EN57CH20-Vincent ARI 31 October 2011 9:14 Essential Oils in Insect Control: Low-Risk Products in a High-Stakes World Catherine Regnault-Roger,1 Charles Vincent,2,∗ and John Thor Arnason3 1UMR CNRS UPPA 5254 IPREM-EEM, Universite´ de Pau et des Pays de l’Adour, F64000 Pau, France; email: [email protected] 2Centre de Recherche et de Developpement´ en Horticulture, Agriculture et Agroalimentaire Canada, Saint-Jean-sur-Richelieu, Quebec J3B 3E6, Canada; email: [email protected] 3Faculty of Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada; email: [email protected] Annu. Rev. Entomol. 2012. 57:405–24 Keywords First published online as a Review in Advance on biopesticide, botanical, terpenes, phenolics, repellent, fumigant September 19, 2011 The Annual Review of Entomology is online at Abstract ento.annualreviews.org In recent years, the use of essential oils (EOs) derived from aromatic plants This article’s doi: as low-risk insecticides has increased considerably owing to their popularity 10.1146/annurev-ento-120710-100554 with organic growers and environmentally conscious consumers. EOs are Copyright c 2012 by Annual Reviews. easily produced by steam distillation of plant material and contain many All rights reserved volatile, low-molecular-weight terpenes and phenolics. The major plant 0066-4170/12/0107-0405$20.00 Annu. Rev. Entomol. 2012.57:405-424. Downloaded from www.annualreviews.org families from which EOs are extracted include Myrtaceae, Lauraceae, Lami- ∗Corresponding author aceae, and Asteraceae. EOs have repellent, insecticidal, and growth-reducing effects on a variety of insects. They have been used effectively to control preharvest and postharvest phytophagous insects and as insect repellents for biting flies and for home and garden insects. The compounds exert their ac- tivities on insects through neurotoxic effects involving several mechanisms, notably through GABA, octopamine synapses, and the inhibition of acetyl- Access provided by Chinese Academy of Agricultural Sciences (Agricultural Information Institute) on 10/21/16. For personal use only. cholinesterase. With a few exceptions, their mammalian toxicity is low and environmental persistence is short. Registration has been the main bottle- neck in putting new products on the market, but more EOs have been ap- proved for use in the United States than elsewhere owing to reduced-risk processes for these materials. 405 EN57CH20-Vincent ARI 31 October 2011 9:14 INTRODUCTION Risks associated with the use of synthetic insecticides have led to the growth of an environmental Repellent: movement seeking sustainable alternatives in pest control. Evidence of this includes the growth a compound applied to of organic agriculture products that are now found in mainstream supermarkets and the banning skin, clothing, or other of ornamental use of synthetic pesticides in many local jurisdictions (22). Among the alternatives substrates that for pest control, biopesticides had a global market value of approximately US$1 billion in 2010. decreases normal contact time of This market is expected to grow to US$3.3 billion in 2014 (63). Biopesticides encompass a large arthropods with the number of technologies, from microbials to botanicals. Among the botanicals, essential oils (EOs) treated surface are a major category that began to develop with research in the 1980s (90). They are derived Steam distillation: from aromatic plants that, in the course of evolution, developed myriad constitutive and induced a type of distillation in chemical defenses against herbivorous insects (117). The EO market has had the strongest growth which water is added of all the botanical pesticide markets in recent years. Safety and regulatory issues have played a role to the distillation in this growth. Their widespread use as herbal medicines in Europe, Japan, and North America mixture, to allow volatile aromatic has increased confidence in their safety. However, EOs, like many natural products, are not always compounds to be subject to rigorous testing or formal registration (113). distilled with water EOs have many industrial applications: perfumery, cosmetics, detergents, pharmacology, fine vapor at temperatures chemistry, and food production products. As a result, overlapping markets sometimes add im- that avoid degradation portant scientific information but also complicate their interpretation for the biopesticide area. Although EOs have a promising future within the biopesticide market, progress on registration must be achieved before EOs occupy a significant market share of global pesticide markets. Among key reviews on EOs, some are general (26, 61), whereas others focus on microorgan- isms (17, 29, 52), cellular biological activities (7), or insects (5, 46, 48, 50, 