Essential Plant Oils and Their Beneficial Properties

Definition and Properties

Essential plant oils are defined as any volatile oil(s) with strong aromatic components that give distinctive odor or flavor to a plant. These aromatic compounds are metabolic byproducts and are often referred to as “volatile plant secondary metabolites.” Essential oils are found in the glandular hairs (trichomes, for example), or secretory cavities of plant cell walls and are present as droplets of fluid on the leaves, stems, bark, flowers, roots, and/or fruits of various plants. Aromatic oils serve a wide range of beneficial functions for the plant such as attracting or repelling insects and grazing animals, preventing diseases, killing molds and mildew, protection from heat and cold, and a means of communicating with nearby plants. In short, constituents of essential oils provide an important defense mechanism for the plant. Although essential oils are used commercially as food additives, flavorings, and fragrances they are produced for the benefit of the plant, not for the pleasure of humans.

Essential oils are usually steam distilled from aromatic plants. Although it is possible to target specific constituents using fractional distillation, steam-distilled essential oils are complex organic cocktails consisting of dozens of organic compounds. Monoterpenes (hydrocarbons with 10 carbon atoms) or sesquiterpenes (hydrocarbons with 15 carbon atoms) are major constituents of essential oils, with other volatile compounds such as benzene derivatives (including phenols), and other miscellaneous hydrocarbons and organic compounds mixed in as well. These are lipophilic (fat loving) molecules with high vapor tension. Typically, these oils are liquid at room temperature and are easily transformed from a liquid to a gaseous state at slightly higher temperatures. Essential oils have low toxicity to mammals, degrade rapidly in the environment, and demonstrate specificity in their biological activity that favors beneficial insects. In addition, target pests are less likely to develop resistance to essential plant oils because they contain a complex mixture of insecticidal compounds with different modes of action. These characteristics make essential plant oils ideal candidates as contact insecticides.

The use of plant essential oils to control pests is not new. Ancient cultures used locally available plants such as Derris and Nicotiniana to control pests. In the United States, pesticidal plants were widely used until the 1940s when they were replaced with synthetic pesticides. With the introduction of synthetic fertilizers at about the same time, modern agriculture was born. However, overuse of these pesticides soon led to problems that were unforeseen at the time of their introduction. Almost eighty years later, the use of synthetic compounds in agriculture is being restricted worldwide because they are carcinogenic, cause developmental malformations, have high residual toxicity, cause hormonal imbalances and infertility, and leave harmful residues on foods. Furthermore, they upset the delicate balance of nature by killing beneficial insects and pollinators, disrupting natural predators, contaminating groundwater, and eliminating beneficial soil microbes. Although these products seemed robust at first, their ability to control pests has become compromised over time. These problems include the evolution of resistant insects and resurgence of treated populations, and the outbreak of secondary pests normally kept in check by their natural enemies (Dubey et al., 2011). The World Resources Institute has reported that more than 500 insect and mite species are resistant to one or more insecticides (WRI, 1994). Pests are becoming tolerant to traditional pesticides resulting in double or triple application rates (Stoll, 2000). When all of these risks and unanticipated consequences are factored in, the rationale for using synthetic pesticides is questionable.

The plant kingdom is the most efficient synthesizer of organic compounds. Research and anecdotal evidence has shown that some of these compounds are very effective insect and pathogen control agents (Isman & Akhtar, 2007). These natural defense mechanisms are the focus of much of our research at GardenCare Naturals. In particular, we are concerned with the biological activity of the 15 essential oils allowed by the EPA as active ingredients in minimum-risk pesticides (also referred to as 25B/4A compliant). The purpose of the minimum-risk designation was to encourage companies to develop natural pesticides by eliminating the high cost of federal registration. These oils are shown in Table 1.1.

Table 1.1. Plant Essential Oils and Constituents Approved for Use in Minimum-Risk Pesticides
castor oil, cedar oil, cinnamon oil, citronella oil, clove oil, eugenol, garlic oil, geranium oil, geraniol, lemongrass oil, mint oil, peppermint oil, rosemary oil, thyme oil, 2-phenethyl propionate

Modes of Action

The biological activity of plant essential oils and their constituents has been studied extensively. Research literature provides excellent reviews of effectiveness, toxicity, comparative analyses of different oils, and meta-analyses of numerous studies and their data (Choi et al., 2003; Tripathi et al., 2009; Brud, 2010; Gora et al., 1988; Bakkali et al., 2008; Xu et al., 1994) It is difficult, however, to draw hard-and-fast conclusions from these published resources. For example, to determine whether rosemary oil is more effective than lemongrass oil it may be necessary to generalize data from research on pests or pathogens other than the one(s) of interest, data obtained using different plant species, outcomes from different formulations of the essential oils or their components (variations in concentration of applied product, how the product was applied – fumigant or contact spray, etc.), type of emulsion – colloidal suspension, microemulsion, nanosuspension, etc.), possible differences in terpene profiles of the oils used, and so on. Consequently, published literature outlined the landscape of our investigations, but in most cases it was necessary for us to conduct our own research to find the answers.

