Insecticidal soap contains unsaturated long-chain fatty acids (derived from animal fats) that dissolve the cuticle (skin) of insects. Insecticidal soap sprays are commercially formulated products sold specifically for insect control. (Don’t confuse these products with herbicidal soaps, which kill vegetation instead of insects, or household soaps, which are detergents.)
To be effective, the insecticidal soap must come in contact with the insects while it’s still liquid—it has no effect after it dries on the plants. Spray only on pests and try to avoid hitting beneficial insects with the spray. Caution: Insecticidal soap can burn some plant leaves. Test each type of plant before spraying the entire plant. Spray a few leaves, then wait 48 hours. If there’s no damage, go ahead and spray the entire plant. Don’t spray on hot days, and rinse the soap off your plants after a few hours if the plants are receiving a lot of sunshine. If you have hard water, mix the soap with distilled water to help the soap dissolve.
Pests controlled: Insecticidal soap sprays are highly effective against mites, aphids, whiteflies, and other soft-bodied insects as well as the softer nymph stages of some tough-bodied bugs.
Oil sprays work by suffocating pests. To be effective, the oil spray must hit the pest directly.
Use “dormant” oils to kill insect pest eggs and disease spores on the bare branches of trees and shrubs during the dormant season. To treat growing plants, use a lighter-weight, more refined horticultural oil (called “summer,” “supreme,” or “superior” oil). Lighter oils evaporate more quickly than dormant oils and are less likely to damage plants.
To avoid plant damage, don’t spray any plants suffering from moisture stress. Also, don’t spray on very hot days. Test the spray on just a few leaves before you spray the entire plant. Wait 48 hours to make sure no leaf spotting or discoloration occurs.
To minimize potential harm to beneficial insects, limit your spraying to small areas where you can see pests lurking, and leave a couple of unsprayed “refuges” for any good bugs you can’t see. Protect nectar-feeding beneficials by not spraying during peak flowering times and by not spraying blossoms. Spray early in the morning, before bees become active. And if you plan to release beneficials, do it after you apply the oil spray.