Pesticides Based on Plant Essential Oils: Phytochemical and

Chapter 2

Pesticides Based on Plant Essential Oils: Phytochemical and Practical Considerations Downloaded by CORNELL UNIV on August 29, 2016 | http://pubs.acs.org Publication Date (Web): August 25, 2016 | doi: 10.1021/bk-2016-1218.ch002

Murray B. Isman* Faculty of Land and Food Systems, University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada *E-mail: [email protected].

Although plant essential oils have long been recognized to possess insecticidal and/or insect repellent actions, commercialization of pesticides based on common plant essential oils dates back less than two decades. In large part commercialization of such pesticides in the U.S.A. was facilitated by exemption of certain oils from regulatory requirements, but other pesticides based on essential oils are beginning to reach the marketplace in the European Union, India and China. Unlike conventional pesticides based on synthetic chemicals, bioactivity of an essential oil in a pest insect cannot always be attributable to the major constituent(s); in some well-documented cases there is internal synergy among constituents of a particular essential oil, with putatively non (or less)-toxic constituents facilitating or enhancing the putatively toxic (active) principles. In addition to chemical considerations, there are a number of practical considerations in commercialization of pesticides based on plant essential oils. These include regulatory requirements and costs thereof (in some jurisdictions), commodity prices of oils used as active ingredients, ongoing availability of oils in large volumes, and chemical consistency of oils. Commercial challenges for essential oil-based pesticides include stability of active ingredient oils in storage and transport, residual action of active ingredients after application, and phytotoxicity on crop and ornamental plants. These considerations and challenges are addressed in this chapter.

© 2016 American Chemical Society Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Recent History of Essential Oils as Pesticides Plant essential oils have a long history of use in the fragrance and flavor industries. More recently, the rise of their use in aromatherapy has expanded markets for essential oil producers. And while there has been documented traditional use of many plants and preparations thereof to repel insects and other pests, there have been hardly any commercially successful repellents (with the possible exception of citronella), let alone insecticides, based on plant essential oils prior to the late 1990s (1, 2). Commercialization of such pesticides in the United States was greatly facilitated by recognition of a formerly little-regarded provision of the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), namely “List 25b – Exempted Products”. Items found on the list, mostly food ingredients or food additives and all “generally regarded as safe” (GRAS), can be used as active ingredients in pesticide formulations and sold without scrutiny and approval by the Environmental Protection Agency (EPA), saving manufacturers months or years normally required for review by the regulatory authority, and the millions of dollars needed to generate toxicological and environmental data supporting the safe use of such products as pesticides. The list, the exact genesis of which remains obscure, includes half a dozen plant oils commonly used as flavoring agents in foods and beverages (3). It is these specific oils and their inclusion in List 25b that have been exploited to produce a wide range of EPA-exempt insecticides, fungicides and herbicides used in agriculture, professional pest control, and consumer (home and garden) markets. EcoSMART Technologies (USA) set the precedent for this movement with its introduction of insecticides for pest control professionals in 1998, followed by the introduction of products for the retail consumer market and agriculture in 2001. The company continues to be the market leader in essential oil-based pesticides to date. As pesticides, essential oils enjoy a broad spectrum of action against arthropods, including pests on crops and ornamental plants, public health pests and disease vectors, and ectoparasites of farm and companion animals (4). Owing to their lack of persistence in the environment, they tend to be compatible with classical biocontrol agents and natural enemies, and generally safe for most nontarget organisms such as fish and wildlife. Their familiarity to most people and long history of safe use in humans is also a factor in their favor. Conversely, they are far less effective than most conventional pesticides, requiring higher rates of application, and their lack of environmental persistence can necessitate frequent reapplication in some contexts.

Essential Oils Used in Pesticides and Commercial Pesticides Containing Essential Oils FIFRA List 25b includes a dozen plant oils, all of which, excluding soybean oil, are normally considered essential oils (Table 1). It also includes three specific compounds, eugenol from clove oil, geraniol from geranium oil, and 2-phenethyl propionate from peanut oil. Curiously missing from the list is orange oil, a large volume commodity (as a byproduct of the citrus industry) with a wide range of uses 14 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

as a cleaning agent and degreaser, and eucalyptus oil, widely used as a flavoring agent and in medicines. Both are used outside of the U.S. as insecticides (5).

