Horticultural soaps and oils: an avenue for pest control?

Insecticidal soaps and horticultural oils are pesticides (insecticides, miticides and fungicides) that are used in greenhouse production systems to suppress populations of certain insect and mite pests, and even prevent fungal infections. Insecticidal soaps and horticultural oils are contact pesticides, which mean that thorough coverage of all plant parts is important for the material to contact the pest. Additionally, repeat applications may be necessary due to their short residual activity. Insecticidal soaps and horticultural oils are oftentimes used as surfactants to reduce the surface tension of water, which increases coverage by allowing the spray particles to spread-out over the leaf surface.

Another benefit of insecticidal soaps and horticultural oils is their low mammalian toxicity (high LD50), which means there are less harmful effects associated with human exposure. Furthermore, 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 (described later in this article).

Therefore, these pesticides can be incorporated into rotation programs to reduce the development of resistance to other pesticides. In addition, insecticidal soaps and horticultural oils may inhibit the ability of aphids to acquire viruses in plants, thus reducing the potential transmission to other plants. However, a concern when using insecticidal soaps and horticultural oils is the potential for plant injury (phytotoxicity), which may be influenced by temperature, relative humidity and stage of plant growth. As such, always water plants the day or night before applying insecticidal soaps or horticultural oils in order to minimize plant injury and make applications when the temperature and relative humidity are appropriate. Below are general descriptions of insecticidal soaps and horticultural oils.

Insecticidal soaps

Insecticidal soaps can provide suppression of a variety of insect and mite pests that feed on greenhouse-grown horticultural crops, including aphids, mealybugs, thrips, whiteflies and spider mites. Soaps are substances derived from the synthesis of an alkali such as sodium (hard soap) or potassium (soft soap) hydroxide on a fat. Fats are, in general, a blend of particular fatty acid chain lengths. Soap is a general term for the salts of fatty acids. Fatty acids are the primary components of the fats and oils present in plants and animals.

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

The mode of action of insecticidal soaps is still not well-understood, although there may be four ways by which insecticidal soaps kill insect and mite pests. First, insecticidal soaps penetrate through fatty acids present in the insect’s outer covering (cuticle), which dissolves or disrupts cell membranes resulting in water loss. Consequently, cell integrity is impaired, causing cells to leak and collapse and respiratory functions are destroyed, resulting in dehydration and death. Second, insecticidal soaps may act as insect growth regulators by interfering with cell metabolism and production of growth hormones during metamorphosis (change in form). Third, insecticidal soaps may block the breathing pores (spiracles), thus interfering with respiration. Fourth, insecticidal soaps may uncouple oxidative phosphorylation or reduce the production of energy by inhibiting adenosine tri-phosphate (ATP).

There are a variety of fatty acids; however, only certain fatty acids have insecticidal activity, which is primarily based on 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). Shorter chain fatty acids (9-carbon chains or less) have herbicidal properties, 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 very effective as an insecticidal soap. In fact, most commercially available insecticidal soaps contain potassium oleate (potassium salt of oleic acid). Water quality may impact the activity of insecticidal soaps as hard water reduces the effectiveness of applications.

32 percent of growers say that one of the aspects of their business they’d most like to improve is pest and disease control. Source: 2016 State of the Industry Report

Insecticidal soaps may be directly and indirectly harmful to natural enemies, including predators and parasitoids thus disrupting biological control programs. For instance, ladybird beetle and green lacewing larvae are killed by wet sprays on treated plant leaves. A study showed that insecticidal soap is harmful to the predatory mite Phytoseiulus persimilis. Another study reported that a 4-percent application rate of insecticidal soap resulted in 80 to 99 percent mortality of the predatory mite, Neoseiulus (= Amblyseius) cucumeris. The larval stages of the parasitoid Encarsia formosa are more susceptible to insecticidal soap wet sprays than the adults.

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

Horticultural oils

There are a number of horticultural oils that contain petroleum, paraffinic, mineral or neem as the active ingredient. Horticultural oils may provide suppression of aphids, mealybugs, thrips, whiteflies and spider mites. Thorough coverage of all plant parts is important in order for the material to contact pests and multiple applications will be required due to the short residual activity of horticultural oils. However, be sure to avoid applying horticultural oils when the temperature is greater than or equal to 80 degrees Fahrenheit and relative humidity is greater than 90 percent. The slower that horticultural oil wet residues evaporate from the leaf surface, the higher the likelihood for plant injury. Therefore, horticultural oil applications should be made when temperatures are between 40 and 75 degrees Fahrenheit.

Horticultural oils will kill the eggs of insect and mite pests with multiple modes of action, including: 1) prevent normal gas exchange through the insect cuticle and 2) 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 (spiracles). However, horticultural oils may also penetrate the insect cuticle and disrupt the functionality of the internal contents. Furthermore, horticultural oils may soften the cuticle resulting in a loss of water (dehydration). Eggs and immature life stages may be more susceptible to horticultural oils than adults. Horticultural oils have also been reported to have repellent activity, which decreases egg-laying and feeding. Horticultural oils, like insecticidal soaps, may be directly and indirectly harmful to natural enemies.

Commercially available horticultural oil products for use in greenhouse 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 in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involved plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu