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“Mosquitoes, malaria and essential oils”
 

International J. of Aromatherapy  (2005) Vol15(1), 30-41
 

 

 T. Burfield & S-L. Reekie

 

 

 

ABSTRACT

Biocidal (insect repellent) preparations used against mosquito bites to prevent infection are reviewed, comparing the use of essential oils and natural aromatic materials with various synthetic topical agents. A synopsis of malaria prevention strategies and insecticidal toxicity is also discussed, in the light of emergent mosquito resistance to synthetic chemical pesticides.

 

The use of natural products for use as anti-malarials is also reviewed, finding that a serious divide occurs between a purely open approach, and the approach via the commercial interests of pharmaceutical and chemical companies. The latter tend to focus on the exploitation of single active chemicals, whilst the activity of whole botanical extracts is overlooked despite the fact that insect resistance to single actives is common; and resistance to whole plant extracts is rare. The European Union’s role in this issue is also discussed.

 

Better formulation technology is needed for topical repellents by providing more effective fixation for the essential oil content and incorporating strategies for controlled release of essential oil vapours, whilst providing solutions for the problem of potential dermal irritancy.

o CaptionFull Article

 

§1. Preamble.

Essential oils as biocides

Use of plant products as insect-control agents can be traced back at least as far as the Romans who used species such as white hellebore (Veratrum album) and black hellebore (V. nigrum). Essential oils & plant extracts are still an important natural resource of pesticides/insectides: Raguraman & Singh (1997), Gbolade (2001); or larvicides: Jacobson  (1983), Adebayo et al., (1999), Murty U.S. & Jamil K. (1987); or insect repellents: Sadik F. (1973), Thorsell et al. (1998); Oyedele et al. (2000); or molluscides (Hostettman 1989). Sadik lists the essential oils of cedarwood, citronella, eucalyptus, pennyroyal, turpentine and wintergreen as having traditional uses for their insect-repellent properties, which are formulated for use as either topical preparations or as combustible products (e.g. incense sticks). These properties are exploited to protect the immediate environment of the user or the user him/herself from the bites of harmful insects, which may be vectors of disease. To the above list we could add catnip, basil, lemongrass, melissa, garlic and peppermint oils amongst others; Duke (1997) also mentions pulegone-containing plants such as Pycananthenum muticum (Mountain mint) and citrus-smelling essential oils like Thymus citriodora. It is as well to remember that these products in general may only be effective close to the emanating biocidal source, in situations with limited air movement, or downwind of the application! Caution may have to be exercised in topical product formulations, since many volatile terpenoids in essential oils can function both insect attractants and deterrents – for example geraniol may act as an attractant for scotylids, honey bees and the Japanese beetle, but as a deterrent for the common house fly and silkworm (Berenbaum & Seigler 1992). The attractant effect of natural aromatics may be put to use in lure & bait traps, where say 1-octen-3-ol (which occurs in lavender oil) might be employed, possibly in conjunction with electrocuting apparatus (EPA 2003). Common terpinic alcohols, which widely occur in essential oils, such as citronellol, geraniol, nerolidol and farnesol, may also be used as sex specific arrestant kairomones, to control mites etc (EPA 2004).

 

Natural aromatic materials as anti-malarials.

Leung & Foster (1996) note the use of aqueous extract of ginger for malaria fever in China  (Buhner 2000 indicates that ginger is used for relief of “chills”). Deertongue, liquorice root and white pepper are also quoted for treating malarial symptoms. The Revolutionary Health Committee of Hunan Province (1977) in their original and extensively plagiarised book A Barefoot Doctor’s Manual provide detailed and specific treatments for solid and deficiency types of malaria, using a mixture of herbs for the solid type including Blupleurum chinense, Scutellaria baicalensis (both of which reduce fever and possibly have other actions) and Artemisia apiacea. External treatment for both forms of malaria includes procedures involving the use of garlic, white pepper and the herbs Atractylis ovata, Angelica anomala and Conioselium univattum. Buhner (2000) separately remarks that the role for eucalyptus oil treatment of treating malaria in traditional medicine is relatively unexplored in terms of published scientific studies.

 

African healers have traditionally used the root of Cryptolepsis sanguinolenta against malaria and other fevers. Buhner (2000) describes a trial carried out at the centre for Scientific Research into Plant Medicine at Mampong-Akwapim in Ghana where its’ effectiveness was favourably compared with chloroquine, clearing parasites faster, relieving symptoms faster and presenting no side effects.

 

Artemisia species with anti-malarial activity are though to include A. apiacea form China and Japan, and A. scoparia and A. caucasia from Turkey. The most important species is A. annua, from which the important anti-malarial principle artemisinin is obtained, which is discussed below.

 

Repellent Natural products – the legislative situation.

