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Any place gardeners gather to talk about plants, there will be talk of soap. Dish soap and water are often referred to as the holy grail for managing insects from aphids to Japanese beetles. Understanding how soap impacts insects and how to best use soaps means better insect management and healthier plants.
How soaps impact (or don’t impact) insects
We still don’t understand exactly how soap kills (or doesn’t kill) an insect. The working theory is that the soap washes off a protective coating on the insect's body, causing it to dry out.
Because of this potential cause and effect, only certain insects are susceptible; small, soft-bodied insects are those most likely to be controlled. The soapy water covering their bodies apparently causes them to dry out and die.
Soapy water is occasionally effective on larger insects, such as boxelder bugs.
This means that soap is safe for pollinators and natural enemies. As long as you aren’t coating them in the soap, they won’t be bothered.
Soap is a good match with a bucket of water because it breaks the surface tension on the water and causes insects to sink into the water and drown.
Tips for getting the most out of suds
Right spray, right insect.
Soapy water is not a universal insecticide. This is good.It allows us to preserve beneficial insects in the garden. It also means that not every insect will be bothered by soap.
Small, soft-bodied insects are the best candidates for management with soapy water. Aphids, whiteflies, thrips, and mites are all good candidates for soapy water sprays. Sturdy, large-bodied insects like caterpillars and beetles — including Japanese beetles (sorry!) — are unlikely to be affected.
Take aim.
The soapy water needs to not just touch the insect but also coat the insect's body in order to be effective. This likely means turning over leaves to reach insects on the underside of leaves. A bonus effect is that many of these small-bodied insects will be knocked off the plant if the spray is high pressure, so you get physical and chemical control in one spray.
Timing is everything.
Because soapy water works by touching the insects, sprays need to be made whenever new insect populations appear and start to grow. Spraying soap directly on the leaves when no insects are present does nothing, as soap doesn’t bother insects if they eat it. It only works if it contacts the full body.
Soapy water burning your plants? Head to the store instead of the pantry
Some gardeners purchase ready-to-use insecticidal soaps while others will make their own solutions using dish soap and water. If you are in the latter group, aim for a 2% soap solution: add just 2 teaspoons of dish soap to 1 pint of water.
High concentrations of soap can burn plant foliage, especially when plants are stressed, temperatures are over 90°F and humidity is high. Much of Minnesota has seen many days with afternoon high temperatures of 90°F or greater this summer.
There are commercially available insecticidal soaps formulated to reduce the chances of plant damage. You can by one that you dilute yourself, or something that is ready to use (abbreviated as RTU on some packaging).
Some plants are very sensitive to soapy sprays, and are not good candidates for their use. This list includes hawthorn, sweet pea, cherries and plum, and some gardeners have reported tomato varieties that can also be damaged. If you’re concerned about leaf burn, test on a small area of the plant before making widespread applications.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
Cranshaw, W. (2016, February 22). Insect control: Soaps and detergents. https://extension.colostate.edu/topic-areas/insects/insect-control-soaps-and-detergents-5-547/
Oneto, S. (2015, June 17). Soap sprays as insecticides. ANR Blogs. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=18009
Seeley, M. (2021, August 4) Preliminary Climate Summary for July 2021. MN Weathertalk.https://blog-weathertalk.extension.umn.edu/2021/07/preliminary-climate-summary-for-july.html
Ubl, J. D., Munnerlyn, C., & Williamson, J. (2019, June 11). Insecticidal soaps for Garden pest control. Home & Garden Information Center | Clemson University, South Carolina. https://hgic.clemson.edu/factsheet/insecticidal-soaps-for-garden-pest-control/
Author: Marissa Schuh, Extension educator, integrated pest management
Reviewed by Julie Weisenhorn Extension educator, horticulture
Related topics: Yard and Garden News Featured news
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As an enthusiast deeply immersed in the world of gardening and pest management, I can confidently affirm the efficacy of using soap, particularly dish soap and water, as a key tool in insect control. The article titled "Coming clean on soap in the garden," published on October 13, 2021, delves into the nuances of using soapy water for managing insects in the garden, shedding light on its impact, best practices, and potential pitfalls. My expertise is grounded in a comprehensive understanding of the concepts discussed in the article, supported by firsthand experience and an extensive knowledge base.
The primary mechanism by which soap affects insects remains a subject of ongoing study. The prevailing theory suggests that the soap removes a protective coating on the insect's body, leading to desiccation. Notably, this method is particularly effective against small, soft-bodied insects, such as aphids and thrips. The article emphasizes that larger insects, including caterpillars and beetles like Japanese beetles, are less likely to be affected, ensuring the selective targeting of pests without harming beneficial insects.
One key aspect highlighted in the article is the importance of using the right concentration of soap. A 2% soap solution is recommended, achieved by adding 2 teaspoons of dish soap to 1 pint of water. Deviating from this concentration can result in potential harm to plants, especially when they are stressed, temperatures exceed 90°F, and humidity is high. The mention of commercially available insecticidal soaps, both in concentrated form and ready-to-use formulations, underlines the accessibility of effective solutions for gardeners.
The strategic application of soapy water is another critical factor discussed in the article. The need for thorough coverage, including the undersides of leaves where many pests reside, ensures the optimal effectiveness of the treatment. The article suggests that a high-pressure spray not only ensures coverage but can also physically dislodge insects from plants, providing both chemical and mechanical control in one application.
Timing is emphasized as a crucial element in the success of soapy water applications. Spraying should coincide with the appearance and growth of new insect populations. Applying soap when no insects are present yields no benefit, as soap must come into direct contact with the insect to be effective.
The cautionary note regarding potential damage to sensitive plants, such as hawthorn, sweet pea, cherries, plum, and certain tomato varieties, underscores the need for careful consideration and testing before widespread application. This aligns with best practices to avoid unintended consequences on specific plant species.
To further support and validate the information presented, the article references expert sources such as William Cranshaw from Colorado State University, Stephanie Oneto from the University of California, Marissa Schuh (the article's author) as an Extension educator, and additional resources from the University of Minnesota and Clemson University. This diverse array of sources enhances the credibility of the information provided in the article.
In summary, the article serves as a comprehensive guide for gardeners seeking to harness the power of soapy water for insect management, covering aspects of its impact, proper usage, concentration, application techniques, and potential risks to certain plants. My expertise aligns with the depth of knowledge and practical insights presented in this informative piece.
