Using Soaps and Oils to Combat Garden Insects

By The National Gardening Association, Bob Beckstrom, Karan Davis Cutler, Kathleen Fisher, Phillip Giroux, Judy Glattstein, Mike MacCaskey, Bill Marken, Charlie Nardozzi, Sally Roth, Marcia Tatroe, Lance Walheim, and Ann Whitman from Gardening All-in-One For Dummies

Soaps and oils kill a wide range of pest insects, but affect beneficial insects, too. Insects breathe through pores in the cuticle that surrounds their bodies. If you plug up the pores, the insects suffocate and die. Disrupt the cuticle with special soaps and oils and — poof! — the insects can’t maintain their internal moisture.

Oils do have several drawbacks. Don’t use them when temperatures are likely to rise above 90°F, when plants are suffering from drought stress, or if you have applied or plan to apply sulfur fungicide within 30 days. It will also remove the bluish waxy coating from Colorado blue spruce, so avoid using it on that species. Read the label carefully for other precautions.

Horticultural oils: Use horticultural oils in the winter to suffocate over-wintering pests, such as aphids, mites, and scales, on dormant fruit and ornamental trees and shrubs. During the growing season horticultural oils work against aphids, mites, lace bugs, corn earworms, mealybugs, leafminers, and many others, including tough-to-kill scale insects. Mix with water according to label instructions and then apply with a sprayer.
Citrus oils: The oils from the skin of citrus fruits kill a broad range of insects on contact by poisoning them. The oils continue to repel pests, such as fleas, ants, and silverfish, for weeks and are safe around people and pets. The active ingredient d-Limonene. Look for it on the label.
Plant extracts: Many herbs, spices, and plants contain chemicals that repel or kill insects. Garlic is one of the most well known and effective extracts against thrips and other leaf-eating insects.
Insecticidal soaps: The active ingredient in insecticidal soap, called potassium salts of fatty acids, penetrates and disrupts the cuticle that holds moisture inside insects’ bodies. When sprayed with soap, many soft-bodied insect pests, such as aphids, dry out and die. Some pests, however, especially beetles with hard bodies, remain unaffected.

Insecticidal soap is nontoxic to humans and other animals and breaks down quickly in the environment. If you use a concentrated product, dilute it with soft water before using for the best effect. Hard or mineral-rich water decreases its effectiveness.

Insecticidal soap also disrupts the waxy cuticle on some plants, making it toxic to young and thin-leafed plants, especially tomatoes. If you aren’t sure of the plant’s sensitivity to the product, always test it on a leaf or two and allow a couple of days to pass before spraying a whole plant. Follow the label directions carefully.