The EU Biocides legislation does currently not recognise essential oils as a special class of biocide, threatening to inappropriately reclassify them variously as cosmetics, vetinary chemicals etc. with specific biocidal properties (Burfield 2004). Topically applied insect repellents are however specifically regulated under the Biocides Products Directive (BPD) 98/8/EC, Category 19: Repellents and Attractants, introduced in May 2000. Safety evaluations of any materials registered in Annex 1 of the BPD (which includes essential oils), have yet to be carried out (Berend 2004) - in complete contrast to the US situation, where for mosquito & general insect control, citronella oil together with four other fixed vegetable and eleven other essential plant oils have been exempted from EPA pesticide registration. This is because they are “so unlikely to cause harmful effects” (EPA 2000; updated 7/01) and further “when used as pesticides (they) do not present any known risks to humans or the environment”. However the EPA does not approve of any repellent containing more than 10% of essential oils from consideration of possible skin irritancy and other effects (malaria is worse! – T.B.), and formulations with over 3% concentration of essential oils require registration.

 

Malaria – general notes

In 1897 Ronald Ross, a British physician, first associated the transmission of malaria with mosquitoes. We now know that the bites of sixty-six species of Anopheles mosquitoes out of the 380 known species, infect 200 million new victims per annum (Metcalf & Novak 1994), resulting in recurrent illness and death. The above authors further remarked that 250 million people have filariasis from the roundworm Wuchereia bancrofti, which is spread by the common house mosquito Culex pipiens quinquefaciatus.  Malaria kills between one and three million people per year worldwide, most of them pregnant women and children, but the disease has a low public health priority (McGinn, Platt 2002). There is some evidence that mosquitoes are drawn to human subjects by the exudation of body heat and specific chemicals: for example carbon dioxide plumes or other chemicals from the exhaled breath, or chemicals emanating from the skin surfaces, such as isovaleric and lactic acids. Visual clues (dark colours, body shape) and moisture emanation may also play a role. Pregnant women in the later stages of pregnancy are more susceptible to targeting by mosquitoes since they exhale greater volume of air than non-pregnant women of the same body weight, and since their skin flora is different due to higher body temperature and increased sweating rates (FDA Consumer 2000). Recently Makubaba et al. (2004) concluded that although human breath contains mosquito attractants (carbon dioxide, ammonia, fatty acids) it also contains allomonal (i.e. repellent) compounds, making individuals vary in their individual attractiveness to feeding mosquitoes.

 

In simple terms the mode of disease transmission operates as follows: the gametocytes (malarial parasites) in the blood of a malaria patient are transmitted into the body of an (anopholeles) mosquito during feeding on an infected patient. Within the mosquito these gametocytes develop into sporozites, which are subsequently transferred into the bloodstream of a new victim. After receiving a mosquito bite, the subject will produce a reaction to the injected saliva which contains coagulating proteins, but the sporozites proceed via the circulation to the liver where they develop and invade the red cells, causing recurrent chills and fever patterns – either daily or at intervals of three days.

 

During the 16th to 18th Centuries, before widespread draining of marshy areas, vivex malaria (from Plasmodium vivex) was once endemic in the UK (known as “the Ague” or  “Marsh fever”). Nowadays, global warming add to fears of a re-emergence, with an increase in deaths from malaria to 16 in the UK for the year 2003 compared with 2,000 people returning to the UK with malaria after holidaying in malaria-prone areas (Boseley 2004). This phenomena has highlighted several problem areas. Individual cases of malaria occurring in populations living near to airports has lead to realisation that live mosquitoes can be carried in aircraft (Inter Press Service 2000). Aircraft containing mosquitoes have also been identified as part of the transmission cycle of the West Nile Virus from birds to humans which have caused outbreaks recently in New York, New Jersey and Delaware (Kahn 2001). Mosquitoes can also carry viruses causing dengue fever, yellow fever and encephalitis.  It is also apparent that there is reluctance amongst a section of travellers to take convention malarial drugs for unknown reasons (Boseley 2004). Because of core beliefs in aromatherapy expressed as dualism characterised by favourable disposition towards anything natural, organic & holistic (i.e. essential oils!), and a perceived aversion to conventional medicine, chemicals and synthetics (Vickers 1996), it is not unreasonable to assume many aromatherapists may well fall into this category of reluctance to take anti-malarial medication! Boseley (2004) considers public concern to the side effects of drugs such as Lariam (mefloquine) – to which one of the authors (Tony) has experienced personally – is not helping the situation, but the main issue is considered to be ignorance of the disease.