Table 1. Plant Oils on FIFRA List 25b – Exempted Products


Plant oil or derivative

Botanical Source(s)a

Plant Family

Cedar oil

Juniperus spp.

Cupressaceae

Cinnamon oil

Cinnamomum spp.

Lauraceae

Citronella oil

Cymbopogon winterianus

Poaceae

Clove oil, eugenol

Syzygium aromaticum

Myrtaceae

Garlic oil

Allium sativum

Amaryllidaceae

Geranium oil, geraniol

Pelargonium graveolens

Geraniaceae

Lemongrass oil

Cymbopogon citratus

Poaceae

Mint oil

Mentha spp.

Lamiaceae

Peppermint oil

Mentha piperita

Lamicaeae

2-phenethyl propionate

Arachis hypogaea

Fabaceae

Rosemary oil

Rosmarinus officinalis

Lamiaceae

Soybean oil

Glycine max

Fabaceae

Thyme oil

Thymus vulgaris

Lamiaceae

List 25b includes the common names shown in the lefthand column, but provides no strict botanical definitions for these. The botanical names shown in the middle column are representative but not inclusive. For some of these, multiple species are known by a single common name.

a

Agricultural insecticides developed by EcoSMART and currently sold under the EcotrolR and EcotekR brand names contain rosemary and peppermint oils and geraniol as active ingredients. Consumer insecticides sold under the EcoSMARTR brand all include rosemary oil, with different products also containing various mixtures of peppermint, cinnamon, thyme and clove oils and 2-phenethyl propionate. Professional pest control products originally developed by EcoSMART but now sold under the EssentriaTM brand are mostly based on rosemary and peppermint oils and 2-phenethyl propionate. TyraTech recently registered two insecticides in Canada containing thyme oil and wintergreen oil (methyl salicylate; from Gaultheria species) as active ingredients. Other minor brands in the U.S. use cedar or geranium oils as active ingredients. An important exception to these EPA-exempt products is RequiemR, the first EPA-registered insecticide based on a plant essential oil. This product, containing an extract from a variety of wormwood (Dysphania [=Chenopodium] ambrosioides nr. ambrrosioides) was first registered in 2011 by AgraQuest but is currently marketed by Bayer CropScience. It was approved for use by the European Commission in 2015. 15 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In South Africa, Prev-AMR is an insecticide/fungicide containing 50% orange oil as an active ingredient; a similar product containing 60% orange oil is sold in France and the Netherlands under the same brand. In the European Union, orange oil is one of only two plant oils approved for use as insecticides, although clove, spearmint, citronella and rape seed oils (and geraniol) are approved for use as insect repellents. In China there are two registered insecticides containing eucalyptol (= 1,8-cineole, from Eucalyptus globulus and related species), five insecticides containing camphor (from camphor wood oil, Cinnamomum camphora), plus fungicides based on eugenol (from clove oil, Syzygium aromaticum) and carvacrol (from oregano oil, Origanum sativum). In India, eucalyptus leaf extract (rich in 1,8-cineole) is approved for use as an insecticide while in Australia an insecticide/miticide has recently been approved containing both eucalyptus and tea tree (Melaleuca alternifolia) oils as active ingredients. Garlic oil can be found as a major active ingredient in insecticides in the U.S., Mexico and Colombia (5).

