Insecticidal soaps kill soft-bodied pests including mites, aphids, immature scales, psyllids, thrips, and whiteflies. Soaps also kill the eggs and larvae of many pests.
Soaps are contact insecticides; they must be sprayed wet directly on the insect pest to be effective. To treat a severe pest infestation, spray a plant or crop every 2 or 3 days for two weeks. Once the soap spray dries, it has no residual insecticidal effect.
Insecticidal soaps are not toxic to people, pets, or wild life, but it is best not to inhale soap spray or get it in your eyes.
Insecticidal soaps are made from potassium salts of fatty acids, also called soap salts. You can buy insecticidal soaps at garden centers or nurseries or you can make your own.
How to make insecticidal soap: Make insecticidal soap by adding 2 tablespoons of a vegetable-oil-based liquid soap such as castile soap to 1 gallon of water. Do not use laundry detergent or liquid dish soap. (Soaps are made with olive oils and vegetable oils—renewable resources. Detergents are made from petroleum-based oils, nonrenewable resources.) Use a soap with no additives or perfumes which may harm plants.
Applying insecticidal soap: Apply insecticidal soaps with a spray bottle. Spray directly on insect pests. Spray the tops and undersides of leaves where pests lay eggs and hide and all growing points of plants—tender tissue that insect pests feed on.
Insecticidal soaps kill by damaging the outer cell membranes or coating of the insect causing dehydration and paralysis. All soft-bodied insects sprayed with soap will be harmed or killed including beneficial insects, so be careful to spray only the insects you want to kill.
Before spraying an entire plant or crop, test spray a leaf or two on one plant from a crop then wait a day or two to make sure the soap does not damage the plant. Some very tender plant leaves may be burnt by soap sprays.
Here are tips for using insecticidal soaps:
- Spray early in the morning when plants are still wet with dew or late in the afternoon or after sunset when temperatures are moderate. Avoid spraying at midday in the hot sun—the soap will dry before it can work.
- Spray both sides of the plant’s leaves; many pests feed on the undersides of leaves or take shelter there when they detect movement or light changes that might indicate danger.
- Do not treat heat- or water-stressed plants; the soap coating on leaves may hinder transpiration the exchange of gases and moisture through leaf pores.
- Soaps may damage some plants, especially those with dull leaf surfaces or hairy surfaces.
- Soaps are less effective or may have no effect on adult pests with hard outer coatings or shells such as beetles.
- Insecticidal soaps only work while wet. To be sure you get all or most of the pests on a plant, spray several times a week for three weeks. Keep an eye out for pests to know when it is time to spray again.
- Spray plants every 2 to 3 days for two weeks or more for severe infestations.
- Store-bought insecticidal soap usually calls for two teaspoons of soap per pint of water. Always follow the directions on the label.
- The least invasive pest control is to spray or wash infected plants with water to dislodge harmful pests.