 

Resistance to the first-defence synthetic drug against malaria, chloroquine, is now up to 90% for many parts of Africa, with resistance to sulfadoxine pyrimethamine steeply rising also (The Guardian 2004). With increasing resistance to the few anti-malarial drugs becoming acutely problematic, attention has been focussed more on the elimination of mosquitoes. As long ago as a 1955 WHO initiative was established (“The Global Malaria Eradication Problem”) – using pesticides such as DDT. Some success in establishing either mosquito-free zones or a severe reduction in their numbers in thirty-seven countries was achieved by 1961 (McGinn, Platt 2002) but this was tempered by the finding of evidence of the bioaccumulation of DDT and evidence of chronic toxicity. The campaign was abandoned in 1969 when re-emergence of malaria due to widespread resistance to DDT became apparent.  In this period the publication of Rachel Carson’s book Silent Spring (1962) painted an austere picture of the environmental consequences of unrestricted worldwide pesticide use.

 

Anti-malarial essential oils.

The potentially important role for phytomedicines in combating disease in the light of extensive multi-antibiotic resistance, has been reviewed by Carson & Riley 2003. The use of artesunate (an arteminsinin derivative, see below) for malaria treatment is cited as an example.

 

The plant Artemisia annua has been extensively used in Chinese medicine for over a thousand years as an anti-malarial (Trig 1989), wild plants containing up to 0.5% concentration of the anti-malarial principle artemisinin, which can be quadrupled by selective breeding. Focus on derivatives of artemisinin which is toxic to malaria parasites at nanomolar concentrations (Samuelsson 1999), the component being found more specifically in the extracts/essential oil of the leaves & flowering tops of A. annua and other Artemisia spp., is being actively considered by malaria experts as a possible solution to this problem. There is evidence to suggest that the activity of the whole herb may be greater than that of the essential oil (McCaleb 1993). Buhner (2000) also remarks that arteminsinin-free extracts of A. annua are comparably as effective an anti-malarial as arteminsinin at twice the dose.

 

There is a most important principle here. Brinker (1999) discusses a basic problem surrounding medicinal herbs where a focus on single-constituent plant derived drugs can reduce the potential overall benefits for humankind. This is the case here, where Berend (2004) advises that the single compound artemisinin from Artemisia annua is supported by corporate interest and is regulated within the framework of Directive 2001/83/EC on Human Medicinal Products - whereas the unpatentable but medicinally effective extracts of A. annua are commercially unsupported both in the latter directive and in the BPD. From the work of McCaleb (1993) & Buhner (2000) cited above, we already know of the effective biocidal effects of whole A. annua extracts, even those not containing artemisinin! The underlying principle here cannot be emphasised enough. Put simply, there is a serious divide which occurs between the commercial interests of pharmaceutical companies (focussing on one marketed and often patentable ingredient) and a more open approach & holistic approach (which considers the beneficial medicinal potential of the whole herb, to humankind). More than this, the EU Commission apparently supports the “corporate science” approach to the extent that impossibly high registration costs effectively seal off potential consumer free-choice for herbal and phytochemical products for the foreseeable future, since these small industries involved cannot afford the supporting costs involved. 

 

Lets take another illustration of this iniquitous situation, albeit slightly off-topic where Awang (1997) comments on attempts to standardise botanical extracts of feverfew Tanacetum parthenium on a single active (extracts of the herb are a prophylactic for migraine headaches). Awang discusses why it is not safe to explain away the therapeutic action of feverfew on the major sesquiterpene lactone present (parthenolide), as activity would seem to lie with other unidentified components.

 

§2. Malaria prevention strategies and insecticide toxicity.

A US mosquito management scheme (Rose 2001) outlines a policy which features surveillance (traps, biting counts, aerial photography), source reduction (marsh drainage, open water management), larvicide, and biological control (mosquito eating fish!), as well as public relations and education. On the latter point, as an aside, the author was shocked to discover on a visit Zanzibar in 2001 - where statistically, one third of the population will die of malaria - that the majority of the endemic peoples especially those in remoter locations, were unaware that the mosquito is the vector of the disease.

 

To prevent the frequency of getting bitten by Anopheles mosquitoes, topical application of the synthetic chemicals dimethyl phthalate and 2-ethyl-1,3-hexanediol were used by the US armed forces in the Pacific during World War  II. Dimethyl phthalate is also active against other species of biting mosquitoes such as Aedes and Culex spp., but toxicological worries about phthalates make this strategy unworkable, so that the most popular synthetic chemical topical agent used in preventative formulations nowadays is DEET, which is chemically N,N-diethyl-m-toluamine; however this substance is also considered a hazardous material by many.