Phytochemical Considerations: Relationships Between Chemistry of Essential Oils and Toxicity to Insects Commercial botanical insecticides often state a single natural product as the active ingredient, when in fact most plant defensive chemistry consists of suites of natural products derived from a common biosynthetic pathway. Examples include azadirachtin from neem, Azadirachta indica (azadirachtin and a seies of closelyrelated limonoid triterpenes), rotenone from Derris and Lonchocarpus species (rotenone and a series of related isoflavonoids), and pyrethrum from Tanacetum cinerariaefolium (pyrethrins and related esters of chyrsanthemic acids) (6). Within a plant essential oil, the major constituents, monoterpenoids and sesquiterpenoids, can be more diverse and complex. Many plant essential oils comprise up to 80 or more such compounds. In some common essential oils a single constituent can dominate, for example eugenol in clove oil (typically ≥ 80% by weight), cinnamaldehyde in cinnamon oil (≥ 80% by weight), or d-limonene in orange oil (≥ 90% by weight). In others, two constituents can dominate, for example menthol and menthone in peppermint oil (together, 65-85% by weight) or thymol and p-cymene in thyme oil (together 65-80% by weight). In yet other oils, e.g., rosemary oil, 3-5 constituents (or more) collectively make up the majority of the oil by weight. Major constituents of some common essential oils used in pesticides are shown in Table 2. It should not be surprising that in the latter situations, attributing insect toxicity to one or more constituents of an essential oil is anything but straightforward. In an earlier study we attempted to correlate toxicity of 10 commercial rosemary oils to the cabbage looper (Trichoplusia ni) and the true armyworm (Pseudaletia unipuncta) with their chemical compositions (7). We observed slight but significant correlation between two minor constituents (d-limonene and α-terpineol) and toxicity to the cabbage looper, but no significant correlations between constituents and toxicity to the armyworm. These results, 16 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and earlier results testing rosemary oil constituents against the twospotted spider mite (Tetranychus urticae) (8) suggested synergistic interactions among constituents in the oil with respect to toxicity. Other investigators have also reported this phenomenon (9–11). We later confirmed this ‘internal’ synergy among essential oil constituents in a study of the toxicity of the essential oil of Litsea pungens to the cabbage looper (12). More recently we observed in this same insect that most of the toxicity produced by topical administration of rosemary oil can be mimicked or reproduced by a simple binary mixture (in their proportions naturally occurring in the oil) of 1,8-cineole and camphor, two major constituents of the oil (13) Figure 1. Most recently we provided evidence for the mechanism underlying the synergistic interaction between these two compounds: 1,8-cineole facilitates the entry of camphor through the insect’s integument into the bloodstream, where the latter compound is more toxic than the former (14) Figure 2. It is conceivable that other examples of such synergy will be found to be a consequence of enhanced penetration of the putatively toxic constituents by other constituents that, when tested individually, appear to be themselves inactive.

Table 2. Major Constituents of Some Common Plant Essential Oils Plant essential oil

Major constituent(s) (% by weight)a

Rosemary

1,8-cineole (52), α-pinene (10), camphor (9), β-pinene (8)

Peppermint

Menthol (45), menthone (30)

Clove

Eugenol (80)

Cinnamon

Cinnamaldehyde (85)

Thyme

Thymol (58), p-cymene (20), γ-terpinene (8)

Lemongrass

Geranial (48), neral (38)

Eucalyptus

1,8-cineole (85)

Orange

d-limonene (98)

a unpublished data, Berjé Inc. (Carteret, NJ, U.S.A.) and EcoSafe Natural Products Inc. (Saanichton, BC, Canada)

Exacerbating this complex relationship between chemical composition of essential oils and their toxicity to pests is natural chemical variation in composition of the oils that can be a consequence of genetic, geographic, seasonal, climatic or other influences. For example, seasonal variation in chemical content and composition of essential oils has been reported for rosemary grown in southern Spain (15) and basil (Ocimum basilicum) grown in Pakistan (16). Essential oil composition of clove oils varied with country of origin (17). Chemical composition of citronella (Cymbopogon winterianus) varied with time of harvest after transplantation (18), and with daytime temperature. Temperature was also found to be the strongest bioclimatic factor influencing chemical composition 17 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of rosemary oil in the Balkan Peninsula (19). Even significant diural changes in chemical composition have been observed in the essential oil of Ocimum gratissimum growing in Benin (20). Certain species of essential oil-producing plants exist as distinct chemotypes or races that can differ markedly in chemical composition (e.g., wormwood, Artemsia absinthium) (21). To a large extent this variation is managed or mitigated by major producers of plant essential oils who rely on well established supply chains starting with plant biomass that is propagated, grown and harvested consistently following good agricultural practices. Nonetheless essential oils as pesticides constitute a conundrum for regulatory authorities owing to their potential chemical variation, the likelihood that toxicity cannot be attributed to a single, major constituent, and the fact that – at least for use as pesticide active ingredients – there is no strict chemical definition for a particular essential oil. Even the botanical definition of an essential oil can be subject to question; the taxonomic boundries of some of the most common essential oils remain unclear or controversial at best. Faced with this dilemma, a regulatory agency could either recognize a major (and presumably toxic) constituent of the essential oil as the active ingredient of a pesticide product, or alternatively, recognize the intact oil itself as the active ingredient, the lack of definition of the oil noted above notwithstanding.

