Plant essential oils for pest and disease management
Introduction
In spite of widespread public concern for long-term health and environmental effects of synthetic pesticides, especially in Europe and North America, natural pesticides, both of microbial and plant origin, have yet to have much impact in the marketplace. Bioinsecticides, dominated by Bacillus thuringiensis-based products, and botanical insecticides, dominated by pyrethrum-based products, each command little more than 1% of the global insecticide market. However, recent government action in the United States, in the form of the Food Quality Protection Act of 1996, will dramatically restrict the use of many conventional insecticides upon which growers have depended for decades (e.g. organophosphates and carbamates). In turn, this will create a significant market opportunity for alternative products, in particular “reduced-risk” pesticides which are favored by the Environmental Protection Agency in the USA.
Against this backdrop, natural pesticides based on plant-essential oils may represent alternative crop protectants whose time has come. Essential oils, obtained by steam distillation of plant foliage, and even the foliage itself of certain aromatic plants (notably in the families Myrtaceae and Lamiaceae, but in other plant families as well) have traditionally been used to protect stored grain and legumes, and to repel flying insects in the home. Though some of the claims made for these crude preparations have yet to be substantiated through controlled experiments, scientific investigation into the biological activities of these materials proliferated in the past decade. The emerging picture is that certain, specific oils and their chemical constituents have demonstrable contact and fumigant toxicity to a number of economically important insect and mite pests, as well as to plant pathogenic fungi.
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Biological activities of essential oils and their constituents
Contact and fumigant insecticidal actions of plant essential oils have been well demonstrated against stored product pests. Among 22 essential oils tested as fumigants against the bean weevil Acanthoscelides obtectus (Bruchidae), those of Thymus serpyllum (rich in the phenols thymol and carvacrol) and Origanum majorama (rich in terpinen-4-ol) were the most toxic (Regnault-Roger et al., 1993). In a more detailed study, Shaaya et al. (1991) evaluated the fumigant toxicity of 28 essential oils and
Toxicity to insects
The aforementioned studies with insects convincingly demonstrate the fumigant toxicity of essential oils and their constituents. Knockdown activity and lethal toxicity via contact has been demonstrated in the American cockroach (Periplaneta americana) (Ngoh et al., 1998), the German cockroach (Blattella germanica) and the housefly (Musca domestica) (Rice and Coats, 1994, Coats et al., 1991). These studies latter point to an obvious neurotoxic site-of-action. Certain essential oil monoterpenes
Health and environmental impacts
Perhaps the most attractive aspect of using essential oils and/or their constituents as crop protectants (and in other contexts for pest management) is their favorable mammalian toxicity. Some of the pure essential oil compounds are slightly toxic, with rat acute oral LD50 values of 2–3 g kg−1 (viz. carvacrol, pulegone), but an essential oil insecticide consisting of a proprietary mixture of essential oil constituents (EcoSMART Technologies Inc.), resulted in no mortality when fed to rats at 2 g kg
Commercialization of essential oil-based pesticides
In a recent review paper on neem and other botanical insecticides, three barriers to the commercialization of new products of this type were identified: (i) the scarcity of the natural resource; (ii) the need for chemical standardization and quality control; and (iii) difficulties in registration. As the essential oils and their purified constituents have a long history of global use by the food and fragrance industries, and most recently in the field of aromatherapy, many of the oils and/or
Conclusions
Certain plant essential oils and/or their constituents have a broad spectrum of activity against insect and mite pests, plant pathogenic and other fungi, and nematodes. As such, they have considerable potential as crop protectants and for pest management in other situations (e.g. urban pest control). Current information indicates that they are safe to the user and the environment, with few qualifications. As a cautionary note, the essential oils that are most efficacious against pests are often
Acknowledgements
I thank Drs. David Lindsay and Essam Enan for reviewing the manuscript. Laurin Hummelbrunner, Andrew Wan and Emma Gelok provided excellent technical assistance. Plant essential oil research in my laboratory has been funded by EcoSMART Technologies Inc. (Franklin, TN, USA).
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