 

              

 

The problems of mosquito resistance to synthetic chemical pesticides and environmental pollution have complicated the policy of routine insecticidal approaches to the problem of mosquito control. In certain cases resistance mechanisms to insecticides have been elucidated e.g. to organophosphate insecticides in Culex spp. has been suggested to operate via the amplification of an esterase gene (Mouches et al. 1986). However, we do have naturally available, non-persistent, insecticides. Of the phytoalkaloids, we have nicotine from various Nicotiana ssp. From Quassia amara we have various quassinoids, which also show anti-malarial activity. We have rotenone and other actives from the roots of Derris (traditionally used in China) and from Lonchocarpus spp., and the natural pyrethrins from Chrysanthemum cinerariifolium flowers need no introduction, having been used for centuries against infestations of flies, mosquitoes etc. However natural pyrethrins are not particularly light-stable and are metabolised rapidly, so that for agricultural & domestic use, synthetic pyrethroids such as (+)-trans-allethrin are often used in commercial formulations together with synthetic synergists such as piperonyl butoxide or MGK 64. Since piperonyl butoxide affects the production of liver enzymes, this material should not be used in homes who’s inhabitants include seriously ill people on medication. Other agents such as permethrin are now suspected of serious health damaging effects. The use of mosquito coils containing pyrethrins for mosquito control may however have consequences for health: Goldsteins (2003) concludes that the emitted smoke contains carcinogens and suspected carcinogens, as well as fine particulate matter, stating that the smoke from one coil contain the equivalent amount of formaldehyde emitted by 51 cigarettes. Under-estimation of the health risks of such insecticides are unfortunately seen in many published papers (see for example Mafong et al. 1997). Research by the Duke University as featured in the Journal of Toxicology and Environmental Health concluded that combined exposure to DEET and permethrin, causes severe cellular damage.

 

The triterpenoid azardirachtin is another natural biocide occurring at up to 0.4% in seed-kernels of the neem tree Azadirachta indica and which is active against a large number of insect pests. This effect is not by instant knockdown, but by blocking the action of the moulting hormone ecdyson and by anti-feeding effects. Feng & Isman (1995) showed that after 40 generations repeated treatment with azardirachtin and neem oil against green peach aphids, a ninefold resistance was developed to azardirachtin but none towards neem extract, providing further proof that pure single chemical isolates of botanicals are often less effective than crude extracts. Of the inorganic materials, silica gel is used as a spray against insects, and causes the insect to dehydrate in low humidity situations via the waxy layer on the insect cuticle. Boric acid powder also used against infestations of ants and roaches (mixed with sugar) and seems to have a widespread global use amongst householders, against best advice of not using it in the home (boron is an accumulative poison).

 

In India, karanga seed oil (Pongammia glabra) is used as a foliar spray against a number of pests. It is believed that the liminoid karanjin which is present at up to 2% may be important for it’s insecticidal properties (see http://www.plasmaneem.com/karanja.htm).

 

Once hailed as a breakthrough, the strategy of breeding transgenic mosquitoes, say by introducing retroviral vectors which could be used to integrate and express foreign genes in the malaria mosquito, Anopheles gambiae in order to combat malaria has been muted by various workers such as Mafong et al. (1997). The idea has now been revealed to be ecologically flawed (Enserink 2002; Highfield 2003) since the induced genes appear to fade within 16 generations.

 

As mentioned above, qinghaosu (artemisinin) is an anti-malarial sesquiterpene lactone, which is isolated from Artemisia annua, being active against multiple-drug resistant forms of Plasmodium falciparum. Earliest reports of the anti-malarial activity of the plant were included in a document found in the Mawangdui Han Dynasty Tomb dated at 168BC. Artemisinin is in fact a peroxide and a d-lactone, but nevertheless shows remarkable stability. The peroxide group is essential for anti-malarial activity, which manifests as a potent blood schizontocide. Resistance to the single-active artemisinin has been reported (Trigg 1989), but work on more potent artemisinin derivatives is on-going.

 

Worries about scientists in conflict of interest situations distorting scientific objectivity can be glimpsed via a damning letter by Epstein et al. (http://www.chem-tox.com/letters/banned.htm) criticising the conduct of the National Research Committee (NRC) in the US. According to the authors, the NRC allegedly trivialised the cancer risks to infants from food contaminated with carcinogenic pesticides in its 1996 report on “Carcinogens and Anticarcinogens in the Human Diet”. The distinguished authors of the letter point out that (only) highly qualified and independent scientists acceptable to, or working with, NGO’s should play a role in any science advisory body. In similar vein, Burfield (2004) has criticised the EU Commission for its “corporate science” approach to chemical products legislation which effectively bypasses the democratic process and which has, in many instances, removed the freedom of the individual to choose safe natural products, against the wishes of many EU citizens.

 

Essential oils as insect repellents in general and mosquitoes in particular.