Figure 1. Toxicity of rosemary oil and its major constituents to 3rd instar cabbage looper (Trichoplusia ni) via topical administration. Based on data from reference (14). 18 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. Penetration of topically applied 1,8-cineole and camphor, administered individually and as a binary mixture, in the cabbage looper, Trichoplusia ni. I = applied individually; M = applied as a mixture. Data from reference (14).

Practical Considerations Regulatory Concerns In the developed world, regulatory approval remains the final, and often most difficult, barrier to overcome in commercializaing a pesticide. Without such approval, no product can be sold and no revenue realized. The only obvious exception to this is the exempt active ingredient status afforded to certain essential oils in the USA as noted above (Table 1). In other jurisdictions, approval for essential oil-based pesticides has been problematic because pesticide regulatory guidelines developed historically to evaluate synthetic pesticides where there is a single active ingredient with no ambiguity. The EU is only now preparing to publish criteria for “low risk active substance” that some essential oils may meet, possibly clearing a path for approval of more pesticidal products based on such oils (22). Regulatory approval is based on review of data on product chemistry, environmental fate, and toxicology to laboratory animals and nontarget organisms including fish, wildlife and pollinators. Some regulatory agencies require efficacy data while others do not. For essential oils used as flavoring agents in foods and beverages, as fragrances in consumer products and cosmetics, and medicinally in aromatherapy, product chemistry and laboratory animal toxicology are often well established and documented, although such data is usually considered proprietary and therefore not freely accessible. Less well documented is the environmental fate of plant essential oils, although published studies consistently indicate that these materials are minimally persistent in the environment owing to their volatility and consequential evaporative loss (23), except in closed containers 19 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


where fumigation is the goal. Fewer still are investigations of the effects of plant essential oils on nontarget organisms, although there is some evidence that they can be used with a margin of safety towards biocontrol agents and fish (24, 25). Essential oils have been evaluated and sometimes used to manage Varroa devastator, an ectoparasitic mite in honeybee colonies, but within the closed confines of a hive the margin of safety to bees is rather narrow (26). Given the widespread use of essential oils in foods, beverages and consumer products, it is not surprising that most of the commonly used oils have minimal or low toxicity to lab animals, typically with rat, oral acute LD50 values >2000 mg kg-1. One notably exception is pennyroyal (from the mint Mentha pulegium) with a rat LD50 of 400 mg kg-1. The major constituent, pulegone, has a rat LD50 of 470 mg kg-1. Most importantly, there are reports that pennyroyal is a potential abortifacent in humans (27). Among the more common essential oils, carvacrol, a major constituent of oregano (Origanum vulgare) oil and a minor constituent of thyme (Thymus vulgaris) oil, is a strong corrosive and skin sensitizer. A more thorough review of the toxicology of essential oils can be found elsewhere (28).

