Soaps & oils: tools for your IPM program

Features - Plant health

Understand the pros and cons of using horticultural soaps and oils for pest control in vegetable crops.

September 19, 2022

Horticultural soaps and oils may help control pests, like mealybugs.
Photo by raymond cloyd

Insecticidal soaps and horticultural oils are pesticides (insecticides, miticides and fungicides) that can be used in greenhouse vegetable crop production systems to manage populations of certain insect and mite pests, and even prevent fungal infections. Insecticidal soaps and horticultural oils are contact pesticides, and thorough coverage of all plant parts is important to ensure contact with pests. Repeat applications will be required due to the short residual activity of these soaps and oils. They are commonly used as surfactants to reduce the surface tension of water, which increases coverage by enhancing the spread of the spray particles over the leaf surface.

A benefit of insecticidal soaps and horticultural oils is their low mammalian toxicity (high LD50), meaning there are less harmful effects associated with human exposure. The probability of insect and/or mite pest populations developing resistance to insecticidal soaps and horticultural oils is very low due to their multiple modes of action.

Insecticidal soaps and horticultural oils can be incorporated into rotation programs to reduce the development of resistance to other pesticides. They may inhibit the ability of aphids to acquire viruses from plants, which may reduce transmission and subsequent spread of viruses to other plants. Insecticidal soaps and horticultural oils have the potential to injure plants, which may be influenced by temperature, relative humidity and stage of plant growth. Always water plants the day or night before applying these soaps and oils to minimize plant injury.

Insecticidal soaps

Soft-bodied insect and mite pests, such as aphids, mealybug nymphs, thrips, whiteflies and spider mites are susceptible to soap applications. In general, insecticidal soaps have minimal activity on beetles and other hard-bodied insects due to the insect’s thickened cuticle, which inhibits penetration of the soap solution. Insecticidal soaps are only effective when insect and mite pests come into contact with wet sprays. Dried residues on plant surfaces have minimal (if any) activity on insect or mite pests because the residues degrade rapidly. Insecticidal soaps are most effective on larvae, nymphs and adults of soft-bodied insects and mites with minimal activity on eggs.

The mode of action of insecticidal soaps still isn't well understood, but there are four ways insecticidal soaps kill insect and mite pests. They penetrate through fatty acids present in the insect’s outer covering, which dissolves or disrupts cell membranes, resulting in water loss. Cell integrity is impaired, causing cells to leak and collapse, and respiratory functions are destroyed, leading to dehydration and death. Insecticidal soaps may act as insect growth regulators by interfering with cell metabolism and production of growth hormones during metamorphosis. Insecticidal soaps may block the breathing pores, interfering with respiration. Insecticidal soaps may uncouple oxidative phosphorylation or reduce production of energy by inhibiting adenosine tri-phosphate (ATP).

There are a variety of fatty acids, but only certain fatty acids have insecticidal activity, which is associated with the length of the carbon-based fatty acid chains. Most insecticidal soaps with insect and mite pest activity are composed of long chain fatty acids (10- or 18-carbon chains); whereas, shorter chain fatty acids (9-carbon chains or less) have herbicidal activity, so using materials containing short chain fatty acids can kill plants. For example, oleic acid, an 18-chain carbon-based fatty acid that is present in olive oil and other vegetable oils, is effective as an insecticidal soap. Most commercially available insecticidal soaps contain potassium oleate. Water high in dissolved minerals such as calcium and magnesium can reduce the effectiveness of insecticidal soap applications.

Insecticidal soaps may be directly and indirectly harmful to biological control agents, including predators and parasitoids, which can disrupt biological control programs.

M-Pede, which contains potassium salts of fatty acids (49% active ingredient), is a commercially available insecticidal soap product registered for use in greenhouses.

Horticultural oils

There are several horticultural oil pesticides that contain mineral or clarified hydrophobic extract of neem oil as the active ingredient. Horticultural oils may help manage populations of aphids, mealybugs, thrips, whiteflies and spider mites. Thorough coverage of all plant parts is important to ensure contact with insect and/or mite pests and multiple applications will be required due to the short residual activity of horticultural oils. Avoid applying horticultural oils when the temperature is =80° F and relative humidity is >90%. The longer it takes for horticultural oil wet residues to evaporate from the leaf surface, the higher the potential for plant injury. Apply horticultural oils when temperatures are between 40-75° F.

Horticultural oils kill the eggs of insect and mite pests by means of multiple modes of action. They prevent normal gas exchange through the insect cuticle and interfere with water balance inside the egg, soften or dissolve the egg covering, or interfere with hormone or enzyme activity. When used against larvae, nymphs or adults, horticultural oils act by means of suffocation by blocking the breathing pores. Horticultural oils may also directly penetrate the insect cuticle and disrupt the functionality of the internal contents. In general, eggs and immature life stages are more susceptible to horticultural oils than adults. Horticultural oils are reported to have repellent activity, which can decrease egg-laying and feeding. Horticultural oils may be directly and indirectly harmful to biological control agents.

Commercially available horticultural oil products for use in greenhouse vegetable production systems include Ultra-Pure Oil and SuffOil-X (active ingredient=mineral oil) and Triact (active ingredient=clarified hydrophobic extract of neem oil).

Raymond is a professor and extension specialist at Kansas State University, rcloyd@ksu.edu.