(a) General

It may be useful to attempt a brief review of some of the scientific literature available on essential oils as mosquito repellents. In terms of popularity, citronella oils from Cymbopogon nardus and C. winterianus have been reported to be the most widely used insect repellents (Hermes & James 1961). These oils naturally contain (4R)-(+)-(b)-citronellol and (mainly) (3R)-(+)-b-citronellal which appear to be the principle actives. Although these oils are effective and of relatively low cost, not everyone cares for the somewhat crude odour profile of citronella oils, especially the Sri Lanka sourced oil (from Cymbopogon nardus). However in terms of efficiency, Dr. David Sullivan of Johns Hopkins Bloomberg School of Health in Baltimore, Md., reports that Soy oil is in fact the most useful of the plant-based mosquito repellents (United Press International 2003).

 

In parts of Scotland, the Highland Midge Culloides impunctatus can make life miserable for locals and holidaymakers alike. The use of endemic aromatic plant Myrica gale, the volatiles from which having a high 1,8-cineole content, has been well reported for midge repellency (Simpson et al 1996; Stuart 1990; McGhee 1975). The essential oil for use against midges is sold under the name of “Myrica” by Scotia Pharmaceuticals. 

 

Differences in susceptibility across a range of different mosquito species, to six separate agents (diethyl toluamide [DEET], dimethyl phthalate, ethyl-hexanediol, permethrin, citronella and cedarwood oils) were investigated by Curtis et al. (1987). The authors found that general susceptibility decreased in the series Anopholes stephensi, A. gambiae, A. albimanus  and A. pulcherrimus. DEET-impregnated anklets gave 84% protection in 2-hour tests against Culex quinquefasciatus and A. funestus but protection was less impressive against A. gambiae, A. coustani and Mansonia spp.

 

As well as differences in species susceptibility, the choice of a superior natural repellent is the driving force of many published articles. It is widely assumed that repellents act by blocking receptors on the hairs of mosquito antennae which detect body heat, moisture and carbon dioxide, causing disorientation and loss of targeting ability. Conclusions about the absolute efficacy of individual oils as repellents are hard to deduce from the mass of data, since the botanical, geographical, chemotype and composition of the oils used are rarely defined. All-day repellency from a simple single topical application of an essential oil or mixture is a high expectation, given the relatively high rate of evaporation/absorbtion of essential oils from, or into, the skin. Thus synthetics with a longer dermal surface residence time, such as DEET, may offer an advantage.           

 

Nevertheless, several articles multi-screen a number of essential oils for mosquito repellency, with mixed outcomes. Girgenti & Suss (2002) used five commercial repellents based on essential oil mixtures including containing citronella, clover, eucalyptus, geranium, lavender, peppermint, sandalwood and thyme, reporting poor repellency (less than or equal to one hour) against the mosquito Aedes aegypti, whereas a synthetic repellent afforded better protection. Similar conclusions were drawn by Primavera Laboratories (1999) reporting on their trials leading to an insect repellent patent. The company reportedly used seven subjects in a total of 1210 trials using Aedes aegypti mosquitoes. The best combination was said to consist of 7% citronella (oil), 9% geraniol, 9% terpineol (no isomer stated, presumably a-), and 75% “Chinese crystal” at about 1-5%, where the identity of “Chinese crystal” was stated to be a natural 3-hydroxy-a,a,4-trimethylcyclohexanemethanol isolated from a particular Chinese essential oil. Twenty essential oils were screened by Cora et al. (1993) against the mosquito (Aedes aegypti), fruit fly (Drosophila melanogaster) and the house fly (Musca domestica), where the oils of Juniperus communis, Valeriana officinalis, Thymus vulgaris, Solidago graminifolia, sylvestris, Coriandrum sativum, Larix decidua, Pseudotsuga menziesri, Tanacetum vulgare and Abies alba were all found to be highly repellent to mosquitoes.

 

Tawatsin et al. (2001) investigated the repellency effects of essential oils of  turmeric (Curcuma longa), keffir lime (Citrus hystrix), citronella grass (Cymbopogon winterianus) and hairy basil (Ocimum americanum), against  the msquitoes Aedes aegypti, Anopheles dirus and Culex quinquefasciatus both in caged and open room conditions. The oil of Keffir lime either alone or with 5% vanillin added, repelled mosquitoes in cage conditions for up to three hours. Turmeric, citronella and hairy basil oils especially with the addition of 5% vanillin, repelled the three species under cage conditions for up to eight hours, comparing well against the DEET standard which provided protection for at least eight hours against A. aegypti and C. quinquefasciatus, but for six hours against A. dirus. Incorporating 5% vanillin with DEET gave protection against the three mosquito species for at least eight hours. The large room experiment confirmed the findings of the cage experiment. The authors concluded that it is possible to formulate with essential oils as a DEET replacement.