Cost and Availability The agrochemical business is extremely competitive. Even a pesticide with the least environmental and health impacts, or any other desirable traits, will not be successful in the marketplace if it is not cost competitive with other pesticide products available to the user. This is especially the case for pesticides used in agriculture or by pest control professionals, where volumes used are greater and costs of application are significant. What makes the material costs of certain plant essential oils low enough to be attractive for use in pesticides is the large scale international trade in those oils as commodities for use in the flavoring and fragrance industries. It is these latter, well established uses and supply chains that dictate prices; their use in pesticides thus far has been a minor alternative market for producers that has yet to grow large enough to influence prices. Prices for the major essential oils tend to be relatively stable owing to these well developed supply chains. The greatest source of perturbations in price are occasional crop failures in a principal supply region as a result of abnormal weather events. On the other hand, some oils have seen a recent trend toward price reduction as production has moved to geographic regions where land and labor are less expensive, viz. China, India and Brazil. In contrast, the production of certain oils remains largely restricted to regions where the source crops were first successfully cultivated, for example clove and patchouli oils in Indonesia. Overall it must be said that posted prices for any particular essential oil can vary widely depending on the quality, source and volume of the oil in question. Among the least expensive of essential oils is orange oil, in part because the biomass (fruit peels) used for extraction is effectively a waste product of another, very large, industry (orange juice), and in part because the oil can be obtained by cold pressing of the fruit peels rather than via hydrodistillation. Recent commodity prices for the most highly traded essential oils, including most of those use in pesticides, are listed elsewhere in this volume (29). 20 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Commerical Challenges Apart from the obvious issues of cost, availability and consistency, there are a number of challenges for those producing pesticides based on plant essential oils. The first of these is long term stability of a formulation containing essential oils, in storage and transport. Typically an agrochemical product is expected to remain stable for up to two years in storage; short term stability trials at elevated temperatures (e.g., 50°C for 90 days) attempt to simulate this. Though relatively stable, some oils are susceptible to oxidation in the absence of any antioxidants, even though certain oils themselves have been reported to have antioxidant properties. Given the volatility of many constituents of essential oils, the packaging material (bottles) used can be a factor in maintaining chemical consistency and stability of a product containing an essential oil, with the least porous and most chemically inert materials preferable. Another challenge is persistence of an essential oil pesticide, i.e., how long the product remains biologically active against target pests after application to the target area. Often the direct insecticidal action of an essential oil-based pesticide is lost within hours of application, but field trials on horticultural crops suggest that a single application can provide crop protection for up to three weeks (1). Presumably this is because residual concentrations of the oils that are too low to kill insects are sufficient to deter/repel pests for a much longer period of time. Laboratory studies on the behavioral effects of essential oils in insects provide some evidence for this, but direct observations of behavioral effects on pests in the field are lacking. Nonetheless it is desirable to extend the residual life of essential oil-based pesticides, at least in outdoor contexts where efficacy is clearly not based on fumigant activity. Recent advances in pesticide formulation, especially the use of microencapsulation and nanoformulation, appear to provide avenues to solve this problem, possibly extending the residual activity of essential oil-based pesticides from hours to days (or even weeks) in the field (30). A particular challenge for the use of some essential oil-based pesticides in agriculture or for home and garden use is phytotoxicity when application is made directly onto plants. Almost any oil, whether from a natural source or from petrochemicals, can be phytotoxic if applied at concentrations exceeding 2% (as an aqueous emulsion), and in some cases at a concentration as low as 1%. Most of the essential oil-based pesticides in current use are recommended to be used at concentrations in the 0.5-1.0% range, depending on the crop and pest. At higher concentrations (e.g. ≥ 5%), clove oil is sufficiently phytotoxic to be used as an herbicide (31). As the margin of error for certain plants can be quite small, empirical testing of any product on the intended crop under realistic conditions is highly recommended. Formulation is again an avenue to mitigate or eliminate phytotoxicity where the risk to a target crop is considerable. Finally, in agricultural applications it is becoming common practice to tank mix different pesticides, or to use tanks and spray equipment sequentially for different pesticides with limited cleaning. For these reasons it is important to establish the chemical compatibility of an essential oil-based pesticide with other products that growers also use or have a high probability of use in tank mixtures. 21 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Future Sources Research interest in the insecticidal and repellent effects of plant essential oils has grown dramatically since 2000. By 2011, 23% of all published papers on botanical insecticides focused on essential oils, representing almost 5% of all published papers on insecticides (32). Between 2004 and 2014, over 3,600 papers were published on essential oils associated with insects and arthropods, according to the Web of Science. The vast majority of these reported on the screening and bioactivity of relatively exotic essential oils to insects, so the ‘discovery’ end of the R&D spectrum is well represented in the literature. Unfortunately there is a relative dearth of papers toward the ‘development’ end of the spectrum, although to be fair, a high proportion of the research effort in that regard is expected to be protected information (i.e., intellectual property) that might only reach the scientific literature after patent protection has been secured. But it is easy to observe that only a handful of essential oil-based pesticides have seen successful commercialization, relative to the expansive scientific literature devoted to the area. That being said, and noting the practical considerations and challenges described above, what are the prospects for the introduction of new pesticides based on plant essential oils not currently in wide use in other industries? As of this writing, even the most successful essential oil-based pesticides occupy only a paper-thin share of the global insecticide market, although all indications are that they are at a very earlier stage in that market and are poised for faster market growth over the next decade than conventional pest management products. Perhaps if these predictions have any accuracy, essential oil-based pesticides will become significant participants in the rapidly expanding biopesticide market and in so doing make the prospects for the introduction of pesticides based on exotic oils more economically attractive. One such example that has been subject to an extensive research effort in recent years is the essential oil from wormwood, Artemisia absinthium (Asteraceae) growing in Spain. This aromatic plant with a long history of medicinal use is also the botanical source of absinthe, a bitter liqueur banned for many years owing to its human toxicity, attributable to its major constituents, αand β-thujone. Thujone-rich oils have also been demonstrated to be insecticidal. Although commercial samples of wormwood oil are often rich in thujones, there are numerous chemotypes of the species, some of which lack these compounds entirely (21). A series of thorough investigations of domesticated plants collected from two locations in Spain and grown under both field conditions and controlled conditions (in a growth chamber, greenhouse and in aeroponic culture) have documented chemical variations in their essential oils (33). The major constituents of essential oils produced from these cultivated plants, which lack thujones, are cis-epoxycimene, chrysanthenol and chrysanthenyl acetate. Though less active than thujones as antifeedants to the green peach aphid (Myzus persicae), bird cherry oat aphid (Rhopalosiphum padi) and the cotton leafworm (Spodoptera littoralis), oils containing the above noted compounds are effective antifeedants. Interestingly, bioactivity to insects was not well correlated to concentrations of any of the three compounds, providing another example of internal synergy 22 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


