Insect pests plague both houseplants and garden plants, reducing yields, creating unsightly symptoms and sometimes killing the plant. Most gardeners, at some point, use an insecticide. However there are concerns about the safety of such products, which many be harmful to people, pets and the environment. Insecticidal soap has provided one possible remedy that combines safety with effectiveness.
Although the mechanism of insecticidal soap isn't perfectly understood, horticulturalists believe that it works in two ways. First, the soap removes the waxy coating on certain insects' bodies, which causes it to dehydrate and die. Insecticidal soap also seems to disrupt the function of the insect's cell membranes, according to Colorado State University.
According to W.S. Cranshaw of the Colorado State University Extension, insecticidal soaps have been used for 200 years. The ease by which they can be made at home with simple ingredients that most homeowners have on hand has added to their appeal. However, commercial varieties are also available. The University of Maryland Extension's website recommends commercial insecticidal soaps because they are formulated to reduce risk of harm to plants. Modern soaps and detergents contain additives like lotions and antibacterial agents that could damage some plants.
With a few exceptions, insecticidal soap is only effective against small, soft-bodied insects like scale crawlers and thrips, so correctly identifying the pest affecting your plant is important before deciding to use insecticidal soap. Insecticidal soap does not have a residual effect and works only when it completely soaks the insect pest, so thorough spraying of all plant parts, including under the leaves, is essential to using insecticidal soap effectively. Hard water also reduces the effectiveness of insecticidal soap, according to Washington State University.
Insecticidal soap may harm some types of plants. Hawthorn, cherry, plum, portulaca, sweet pea and some tomato varieties in particular may be sensitive. Always read and follow the instructions when using a commercial product, and try homemade insecticidal soaps on a small part of the plant before drenching the whole thing. Since insecticidal soaps don't have a residual effect on pests, the Colorado State University Extension also recommends rising plants a few hours after application to reduce harm.
W.S. Cranshaw, Ph.D., an entomologist at Colorado State University, offers a simple recipe for homemade insecticidal soap. Use 1 tsp. of detergent or soap per pint of water, shake well and use a sprayer to apply to all parts of the plant. Whenever possible, avoid soaps and detergents with additives.