Insecticidal soap is an effective insect spray for small soft bodied garden pests such as aphids, spider mites, young scales, whiteflies, and mealybugs. Some larger garden insects like box elder bugs can be eliminated in their small nymph stages by spraying with insecticidal soap. Soap is effective because it interferes with the insect's cell membranes and also destroys the protective wax coating on their bodies. The soap must coat or come into contact with the insect to be effective. Insecticidal soap as an insect spray is a good choice because it does not kill beneficial insects such as lacewings, ladybugs, and bees. Do not use detergent or dishwashing machine soap. Choose soap that has the fewest additives such as fragrances, anti-bacterial agents or lotions. Often the least expensive dishwashing soap has the fewest additives.
Add 4 tsp. of liquid dishwashing soap to a quart spray bottle filled with water. This creates the 2 to 3 per cent solution that is commonly recommended as an insecticidal spray.
Spray directly onto the insect-infected areas of the plant, including the underside of leaves. The insecticidal soap must soak the leaves and insects to be effective. Insects that are inside curled leaves, such as aphids, will not be effectively controlled unless they are wet.
Wash the insecticidal soap off the plants several hours later. The insects have been affected and removing the residual soap benefits the plants.
Inspect your plants the day after spraying. Look for signs of damage to the leaves such as wilt or burn. These symptoms indicate that the insecticidal soap solution was too strong or the soap had too many chemical additives. Discontinue the spraying program if it is injuring plants.
Repeat spraying every four to seven days until the insects are eradicated. Continue to watch for detrimental effects on the plants.