Plant Essential Oils and Formamidines As Insecticides/Acaricides: What Are the Molecular Targets? Wolfgang Blenau, Eva Rademacher, Arnd Baumann
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Plant essential oils and formamidines as insecticides/acaricides: what are the molecular targets? Wolfgang Blenau, Eva Rademacher, Arnd Baumann To cite this version: Wolfgang Blenau, Eva Rademacher, Arnd Baumann. Plant essential oils and formamidines as in- secticides/acaricides: what are the molecular targets?. Apidologie, Springer Verlag, 2012, 43 (3), pp.334-347. 10.1007/s13592-011-0108-7. hal-01003531 HAL Id: hal-01003531 https://hal.archives-ouvertes.fr/hal-01003531 Submitted on 1 Jan 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Apidologie (2012) 43:334–347 Review article * INRA, DIB and Springer-Verlag, France, 2011 DOI: 10.1007/s13592-011-0108-7 Plant essential oils and formamidines as insecticides/ acaricides: what are the molecular targets? 1 2 3 Wolfgang BLENAU , Eva RADEMACHER , Arnd BAUMANN 1Institut für Bienenkunde (Polytechnische Gesellschaft), Goethe-Universität Frankfurt am Main, Karl-von-Frisch-Weg 2, 61440 Oberursel, Germany 2Institute of Biology, Freie Universität Berlin, 14195 Berlin, Germany 3Institute of Complex Systems—Cellular Biophysics-(ICS-4), Forschungszentrum Jülich, 52425 Jülich, Germany Received 16 May 2011 – Revised 29 August 2011 – Accepted 21 October 2011 Abstract – The parasitic mite Varroa destructor is the main cause of the severe reduction in beekeeping during the last few decades. Therefore, efforts have been made to develop chemical treatments against the parasite. In the past, synthetic products were preferentially used to combat Varroa mites. Nowadays, mainly plant essential oils and organic acids are applied because they are safer and impose less unfavorable effects on the environment. Essential oils contain mixtures of mostly volatile and odorous terpenoid constituents. The molecular targets of these substances are tyramine and/or octopamine receptors that control and modulate vital functions ranging from metabolism to behavior. Disturbing the native function of these receptors in the mite results in deleterious effects in this parasite. This overview considers not only tyramine and octopamine receptors but also other potential targets of essential oils including ionotropic GABAA receptors, TRP type ion channels, and acetylcholinesterase. GABA / G protein-coupled receptor / octopamine / thymol / tyramine 1. PLANT ESSENTIAL OILS— as acetylcholinesterase (AChE) or that manipulate AN INTRODUCTION the overall metabolic status of the parasite (for a review, see Isman 2006). Substances The availability of environmentally safe and such as synthetic pyrethroids inhibit voltage- efficient chemicals against arthropod pests is an gated sodium channels that are important for important aspect for veterinary and crop the firing of neuronal action potentials. Phenyl- industries. At present, chemical compounds pyrazolines or polychloroalkanes (e.g., lindane) that rapidly deliver their insecticidal effect block the activity of GABAA channels, important are preferentially used to achieve a timely members of the ligand-gated ion channel family reduction of pests and ectoparasites on their involved in inhibitory signal generation in respective hosts. This condition is fulfilled by neurons. Organophosphates inhibit the enzymatic compounds that modify the activity of voltage- activity of AChE, an enzyme controlling the gated and/or ligand-gated ion channels in the concentration of the excitatory neurotransmitter central nervous system (CNS) and by compounds acetylcholine (ACh) in the synaptic cleft. Iso- that impair the activity of neuronal enzymes such flavonoids such as rotenone poison mitochondria and thus cause the breakdown of cellular energy Corresponding author: W. Blenau, production. Metabotropic (G protein-coupled) [email protected] octopamine receptors are the target of formami- Manuscript editor: Bernd Grünewald dine pesticides such as amitraz which manipulate Molecular targets of plant essential oils 335 cellular signal transduction processes. Despite the Similar to their heterogeneous composi- efficient and wide use of these substances, they tion, plant essential oils display a broad seem to harbor certain disadvantages. (1) Parasites spectrum of biological activity. They can be can develop resistance to the chemicals, probably toxic for insects and microbes, but can also because the generation times of the arthropod act as insect antifeedants and repellents pests are short. (2) Cross-reactivity of such (Anthony et al. 