How to Use Soap to Kill Garden Pests

Soap sprays are considered contact pesticides, meaning they must come into contact with the pest you are trying to remove for them to work. They do not work as a deterrent or preventative for garden pests, and they do not work on all pests. Soap sprays work best on soft-bodied insects such as mealybugs, aphids and whiteflies by destroying their protective cell membranes.

Whiteflies and aphids on the stem of a tomato flower.

Whiteflies and aphids on the stem of a tomato flower. Since tomatoes are self-pollinating, you do not need to worry about deterring pollinators such as bees from the flowers.

Not all liquid soap is effective against pests or safe in the garden. Specially-formulated insecticidal soap contains potassium salts of fatty acids for destroying pest cell membranes and degrades in the environment within two weeks. For organic gardening, look for an OMRI-listed brand of insecticidal soap, such as Safer. You can also use liquid soap you may already have at home, such as unscented Dr. Bronner’s or plain Ivory. Be careful not to spray soap too close to the flowers of cucumbers, melons or squash, as these require cross-pollination by bees to produce fruit.

Branded insecticidal soap will have specific dosage instructions on the package, but if you are making your own mixture at home, follow these instructions: to each gallon of water, add 3-10 tablespoons liquid soap and pour into a clean spray bottle. If your tap water is hard, use collected rainwater or purchase water at the grocery store instead, as the minerals in hard water can greatly reduce the effectiveness of the soap spray. If you are using more than 3 tablespoons of soap per gallon, injury on the plants’ leaves, such as brown spots, can occur. To kill the pests, yet reduce injury on the plant leaves, apply the soap spray directly on the pests, in late afternoon or evening, then rinse off early morning.

Soap sprays should be used about twice a week for two weeks to fully remove the infestation, and should be applied more frequently if it rains soon after spraying. Be sure to use the “misting” setting on the spray bottle, and be especially sure to spray the tips and undersides of leaves, which is where many soft-bodied pests will feed. Aphids can also hide in the curled tips of leaves, particularly where the leaves are softest, at the top of the plant.

Why do you have a pest infestation? Plants usually succumb to pests when they are already stressed, such as when they are experiencing a period of drought, extreme heat, low nutrients, or an excess of nitrates. Keep your plants well-watered, particularly while they are flowering or fruiting, and help reduce soil moisture loss by applying a thin layer of mulch such as straw, burlap or newspaper. Be careful not to add more fertilizer than is recommended on the package, or add fertilizer that has a high N value (higher than 4) during fruiting.

Looking for more information about aphids? Check out our previous post HERE.