 

Girgenti & Suss (2002) compared five commercial mosquito repellents – four of which were natural oil formulations (containing citronella, clover, eucalyptus, geranium, lavender, peppermint, sandalwood, thyme, etc.) and one which was a synthetic chemical repellent containing 10% KBR 3023 in cream or lotion. Tests were performed on adult male and female human volunteers in laboratory based testing conditions. The natural oil products showed either poor (protection time less than 1 hour) or no repellency against A. aegypti, in contrast to the synthetic repellent, which performed satisfactorily. It is interesting to speculate separately on why unsatisfactory results were obtained here – reliance on particular items from a specific batch of commercial oil product obviously has its’ pitfalls, but it may be that the problem centres around topical dosing levels (i.e. concentration of the oils in the application vehicle).

 

 

Studies on alcohol extracts may tell us more about other classes of actives capable of isolation from aromatic plants, as well as-, and in addition to-, the essential oil content. Govere et al. (1993) used “essential oils” from local South African plants against Anopheles arabiensis, finding that alcohol extracts of Lippia javanica, Pelagonium reniforme and Cymbopogon excavatus all protected against Anopheles arabiensis mosquito bites; the repellent effect of fever tree oil (Lippia javanica) lasted significantly longer – and also happens to be the only commercially available oil, notwithstanding that several chemotypes of the oil are known.

 

The larvicidal mode of action of essential oils was investigated by Corbet et al. (1995) who noted the susceptibility of mosquito larvae and pupae to surface materials entering their tracheal system, observing that essential oils increased the tendency to tracheal flooding and chemical toxicity. They reasoned that the addition of surfactants would increase the oil concentration in water and spreading pressure, and set out to prove this with a mixture of eucalyptus, turpentine and cineol, and Arosurf, an insoluble surfactant.  The authors were able to establish that all three substances were larvicidal, and their action was greater when Arosurf and a detergent were added. The initial level of activity of the oil was greater than for a commercial larvicide, but mortality over a 1-2 day period was lower. This seems to be a promising line of investigation using moderately cheap aromatic raw materials. The potential of garlic oil as a non-toxic mosquito larvicide has been reviewed by Klocke (1989).

 
(b) Cymbopogon oils.

The very common usage of citronella oil as an insect repellent has already been mentioned. Tiwari (1966) tested lemongrass oil against the housefly Musca nebulo and female mosquitoes of Culex fatigans and Aedes aegypti assessing the contact toxicity, fumigant action and repellency of the oil. The findings of the latter tests indicated that lemongrass oil was less efficient than dimethyl phthalate and protection lasted only 40 mins as opposed to 300 mins. for dimethyl phthalate. In contrast to these findings, Ansari & Razdan (1995) studied the effects of Cymbopogon martini var. sofia, C. citratus, C. nardus and Cinnamomum camphora against local Indian mosquito spp. (Anopheles cujicifacies & Culex quinquefasciatus) by exposing the feet forearms and faces of local volunteers from dawn till dusk, dosed with 1 ml of each of the oils. Repellency of each of the Cymbopogon oils lasted 11 hours against Anopheles cujicifacies and 6-7 hours against Culex quinquefasciatus, (but less for the Cinnamomum oil). The oils were found to be comparable in efficacy to dimethyl phthalate and dibutyl phthalate.

 

Citronella oil in the form of candles containing 3% oil and incense containing 5% oil were investigated by Lindsay et al. (1996) for efficiency in protecting eight similarly clothed subjects against the bites of Aedes spp. carried out in a deciduous woodlot in Guelph, Ontario, Canada for two weeks in summer. Grid positioning was used and two citronella candles, 2 citronella incense, 2 plain unscented candles, or no candles (i.e., non-treated controls). Counting biting rates over 5 minutes in 16 sessions at two different positions on the grid each evening led the authors to conclude that significantly fewer bites were received by subjects at positions with citronella candles and incense than at non-treated locations, the overall reduction in bites provided by the citronella candles and incense was only 42.3 and 24.2%, respectively.

 

(c) Lavender oils

Ethnobotanical evidence is on repellency well-documented, for example in Malaga, Spain Lavandula lanata is used to repel insects (Laza (1939), and in the Canary Islands, the aerial parts of Lavandula minutolii are used for the sdsame purpose (Darias et al. 1989).

