by – David Trinklein University of Missouri, Division of Plant Sciences
Most gardeners are good stewards of the land and attempt to control pests using tactics with minimal environmental impact. Insecticidal soaps have become an increasingly popular method of controlling certain insects in a very “eco-friendly” manner. Nearly non-toxic to mammals, insecticidal soaps may be applied to food crops until the day of harvest. They also may be used in organic production.
Soaps are salts of fatty acids. This means that the fatty acids, which are obtained from plants and animals, are made soluble for spray application via a chemical process. The latter involves neutralizing the fatty acids with a base such as potassium hydroxide to form fatty acid salts, or soaps.
Not all soap is created equal. One cannot simply make a solution using their favorite bath soap, spray the garden and expect to get good insect control. Nature produces many different fatty acids that can be neutralized with a number of different chemical bases that results in different types of soap. Most insecticidal soaps are derived from long-chain fatty acids which make them effective in the control of pests and less damaging to plants.
The mode-of-action of insecticidal soaps is not clearly understood. One theory suggests the soap is absorbed by the insect pest via its trachea which leads to the disruption of cellular membranes and leaking of cell contents. Another theory infers that the soap dissolves the exterior cuticle of the insect, causing it to dehydrate. Finally, there are those who maintain that the soap physically blocks the breathing openings of the insect which leads to suffocation.
Whatever the mode-of-action, to obtain optimum results from insecticidal soaps, several characteristics of the soaps need to be kept in mind when applying them. One characteristic is that insecticidal soaps are contact poisons. This means the target pest must be wetted with the insecticidal soap solution if control is to be obtained. Insects walking across the residue of soap that has dried will not be harmed.
Therefore, it is important to determine where insect pests are feeding and to cover these areas as thoroughly as possible. As a general rule, most insect pests feed primarily on the underside of leaves. Hence, thorough spraying from below will result in optimum pest control. Some insect pests are very mobile and will flee as spray is being applied. Repeated applications may be necessary to contact “escapees” when they return to feed.
Another factor to consider when applying insecticidal soaps is the nature of the water used to make spray solutions. It has been demonstrated that hard water may reduce the effectiveness of insecticidal soaps. The soap will combine with certain minerals in hard water which results in a compound precipitated from the spray solution. The mineral elements that cause the greatest problem include calcium, magnesium and iron. Therefore, it is best to use soft water when diluting an insecticidal soap concentrate to the proper strength as dictated by its label.
As a test for water quality, mix a quart of insecticidal soap spray solution with your existing water source and allow it to sit for about 30 minutes. If a scum develops on the surface of the solution, your existing water source is too hard and should not be used.
Additionally, to maintain their effectiveness, insecticidal soaps should not be mixed with certain other pesticides. The latter include copper fungicides such as Bordeau mix, liquid copper, lime or sulfur, rotenone-based insecticides and dithiocarbamate fungicides such as maneb, zineb or mancozeb. Also, avoid mixing insecticidal soaps with fertilizer solutions used for foliar feeding.
Although insecticidal soaps do not damage plant leaves easily, phytoxicity has been reported when the soaps are improperly used. Do not apply insecticidal soaps when the temperature is above 90 degrees F. and do not apply them when the sun is shining brightly on the plants. Soaps work best when they remain on the leaves for the maximum amount of time possible. Therefore, early morning or late evening are preferred spray times.
Also, to avoid phytotoxicity, do not apply insecticidal soaps to wilted plants or species that are known to be sensitive to them. Highly sensitive plants that are easily damaged include begonia, bleeding heart, fuchsia, gardenia, Japanese maple, lantana, lily, nasturtium portulaca and sweet pea. When uncertain about the sensitivity of a species, it is best to spray only a small area and check in a day or two to see if phytotoxic symptoms are present. The latter include yellow or brown spotting on the leaves, burned leaf tips and edges or leaf scorch.
In general, using insecticidal soaps according to label directions and when temperature and moisture conditions are proper provides an effective and safe approach to control certain garden and landscape pests. Insects that are labeled for control by insecticidal soaps include aphid, whitefly, thrips, plant bugs, spider mites, broad mites, russet mites, scale and leafhoppers. Just recently, insecticidal soaps were labeled for powdery mildew control.
As with any pesticide, always read and follow label directions when using insecticidal soaps.
