Joel R. Coats, Distinguished Professor, Department of Entomology, Iowa State University provides an introduction and analysis of plant essential oils – nature’s insecticides
As long as humans have been aware of pests, we have searched for ways to control the pests. People have tried using ashes, salts and other minerals, and plants, as some of the early natural approaches. Many species of plants have been brought to bear against insects, ticks, mites, and other arthropods, but one of the most widely distributed botanical types of insecticides has been the plant essential oils.
Numerous aromatic plants contain substantial amounts of terpenes that constitute the oils that are responsible for the aroma, or “essence”, of those plants. Other kinds of plant oils primarily consist of fatty acids or esters of them, but they are biologically quite distinct from the essential oils. Some familiar essential oils are those from mints, cedar, pine, wintergreen, eucalyptus, camphor, rosemary, lavender, lemongrass, citronella, and citrus.
The essential oils have been used for centuries to protect foods, fibre, stored products, humans, and their animals. They are ubiquitous, with hundreds of them commercially available and used as fragrances in many personal care products and cleaning agents, as well as deodorisers and incense; the food and drink industries utilise them as flavours and fragrances as well. Due to their long history of use in those enterprises, the essential oils are generally considered safe. In addition, they are effective as protectants, leave no residue, and generally have agreeable fragrances.
Historically, plant essential oils have been obtained from plants through steam distillation. In this process, plants are harvested while fresh and then subjected to the distillation; water is boiled with the plants, and a condenser then cools the steam and collects the distillate. Many of the terpenes in the plant essential oils co-distil with the water, and the collection vessel typically contains water plus a film or droplets of the desired oil. Current technology for obtaining the essential oils from mints and catnip entails harvesting the live plants in large wagons, then covering the top except for an outlet to a collection container; then steam is injected into the wagon from the bottom and the steam distillation proceeds.
One alternative method of obtaining the essential oils from the plants is pressing the fresh plants or the seeds to express the oil. Another method involves the use of organic solvents to extract the fat-soluble, or lipophilic, materials from the plant tissues; hexane, ethyl acetate, and ethanol have been used, but even liquid carbon dioxide is used, in a process called super-critical fluid extraction. While the steam distillation approach only collects the volatile organic fraction from the plants, a much broader spectrum of organics is harvested through the pressing or extracting methods.
Why do plants produce essential oils?
Why do they produce various compounds that are not directly involved in germination, growth, and reproduction, often termed secondary plant compounds? As scientists explore their functions in the plant, their value becomes clearer; they are frequently part of the plant’s ‘chemical defences’, helping protect them against many herbivores, pathogens, and competitors. Monoterpenes in many oils are: antifeedants, which deter herbivores of all types; insecticides or repellents, which limit the damage the plants may sustain from insects and mites; antimicrobials, which are capable of killing or inhibiting the growth of bacteria and fungi that try to establish on the plants; herbicides, to reduce competition from other nearby plants, often through terpenes leaching out of dropped leaves, awls, or needles.
The protective strategy is not the only way plants use their secondary metabolites; volatiles are often involved in attracting certain pollinators, from insects to bats. Not surprisingly, co-evolution has allowed for the development of multiple layers of complexity, such as insects that exploit the plants volatiles to home in on the best host species for feeding or laying eggs, and there are parasites and predators of insects that use the plant’s volatiles to locate favourable host insects or prey, respectively. A whole world of important chemical signaling is in constant play, although human noses are mostly too insensitive to be aware of it.
What are the properties of the monoterpenes in plant essential oils that imbue them with insecticidal activity? Like most classes of insecticides, the terpenes are lipophilic, which is typically considered critical to the insecticides’ penetration through the insect cuticle and into the body, and they need to have an impact at a target site in the insect. Other relevant properties include their volatility, especially the monoterpenes; due to their low molecular weight and nonpolar (lipophilic) nature, they vaporise readily. As a result, they do not leave a residue, which is a mixed blessing: not exhibiting much residual activity to kill insects, but also causing no concerns for unwanted residues on crops or in a building.
Because of their volatility and their unique fragrances, many of the plant essential oils have insect-repellent properties. In some cases, it has been shown that very low levels of oil can still repel insects long after a lethal concentration has dissipated. Thus, certain uses of the essential oils have a functional residual effect, even though it isn’t a lethal effect at one week or more.
Utility against resistant insects
One of the most perplexing issues in insect pest management continues to be insect populations that develop resistance to conventional insecticides, especially after they have been used multiple times per year or for many years. Populations of insect pests that can number in the millions or billions have impressive adaptability, due to that enormous gene pool and innumerable opportunities for mutations. Even a few resistant survivors can multiply quickly and eventually become the dominant proportion of the population. Consequently, the growers and consumers are constantly searching for novel products that will control the resistant populations.
Insect toxicology teaches that the optimal strategy for overcoming resistance is to switch to using a different class of insecticides. Insect pests such as mosquitoes, aphids, whiteflies, armyworms, and many others have, in some locations and on certain crops, become resistant to multiple classes of insecticides. One of the most widely available and under-utilised classes of insecticides is the plant essential oils and their constituent terpenes and terpenoids. The world, including the agrochemical industry, is awakening to the promise of this class of natural products which are safe, effective, and structurally varied, and have virtually no residue issues.
*Please note: This is a commercial profile