waxy cuticle layer protects them from losing water and from
viruses, bacteria, funguses, and other pathogens. Even when watered down,
detergents cut through these protective layers. The result is often damaged,
dry, crispy leaves.
Even true soaps can damage plants depending
on which active ingredients they include. Most soaps intended for hygienic uses
are made with sodium hydroxide. This ingredient is a powerful, modern lye. The
sodium in this formula can cause significant damage to plant tissue.
Insecticidal soaps, on the other hand, are intended for use on plants. They are
made with potassium hydroxide, which can be used without damaging plant tissue.
Misunderstanding #3: Soaps kill the bad bugs without hurting the good bugs.
Research shows: Soaps and detergents don't discriminate between good bugs and bad bugs. The ingredients that damage soft-bodied insects and mites also attack beneficial insects and funguses. Insecticidal soaps, although designed for use on plants, also damage pest and beneficial insects indiscriminately.
When it comes to our gardens, we know that
insects can be helpful, harmful, or harmless. No matter which pest control
option we choose, protecting the good bugs will require education and effort.
More about Home Recipes for Pesticides
On the topic of pest control there is a
lot of conflicting information and room for error. Using products not
registered by the EPA as makeshift pesticides is not recommended by UF/IFAS for
a number of reasons, including safety hazards to plants, animals, and people.
Few of the recipes available online come from research-based sources. In fact,
due to an increased interest in sustainable home food production, many recipes
are simply "click bait," offering advertisements rather than sound
garden advice. Finally, application of unregistered chemicals onto crops sold
as food is illegal.
Any soap, even the insecticidal soaps
intended for garden use, can injure plants if used incorrectly. Often issues
with homemade pesticide formulas are related to inconsistent ingredients and
concentrations.
Ingredients to avoid
Ingredients like vinegar and alcohol are
phytotoxic; this means they cause leaf burn and other plant injuries. These
should not be applied to plant tissues. Even familiar products, like Procter
& Gamble Ivory® soaps, frequently change their ingredients. They are not
intended for use on plants and their potential toxicity is unknown.
Worse still, some home recipes for pest
control call for mixing different products. This is dangerous! Common cleaners
that may be safe to use by themselves can be toxic to humans when they are used
together.
Concentration matters
Similarly, home recipes for insecticidal soaps are inconsistent in the rate of application (the amount of soap relative to the amount of water). Some recipes recommend one teaspoon of soap per quart of water; others call for a whole quarter of a cup.
Properly registered and EPA-approved
insecticidal soaps are usually only 1-2% soap by volume. This rate minimizes
damage to the plant but is still effective as a pest control. Labels also
include important information about plant species that are sensitive to the
product's ingredients. Regardless of the formula, plants under drought stress,
in excessive humidity, or in heat above 90 degrees Fahrenheit should never be
treated with soaps.
Regardless of which product you choose, always
read the label first — before you buy and before you use.
If you have questions about soaps or
pesticide use, please contact your county Extension office or the UF/IFAS
Pesticide Information Office for more information about pesticide safety.
This information is available online at
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