Fumigant toxicity of essential oils from the Myrtaceae family and 1,8-cineole against 3 major stored-grain insects
Introduction
In many storage systems, fumigants are the most economical and convenient tool for managing stored-grain insect pests not only because of their ability to kill a broad spectrum of pests but because of their easy penetration into the commodity while leaving minimal residues (Mueller, 1990). For this reason, methyl bromide (MeBr) and phosphine (PH3) are widely used fumigants. Methyl bromide has been used as a fumigant for at least 70 years, its insecticidal properties being first reported by Le Goupil (1932). Methyl bromide is the most commonly used fumigant in soil disinfestation and treatment for quarantine or pre-shipment (Ristaino and Thomas, 1997). Phosphine is used on over 70% of stored grain (Mueller, 1990). Application technology of phosphine such as SIROFLO® and Phosfume® (now renamed ECO2FUME®) are currently employed in many sealed and unsealed storages (Ryan and Russell, 1998). This wide role of MeBr and PH3 is likely to be more restricted in the future. Under the Clean Air Act and Montreal Protocol, the use of MeBr will soon be restricted, in 2005 in developed countries and in 2010 in developing countries, due to its potential ozone depleting properties. Specific uses of MeBr will be exempt from restriction including pre-shipment, quarantine and emergency uses (World Meteorological Organization, 1995). The fumigation of commodities with phosphine is likely to become more widely used in the future because of its efficacy and rapid desorption. Phosphine fumigation, however, may become limited in use due to increasing resistance of stored-grain insects to this material. Resistance has now been reported in many countries (Bell and Wilson, 1995; Daglish and Collins, 1999). In addition, there have been some arguments about the genotoxicity of phosphine (Meaklim, 1998).
The increasing problem with today's fumigants makes it necessary for research to devise other control procedures and to identify new fumigants such as carbonyl sulphide and to reinvestigate the use of old fumigants such as sulphuryl fluoride, ethyl formate and hydrogen cyanide (Chaudhry, 1997; Bell, 2000).
Different types of aromatic plant preparations such as powders, solvent extracts, essential oils and whole plants are being investigated for their insecticidal activity including their action as repellents, anti-feedants and insect growth regulators (Prakash and Rao, 1997; Isman, 2000; Weaver and Subramanyam, 2000). Weaver and Subramanyam (2000) suggested that fumigant activity in botanicals could have a greater potential use than grain protectants in future on the basis of their efficacy, economic value and use in large-scale storages.
More current research showed that essential oils and their constituents may have potential as alternative compounds to currently used fumigants (Singh et al., 1989; Shaaya et al (1991), Shaaya et al (1997); Regnault-Roger et al., 1993; Dunkel and Sears, 1998; Huang and Ho, 1998; Huang et al., 2000; Tunç et al., 2000; Lee et al (2001a), Lee et al (2001b)). Major constituents from aromatic plants, mainly monoterpenes, are of special interest to industrial markets because of other potent biological activities in addition to their toxicity to insects (Kubo et al., 1994; Isman, 2000; Weinzierl, 2000).
Lee et al. (2001b) reported toxicities of essential oils as fumigants from various Korean medicinal and spice plants, and the primary mode of action of their toxicity against the rice weevil, Sitophilus oryzae (L.). Rice weevils are one of the major stored-grain pests worldwide (Sinha and Watters, 1985). Lee et al. (2001a) also reported toxicity of commercially available essential oils and their major compounds against S. oryzae. Based on the results of Lee et al (2001a), Lee et al (2001b), toxicity of 42 essential oils extracted from seven Australian Myrtaceae genera (Eucalyptus, Melaleuca, Callistemon, Leptospermum, Kunzea, Baeckea and Angophora) was tested against the 3 major stored-grain insects: S. oryzae, Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.).
Section snippets
Culturing insects
Experimental insects were cultured on wheat from a single source (variety Rosella) at a moisture content of 11.5–12.5%. Samples of 1 g of S. oryzae (SGRL strain LS2) were cultured on 800 g wheat, 0.86 g of T. castaneum (SGRL strain TC4) on 734 g flour plus 66 g brewer's yeast, and 0.56 g of R. dominica (SGRL strain RD2) on 800 g wheat plus 160 g flour. The parents were removed after 2 weeks and culture media incubated for 8–9 weeks at 25°C. The adults used in the experiment were 2–3 weeks
Oil yields
The yields of essential oils from 42 species of Myrtaceae are listed in Table 1. Melaleuca thymifolia had the highest oil content (7.6%) and Eucalyptus curtisii had the lowest (0.2%). Most species contained a significant amount of oil (⩾1.0%). Eucalyptus macrorhyncha (0.6%), E. polyanthemos (0.7%), M. decussata (0.6%), E. leucoxylon (0.7%), E. caesia (0.9%), E. pauciflora (0.7%), M. lanceolata (0.3%) and Melaleuca sp. (0.6%), however, all had low yields.
Fumigant toxicity of essential oils from the Myrtaceae family against from 24 h exposure to their vapours at 25°C
The toxicity of 42 essential oils is
Discussion
Singh et al. (1989) reported that Mentha citrata Ehrh. (Labiatae) oil had fumigant toxicity to S. oryzae. Also, Shaaya et al. (1991) investigated the toxicity of essential oils extracted from a group of Labiatae plants only described by their common names and therefore leaving some doubt about the actual species involved. They showed oils from sage, bay laurel, rosemary and lavender are toxic to R. dominica, while those from oregano, basil and thyme are toxic to Oryzaephilus surinamensis (L.).
Acknowledgements
The authors wish to acknowledge: J.J. Brophy, School of Chemistry, University of New South Wales for analysis of essential oils; M.I.H. Brooker and W. Anthony, CSIRO Plant Industry Canberra, for identifying plants; staff of Stored Grain Research Laboratory CSIRO Entomology Canberra, for experimental advice and support; and the School of Science and Technology, University of Newcastle for supporting this research.
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