By Marissa Schuh, University of Minnesota Extension educator, integrated pest management
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.
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 percent soap solution: add just two teaspoons of dish soap to omne pint of water.
High concentrations of soap can burn plant foliage, especially when plants are stressed, temperatures are over 90 degrees and humidity is high. Much of Minnesota has seen many days with afternoon high temperatures of 90 degrees 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.