51, 90, 92). We provide a critical treatment of EOs as insecticides. After defining them and discussing their main chem- ical components and properties, we focus on their bioactivities relevant to insect management in medical and agricultural contexts from a sustainable development perspective. Registration considerations in both North America and Europe, which enforce the largest and most rigorous legislation about plant protection products, are addressed because they are a key constraint on marketing EOs as insecticides or repellents. PHYTOCHEMISTRY OF ESSENTIAL OILS Plant EOs have been used since antiquity, but the first written description of distillation dates from the thirteenth century by Ibn al-Baitar in Andalusia, Spain (9), after which they were included in the pharmacopeias of European countries (17). EOs are defined as the products obtained from Annu. Rev. Entomol. 2012.57:405-424. Downloaded from www.annualreviews.org hydrodistillation, steam distillation, dry distillation, or mechanical cold pressing of plants (35). The classical preparation method is based on the Clevenger steam distillation apparatus devel- oped in 1928. Today this method has been adapted and scaled up for industrial production. Steam distillation requires large containers because of the low (generally <1%) yield from biomass and is costly because of high temperatures needed for distillation. Citrus peel is an exception because large quantities of oils can be obtained cheaply by cold pressing and conventional distillation. Modern methods of extraction include microwave-assisted process and supercritical fluid extrac- Access provided by Chinese Academy of Agricultural Sciences (Agricultural Information Institute) on 10/21/16. For personal use only. tion (18, 100), which may have great advantages such as lower oxidation of compounds, a feature especially useful for flower fragrances. However, these methods require some cleanup by fraction- ation because higher-molecular-weight lipids are also extracted. The advantage of the original steam distillation process is that it separates a relatively clean secondary metabolite fraction from plants, including mainly low-molecular-weight volatile phytochemicals, chiefly of terpene and phenolic origin, while excluding most primary metabolites and high-molecular-weight secondary 406 Regnault-Roger · Vincent · Arnason EN57CH20-Vincent ARI 31 October 2011 9:14 compounds, which have highly variable structures and modes of action. Chiasson et al. (18) com- pared three EO extraction methods, microwave-assisted process, supercritical fluid extraction, and steam distillation, on two aromatic plants and found that the three methods did not produce the same products. EOs are produced in 17,500 aromatic species of higher plants belonging mostly to a few families, including the Myrtaceae, Lauraceae, Lamiaceae, and Asteraceae. The synthesis and accumulation of EOs are associated with the presence of complex secretory structures such as glandular trichomes (Lamiaceae), secretory cavities (Myrtaceae, Rutaceae), and resin ducts (Asteraceae, Apiaceae) (101). Depending on the species considered, EOs are stored in various plant organs, e.g., flowers (bergamot orange, Citrus bergamia), leaves (lemon grass, Citronella spp.; eucalyptus, Eucalyptus spp.), wood (sandalwood, Santalum spp.), roots (vetiver grass, Chrysopogon zizanioides), rhizomes (ginger, Zingiber officinale;turmeric,Curcuma longa), fruits (anise, Pimpinella anisum), and seeds (nutmeg, Myristica fragrans). EO constituents belong mainly to two phytochemical groups: terpenoids (monoterpenes and sesquiterpenes of low molecular weight) and, to a lesser extent, phenylpropanoids. Terpenoids are major constituents of EOs. Monoterpenes are biosynthesized via the methyl erythritol phos- phate pathway in plastids, which yields the 5-carbon precursors isopentenyl pyrophosphate and dimethylallyl pyrophosphate, which condense via geranyl pyrophosphate synthase to give the monoterpenes (10-carbon) (13). Although isopentenyl pyrophosphate can transfer between com- partments, the monoterpenes and diterpenes tend to be formed in the plastid, where unique cyclases yield the fenchane, bornane, camphane, thujone, and pinane ring structures (13). The sesquiterpenes (15-carbon) are formed via the mevalonate pathway in the cytosol. Monoterpenes present in EOs may contain terpenes that are hydrocarbons (α-pinene), alcohols (menthol, geran- iol, linalool, terpinen-4-ol, p-menthane-3,8-diol), aldehydes (cinnamaldehyde, cuminaldehyde), ketones (thujone), ethers [1,8-cineole (= eucalyptol)], and lactones (nepetalactone) (Figure 1). Sesquiterpenes have a wide variety of structures, more than 100 skeletons, as the elongation of the chain to 15 carbons increases the