It is well documented that essential plant oils protect plants from insects and pathogens in a variety of ways. They can work as antifeedants, ovicides, repellents or attractants, and interrupt reproduction or modify development (Isman, 2000; Regnault-Roger, 1997; Rice & Coats, 1994; Singh & Upadhyay, 1993; Choi et al., 2002; Choi et al., 2004; Huang et al., 1998; Petrakis et al., 2005). Lipophilic plant oils are known to kill insects by penetrating or degrading the waxy cuticle causing desiccation (dehydration). Evidence suggests that once permeated through the cuticle, certain components in essential oils block neurotransmitters, digestive enzymes, or growth hormones (Sampson et al., 2005; Tarelli et al., 2009). In addition, the rapid effect of essential oils suggests that there may be a neurotoxic effect on pests (Enan, 2001).

It was stated earlier that essential oils are organic cocktails. As such, it is not surprising that an essential oil can have more than one mode of action against target pests and pathogens. For example, peppermint oil works as an insecticide and it is antimicrobial (kills mildew) as well. With regard to peppermint oil, the underlying mechanisms responsible for its biological activity are unclear. It seems to work as a fumigant. However, our research has shown it to be one of the most effective oils and it is among the least toxic to plants. This research prompted us to develop our first product, Peppermint Fury™.

Writing this article was a challenge because there is so much information to cover. Entire books have been written on the topics reviewed here. Knowing that our readers and beta testers are sophisticated in their search for knowledge, we have provided some references for further study. It is a complex subject with important implications and new information is being added as research findings emerge from around the world.

References

Bakkali, F.S., Averbeck, D. Averbeck, and D. Idaomar. 2008. Biological effects of essential oils: A review. Food and Chemical Toxicity. 46: 446-476.

Brud, W.S. 2010. Handbook of essential oils: Science, technology, and applications. CRC Press, Boca Raton, USA. ISBN #978-14200-63158.

Choi, W., B. Park, S. Ku, and S. Lee. 2002. Repellent activities of essential oils and monoterpenes against pulex pipiens paliens. Journal of the American .Mosquito Control Association, 18, 348-351

Choi, W., E.H. Lee, B.R. Choi, H.M. Park, and J. Ahn. 2003. Toxicity of plant essential oils to Trialeurodes vaporariorum. Horticultural Entomology, 96: 1479-1484.

Choi, W., S. Lee, H. Park, and J. Ahn. 2004. Toxicity of plant essential oils to Tetranychus urticae and Phytoseiulus persimilis. Journal of Economic Entomology, 97: 553-558.

Dubey, N.K., R. Shikla, A. Kumar, P. Singh, and B. Prakash. 2011. Global scenario on the application of natural products in integrated pest management. In Natural Products in Pest Management by Dubey, N.K., CABI, Oxfordshire, UK, ISBN #9781845936716, pp. 1-20.

Enan, E. 2001. Insecticidal activity of essential oils: octopaminergic sites of action. Comparative Biochemical Physiology, 130: 325-337.

Gora, J., A. Kurowska, D. Kalemba. 1988. Effect of essential oils and some of their constituents on insects. Wiadomosci Chemiczne, 42: 565-575.

Huang,Y., S.K. Hee, and S.H. Ho. 1998. Antifeedant and growth inhibitory effects of of alpha-pinene on the stored product insects Tribolium castaneum and Sitophilus zeamais. International Pest Control, Jan/Feb: 18-20.

Isman, M.B. 2000. Plant essential oils for pest and disease management. Crop Protection, 19, 603-608.

Petrakis, P.V., V. Roussis, D. Papadimitriou, C. Vagias, and C. Tsitsimpikou. 2005. The effect of terpenoid extracts from 15 pine species on the feeding behavioral sequence of the late instars of the pine processionary caterpillar Thaumetopea pityocampa. Behavioral Process, 69: 303-322.

Regnault-Roger, C. 1997. The potential of botanical essential oils for insect pest control. Integrated Pest Management, Rev. 2: 25-34.

Sampson, B.J., N. Tabanca, N. Kirimer, B. Demirci, K. Baser, I. Khan, J. Spiers, and D. Wedge. 2005. Insecticidal activity of 23 essential oils and their major compounds against adult Lipaphis pseudobrassicae. Pest Management Science, 61: 1122-1128.

Singh, G., and K. Upadhyay. 1993. Essential oils: A potent source of natural pesticides. Journal of Scientific and Industrial Research, 52: 676-683.

Stoll, G. 2000. Natural crop protection in the tropics: Letting information come to life. 2nd Ed., MargrafVerlag, Weikersheim, Germany, ISBN #13-9783823613176376.

Tarelli, G., E. Zerba, and R. Alzogaray. 2009. Toxicity to vapor exposure and topical application of essential oils and monoterpenes on Musca domestica. Journal of Economic Entomology, 102: 1383-1388.

Tripathi, A., S. Upadhyay, M Bhuiyan, and P. Bhattacharya. 2009. A review on prospects of essential oils as biopesticide in insect pest management. Journal of Pharmacognosy and and Phytotherapy, vol. 1(5).

WRI, 1994. World Resources, 1994/1995. Oxford University Press, USA, ISBN #13-978-0195210453, p. 416.

Xu, H., and S. Chiu. 1994. Application of essential oils for the control of insect pests. Tianran Chanwu Yanjiu Yu Kaifa. 5: 82-88.