A large infestation of aphids can cause damage to the leaves, stems and even the roots of a plant. When travelling from plant to plant, they can also transmit disease. Before using toxic commercial pesticides, there are some less toxic alternatives you can try to control the aphids on your plants. One option is to make a homemade insecticidal soap.
There are several species of aphids that attack different parts of the plant. They are usually well camouflaged to blend in with the plant and may be different colours. They are small, teardrop-shaped and, like all insects, have six legs and two antennae. They may or may not have wings. To positively identify the bugs as aphids, look for the cornicles. These are tubelike structures that protrude out of their back end. Also, they do not move quickly or at all when you are inspecting the plant.
Insecticidal Soap Works Better
According to UC Davis, "applying nonpersistent insecticides like soap or oil may provide more effective long-term control." This is because the soap only kills the pests that it touches. It does not persist on the plant to kill beneficial insects, which may eat aphids, that later visit the plant.
Water and Dishwashing Soap
This is the most simple insecticidal soap recipe to make and use. You can rinse out an empty gallon milk container to make the insecticidal soap. Fill the container with water, then add 2 tbsp of dishwashing soap. Secure the lid and gently mix the solution. Fill a squirt bottle with the solution to spray the plants.
Increase the affect of your solution by adding herbs to the mix. Try different herbs to determine the one that works best on this particular species of aphid. Make a tea using wormwood, nasturtiums, elderberry or rhubarb leaves. Boil one gallon of water, add the tea leaves and let the "tea" cool. Once it has cooled, strain out the leaves and make the soap solution as above using the tea water.
Applying the Insecticidal Soap
With any of these recipes, the application of the insecticidal soap is the same. Spray the plant where the aphids are located. Soak the leaf or stem, fully covering the aphids. Spray both the top and bottom of the leaf. When you are finished, the plant should be dripping. You may have to apply this solution several times, at least every three days, to kill all the aphids.