2005;Isman2006; Nerio et substances might also be significant in beneficial al. 2010;Vigan2010).Thetoxiceffectof insects such as the honeybee. In addition, these essential oils is most likely mediated by substances might cause undesired allergic or toxic neurological mechanisms. Indeed, plant essen- effects in humans. For such reasons, there is a tial oils have been suggested to exert their continual demand for compounds that, on the one bioactivity by interacting with various molec- hand, efficiently reduce parasites or pests and, on ular targets including tyramine and octop- the other hand, are safe for the host, the amine receptors (Enan 2001, 2005a, b;Price environment, and non-target species, including and Berry 2006), ionotropic GABA receptors man, that come into contact with such insecti- (Priestley et al. 2003; Tong and Coats 2010), cides/acaricides. Over the years, plant essential and AChE (Grundy and Still 1985; Ryan and oils have been considered as an attractive alterna- Byrne 1988; Keane and Ryan 1999). As tive that might fulfill these criteria. previously mentioned, octopamine receptors Plant essential oils are complex mixtures of have also been identified as a target site of the odorous substances obtained from botanical raw formamidine class of insecticides (Evans and materials by water vapor extraction, dry distil- Gee 1980;Goleetal.1983;Downeretal. lation, or mechanical treatment without heating 1985;Dudaietal.1987;Chenetal.2007). (Isman 2006; Vigan 2010). Because of their In mammalian tissues, p-tyramine, β- odorous character, plant essential oils are widely phenylethylamine, tryptamine, and octopamine used as fragrances and flavorings in perfume are present at very low (nanomolar) concen- and food industries. To identify and to further trations and are therefore referred to as “trace characterize the individual components of es- amines.” Trace amine-associated receptors sential oils, either separation by gas chromatog- (TAARs) have recently been discovered in raphy or HPLC has to be performed. These mammals (for a review, see Zucchi et al. analyses have identified monoterpenes, sesqui- 2006). It is worth emphasizing, however, that terpenes, and related aromatic compounds as the thus far, only two members of the TAAR family main constituents of plant essential oils. Terpe- have been shown to be responsive to trace noids are derived from five-carbon isoprene amines. In addition, mammalian TAARs are units. Coupling of two isoprene units leads to not closely related to arthropod tyramine and ten-carbon structures known as monoterpenoids, octopamine receptors. Thus, it appears that in whereas coupling of three isoprene units leads mammals, TAARs evolved independently to sesquiterpenoids (Table I). Terpenoids can be from the arthropod tyramine and octopamine linear (acyclic) or contain cyclic structures receptors and acquired the ability to interact (Table I). Further biochemical modifications by with different amines, including the decar- endogenous plant enzymes can cause oxidation, boxylated thyroid hormone derivatives known rearrangement, or additional cyclization (Table I) as thyronamines, several volatile amines, and that increases the variability of essential oil possibly other as yet unidentified endogenous constituents (Chappell 1995; Holstein and Hohl compounds (for a review, see Zucchi et al. 2004). At approximately 90%, the mono- and 2006). Therefore, arthropod tyramine and sesquiterpenoids outnumber the other constitu- octopamine receptors might emerge as prom- ents of plant essential oils. Nevertheless, some ising targets for insecticides/acaricides (e.g., aromatic compounds are worth mentioning plant essential oil components) with no or low (Table I). toxicity in vertebrates. 336 W. Blenau et al. Table I. Naturally occurring essential oil components and synthetic substances mentioned in the text. Compound Molecular Substance class Sources (examples) formula Linalool [licareol=(R)-(−)- C10H18O Monoterpenoid alcohol, Coriandrum sativum linalool; coriandrol=(S)- linear (coriander) (+)-linalool] Ocimum basilicum (basil) Citrus × sinensis (sweet orange) Citral [geranial=citral A; C10H16O Monoterpenoid aldehyde, Backhousia citriodora neral=citral B] linear (lemon myrtle) Cymbopogon spp. (lemongrass) Leptospermum liversidgei (lemon tea tree) Menthol C10H20O Monoterpenoid alcohol, Mentha × piperita monocyclic, unsaturated (peppermint) α-Terpineol C10H18O Monoterpenoid alcohol, Melaleuca leucadendra monocyclic, unsaturated (cajuput oil) Pinus spp. (pine oil) Citrus × aurantium (petitgrain oil) Carveol C10H16O Monoterpenoid alcohol, cis-(−)-Carveol: