Fish oils are very similar in chemical composition to plant oils. They are mostly by-products of the fish processing industry. Fish oil products are often combined with plant oils, and the fish oil is listed as an inert ingredient. Some fish oil products are certified organic. One limitation of plant and animal oils is that their natural origin contributes to a wide variation in composition and quality. This depends on geographic origin and species from which the oil is extracted. There are no well defined standards for quality and use of plant and animal oils.
Regardless of the source or type, all oil-based products have a similar mode of action. Insecticidal oils kill insects on contact by disrupting gas exchange (respiration), cell membrane function or structure. They also kill them by disrupting their feeding on oil covered surfaces. Their toxic action is more physical than chemical and is short-lived. When used against plant pathogens, oils may smother fungal growth and reduce spore germination on treated surfaces. They are mostly fungistatic, stopping fungal growth rather than killing the pathogens. Stylet oils are highly refined oils and may control insect-vectored plant viruses in addition to insects, mites and fungal pathogens. These oils reduce the ability of aphids to acquire the virus from an infected plant and transmit it to healthy plants. Stylet oils may interfere with the virus's ability to remain in aphid mouthparts (stylets). Some plant oils that contain sulfur compounds, such as neem oil, may possess additional fungicidal activity compared to petroleum oils.
Oil-based pesticides have low residual activity and must be sprayed directly on the insect or mite. To combat plant fungal pathogens, oils generally must be applied prophylactic ally prior to infection. Repeated applications of oils may be needed to achieve desired levels of control. Scale insects can be controlled with horticultural oils. Target Pests and Diseases Oils are most effective against soft-bodied arthropods. They are most commonly used against mites, aphids, whiteflies, thrips, mealy bugs and scale insects. Dormant oil sprays are also used against over-wintering eggs and scales. Horticultural and plant oils are commonly used to suppress certain fungal diseases, like powdery mildew and black spot on rose. Stylet oils may be used to manage insect-vectored plant viruses.
While oil treatments have historically targeted fruit trees and woody ornamentals, several different types of pesticide oils are currently marketed for house plants, flowers and vegetables. Commercial oil products include emulsifiers to enable the oil to mix readily with water. These emulsifiers are generally considered to be inert, but may have some insecticidal properties. Oil formulations are generally designed to be mixed with water at concentrations of 0.5-2.0 percent (volume/ volume). When applying oils, it is best to agitate hand pump sprayers frequently and keep tank spray agitators running. This reduces the risk of oil separation that could result in sprayer clogging, uneven plant coverage and possible plant injury.
Oils will separate from the carrier. Agitation is necessary to keep oils in solution. When mixed with other pesticides, oils can enhance application efficiency. Oils often act as surfactants, and improve plant coverage and penetration of pesticides into leaf surfaces. Always read pesticide product labels carefully to make sure the product can be mixed with oil. Most labels prohibit the use of sulfur pesticides within 30 days of oil treatment. Oils may be incompatible with copper applications in some crops.
Although generally considered safe, oils can injure susceptible plant species. Symptoms of plant injury (phytotoxicity) may be acute or chronic. They can include leaf scorching and browning, defoliation, reduced flowering and stunted growth. Phytotoxicity may be associated with plant stress, ambient temperature and humidity, and application rate. It can vary among plant species and cultivars. To reduce the risk of phytotoxicity, do not treat stressed plants. Apply when conditions are below 85 °F degrees and 90 percent humidity. Applications during the summer season are best in the morning or late evening. The longer wet oil sprays remain on foliage, the greater the chance of phytotoxicity.
During winter it is best to apply oils only when temperatures are above 40 °F. Apply dormant oils or higher rates of summer oils only after stems and buds have become winter-hardened and before buds begin to swell in the spring. Evergreen trees generally should be treated only by summer rates of all-season oils. Some evergreens, especially those with a glaucous (waxy) coat, may become discoloured following an oil application. This usually does not harm the tree or shrub.
When treating a new kind of plant, it is best to apply horticultural oils to part of the plant or to a few small specimens before treating large quantities of foliage. With oils it is especially important to read, and follow label instructions and recommendations. Manufacturers' labels provide useful information about sensitive plant species based on extensive testing. Some plants most commonly listed as being oil sensitive include azalea, carnation, fuchsia, hibiscus, impatiens, photinia, rose, cryptomeria, juniper, Japanese holly and spruce.
Oils have many characteristics that make them desirable to growers and homeowners. For example, they are low in toxicity to humans, wildlife and pets. Since oils are only active for a short time, they do not Scale insects can be controlled with horticultural oils. Oils will separate from the carrier. Agitation is necessary to keep oils in solution. Affect insect predators or parasitoids unless they are exposed to the direct spray. Oils evaporate quickly and do not generally contaminate the soil or groundwater sources. Plant and fish oils are broken down rapidly by microorganisms on plants or soil, and pose minimum risk to non-target organisms. Oils are also considered one of the few classes of pesticides to which insects and mites have not developed resistance.