 

Choi et al. (2002) investigated the essential oil of Lavender officinalis as well as other oils such as Eucalyptus globulus, Rosemarinus officinalis, and Thymus vulgaris for their individual repellent activities against Culex pipiens. The authors found that all the named oils actively repelled adult mosquitoes on hairless mice, with thyme oil showing particularly good activity in the confines of the protocol, giving a protection rate of 91% at 0.05% concentration in topical treatment, significantly extending the duration of protection. On analysis by GC the thyme oil employed showed the following substances in decreasing order of concentration: thymol, p-cymene, carvacrol, linalol, and a-terpinene. Individual assessment of the repellent activities of these substituents to C. pipiens mosquitoes showed that a-terpinene had a potent repellent activity with a protection rate of 97% at a topical treatment concentration of 0.05%; carvacrol and thymol also showed an equivalent level of repellency. A spray-type solution containing 2% a-terpinene tested for its repellent activity against C. pipiens was reported to show stronger repellent activity than N,N-diethyl-m-methylbenzamide (DEET).

 

(d) Basil Oil.

Iwu (1993) reports that Basil oil is used in many parts of Africa to prevent mosquito bites; Kumari et al. (1994) showed that a crude extract from O. sanctum was an effective pupicide against newly emerging pupae of Aedes aegypti.  A review on the general insecticidal activity of basil oil is given by Holm (1999).

 

Basil species have been tested against mosquitoes by M. Bhatnagar et al. (1993) and Dube et al. (1989). The former authors showed that the oils of O. basilicum & O. sanctum and major constituents have insecticidal effects, decreasing in the series: Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus, in laboratory tests. O. basilicum (and its major constituent methyl chavicol) were found the most effective, the authors finding further that the oil of O. basilicum was more efficacious against Alocophora species than other tested fungicides and fumigants such as the commercial preparations of carbendozim, sulphur, mercury and aluminium phosphide.

 

Lukwa (1994) looked into the possibility than plants used traditionally as repellents might also act as larvicides. In the case of Ocimum canum extract, the  LC50 for An. gambiae s.s mosquito larvae was more than 50% lower than for Lippia javanica extract but was deemed higher than levels of commercial larvicides.

 

            

 

(e) Geranium Oil (Pelargonium spp).

Barnard (1999) considered the effect of concentration  (5, 10, 25, 50, 75, and 100%) and particular combinations of essential oils (geranium Bourbon , cedarwood, clove, peppermint, and thyme) on repellency to Aedes aegypti and Anopheles albimanus on human skin. None of the oils was effective at 5 or 10% concentration; cedarwood oil (botanical origin not specified) was ineffective at repelling mosquitoes altogether and A. aegypti was only repelled by high concentrations of peppermint oil. Thyme and clove oils were found to be the most effective mosquito repellents giving 90 to 210 mins. of protection according to the oil concentration applied to the skin. 1:1 mixtures of clove oil with either geranium oil or thyme oil prevented biting by A. albimanus for 75 to 150 mins. The authors felt that clove, thyme, and peppermint oils were potentially irritating to the skin of users, and reported that both human subjects in this study found the odor of clove and thyme oils unacceptable at concentrations equal to or exceeding 25%.

                                                                                                                  

 

Matsuda et al. (1966) investigated the introduced (and genetically engineered) citrosa plant Pelargonium citrosum ('Van Leenii'), which has been promoted as a mosquito repellent which is claimed to be able to repel mosquitoes within a 10 ft2 (0.93 m2) area from oil emission. Oil yield from fresh plant material was calculated at 0.2% +/- 0.1%. Chemical analysis of fresh oils ex greenhouse- or field-grown showed 35.4 +/- 6.2% geraniol, 10.4 +/- 1.6% citronellol, 8.9 +/- 2.0% isomenthone, and 6.8 +/- 3.8% linalool, the authors noting that the morphology and essential oil composition being tyopical of Pelargonium x asperum hybrid types (“rose geranium”). The effectiveness of the citrosa as a repellent against field populations of spring Aedes spp. mosquitoes was evaluated and compared with a 75% DEET formulation. DEET provided > 90% reduction in mosquitoes biting subjects for up to 8 h post-treatment. There was no significant difference between citrosa-treated and non-treated subjects (!).

 

On the other hand, Jeyabalan D. (2003) considered methanol extracts of Pelargonium citrosa leaf, testing for biological, larvicidal, pupicidal, adulticidal, antiovipositional activity, repellency and biting deterrence against Anopheles stephensi. Larval mortality was dose dependent with the highest dose of 4% plant extract evoking 98% mortality, affecting pupicidal and adulticidal activity and significantly decreased fecundity and longevity of A. stephensi. The treatment completely inhibited the larval, pupal and adult developments. At 4% the extracts evoked strong repellent action. They also interfered with oviposition, egg hatchability, and exhibited a growth inhibiting effect against larvae and good repellency against adults of A. stephensi.  The leaf extract treatment significantly enhanced biting deterrence, leading the author to speculate on the usefulness of these naturally based insecticides, as an alternative for synthetic insecticides controlling field populations of mosquitoes.