amongst constituents of an essential oil. These oils and extracts also have valuable bioactivity against the human parasites causing leshmaniasis and trypanosomiasis (33, 34). Rather striking quantitative year-to-year differences (up to 10-fold for individual constituents) in oil composition were found in cultivated plants from the two populations, and these in turn differed from plants grown under controlled conditions and from wild conspecifics (33, 34). However, supercritical fluid extraction of these plants gave much higher yields of the three compounds relative to hydrodistillation or organic solvent extraction, and the SF extracts were significantly more bioactive against all three pests (11, 35). Thujone-free cultivated A. absinthium appears to have good potential for the production of essential oils and extracts that can be formulated as biopesticides and for use as antiparasitic agents, but production practices will require standardization to achieve the consistency seen in other essential oils produced commercially over many decades (29).

Prospectus and Conclusions As pesticides, plant essential oils should continue to make inroads into the marketplace, especially as the arsenal of conventional pesticide products becomes increasingly constrained, and consumers become ever more discerning about pesticide residues in the food supply, the workplace and the outdoor environment. They should find increasing use in organic production of high-value fruits and vegetables, and with increasing global urbanization, in professional pest control, consumer products and for vector control. Jurisidictions where pesticides are highly regulated may in the near future take a more favorable view of products based on essential oils, in particular those oils with a long history of human use and safety. Economics of production, driven by much larger markets (the flavor, fragrance and aromatherapy industries), will continue to dictate which essential oils are likely to be formulated as pesticides. It will indeed be interesting to see if a botanical source like wormwood can overcome the barriers and meet the challenges to become a new crop based on extracts and oils whose main purpose are for use as biopesticides and medicinal products. Apart from purely descriptive biology and toxicology, our understanding of the pesticidal properties of essential oils and their constituents in insects, fungi and plants (as herbicides) remains somewhat rudimentary. As such, there is tremendous scope for selecting and tailoring plants to produce essential oils with enhanced efficacy as pesticides, for blending essential oils and/or constituents to achieve higher pesticidal efficacy or a broader spectrum of action, and in formulation to prolong residual action where desirable.

23 Jeliazkov (Zheljazkov) and Cantrell; Medicinal and Aromatic Crops: Production, Phytochemistry, and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Acknowledgments David Herbst (Berjé Inc.) generously provided valuable insight on the essential oil industry to the author, as did Rod Bradbury (EcoSafe Natural Proucts Inc.) on formulation chemistry. Dr. Rita Seffrin assisted with literature review. Supported in part by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC).

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