Make a standard, 1-percent soap solution by combining 3 tbsp. of mild, liquid dish soap and 1 gallon of water. While this is all that is truly necessary to rid the garden of aphids, it is not uncommon for people to include 3 tbsp. of vegetable oil in the mix, as this increases the viscosity of the liquid and helps it stick to the body of the bug.
Combine the ingredients in a large bucket and stir briskly, until the mixture is thoroughly blended, then transfer the solution to a spray bottle and apply it to the affected plants. Alternatively, stir 1 tbsp. of liquid dish soap into 1 cup of vegetable oil and transfer the mixture to the canister of a hose-end sprayer; as the water flows through the hose it will dilute the concentrate, enabling you to apply the soap solution to large areas evenly and thoroughly. Spray both the tops and bottoms of the leaves, as this is where aphids tend to congregate. Wait three to four hours, then rinse the soap residue away by spraying the plants with clear water. Reapply the solution two to three times a week until the insects are no longer apparent.
How it Works
Soft-bodied insects, such as aphids, are protected by a waxy outer coating, rather than a hard shell. The grease-cutting ingredients in dish-washing detergents erode this coating, and as a result, the insects dehydrate. Additionally, aphids breathe through openings known as spiracles. When covered with soapy water, the resulting residue blocks these openings, causing the insects to suffocate. Accordingly, for a soap solution to be fully effective, it must thoroughly cover the body of the bug.
Minor aphid infestations can be resolved by spraying the plants with a strong stream of water each morning; however, not all plants respond well to daily watering, and excess moisture can lead to additional problems as damp conditions encourage the growth of fungi and bacteria. Aphids can also be kept to a minimum by encouraging the presence of beneficial insects, such as ladybugs and lacewings. Invite these aphid predators into the garden by including flowering plants, such as marigold, flowering tobacco and sweet alyssum, in the garden’s design.