 

 

 

 

A number of different anti-mosquito products were studied by Jensen et al. (2000) in field trials in Illinois, by comparing mosquito landing rates. The products included a sonic repeller device, UV light & 1-octen-3-ol lure -cum electrocution device, pyrtheroid impregnated mosquito smoke coils, citronella candles, the mosquito plant Pelargonium citrosum, and a DEET-impregnated wrist band. The experimentation features unequal trial times with the various devices, which were compared with positive (DEET topical repellent) and negative (no treatment) controls. The authors found that lower landing rates  with the DEET topical repellent compared to all other products tested, with the mosquito coils and the DEET-impregnated wrist bands also significantly reducing mosquito landing rates relative to untreated controls.

 

 

(f) Himalayan cedarwood oil.

Further to reports that the oil of Cedrus deodora can be used as an insect repellent [Khadi Gramodyog 1972-3], Singh (1984) demonstrated a 50% knock down of mosquitoes Anaphales stephani could be achieved at a 0.45% oil concentration.

 

(g) Sweet orange & Lime oils.

Ezenou et al. (2001) used statistical studies of randomised complete block design with four replicates to show that the volatile peel extracts of Citrus sinensis (sweet orange) and C. aurantifolia (lime) possessed insecticidal activities against mosquitoes, cockroaches and houseflies. Insecticidal activity was superior after 60 min than after 30 min following room spraying. C. sinensis volatile extracts showed greater insecticidal potency, and the cockroach was the most susceptible to the peel extracts among the three insects studied.
 

(h) Vetiver oil.

Whole vetiver oil has been used as a larvicide (Murty U.S. & Jamil K. 1987), and as an insecticide (Komagata et al. 1987). Jains et al. (1982) found that isolates from the carbonyl fraction of S. Indian vetiver oil (such as khuismone) are insect-repellent to weevils, mosquitoes etc.

 

 (i) Miscellaneous oils.

Boswellia sacra is reported as a non-vertebrate poison for mosquitoes (Sepsal data-base 2004). Other reports include the use of b-asarone as an active in calamus oil, the usefulness of which would seem limited by the carcinogenicity of b-asarone itself, and the employment of tagetes oil as potent phototoxin against mosquitoes, which appears to produce its effect via the presence of a-terthienyl (Kloche 1989).

§3. Discussion.

Repellency is an important part of a strategy against malarial transmission, although complete day-long protection from mosquito bites using natural essential oils is not clearly not guaranteed from a single topical application – as can be seen from the collection of papers above, protection varies from an hour or less to eight hours (this is hardly surprising from what we know of kinetics of essential oil evaporation from the dermis). The situation can be divided into two areas:

 

Firstly there is the choice of the most efficient insect repellent. Research carried out by Iowa State University and the US Forest Service shows that the gold standard of repellency, DEET, is less than 10 times less effective at repelling mosquitoes than nepetalactone from catnip oil (where it is though that nepetalactone acts as an irritant to the mosquito)! Nepeta cataria (Catnip) contains mainly E,Z-and Z,E-nepetalactone, and crop protection companies have long known of the potential of the isomers of nepetalactone, since they act as aphid sex pheremones. Further, chemotypes of Nepeta cataria containing isomers of the insect repellent substances pulegone and citronellal are also known. It would seem that there is still large potential surrounding the use of natural products in this area.

 

Better formulation technology needs to be employed for topical repellents utilizing essential oils. Better fixation and strategies for controlled release of essential oil vapours from the skin to give extended protection  (8 hours) need to be explored by manufacturers of topical products. Practical problems (skin colouration from oil components, potential irritancy effects etc.) may also need to be addressed. An appreciation by consumers of the possible limitations in protection from the use of topical essential oils is also important, although some of these drawbacks may yet be overcome by appropriate product development.

 

The situation surrounding the future of biocides and anti-malarial drugs is worrying. Because of the way pharmaceutical companies tend focus on single active materials for revenue & profit, including those from botanical sources, other useful actives in whole botanical extracts may be overlooked. As we have seen above, development of biological resistance to single actives is common; whereas resistance to whole plant extracts is rare - although this has to be tempered with the fact that the are inadequate supplies of natural plant compounds as insect control agents anyway. Most worrying are the policies of the various identifiable government expert committees on biocides who may give undue weight to contributions from scientists with vested interests. This corporate-science influenced path, set against an uneven financial playing-field where only big industry can afford to register biocidal products, could conceivably lead to the future commercial unavailability of safe natural biocides in Europe (such as the essential oils) from nature’s rich store. The current biocides situation seems to be a rerun of the situation surrounding the FDA and the Dietary Supplements issue (Blumenthal 2003) where pharmaceutical modelling and implicitly high product registration & support expenses mitigate against smaller companies in the herbal industry. As Bob Dylan once said “You don’t need a weatherman to see which way the wind blows…”

 

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