Mosquito repellents
Authors: Christophers S. Rickard 
Published: May 2009

1. A general survey has been given of the subject of repellents against mosquitoes. Results recorded in published literature prior to the war have been summarized and those from the greatly increased volume of unpublished research during the war briefly outlined.

2. Of the older literature it has been shown that results were often conflicting due to many of the substances tested not being pure chemicals but very commonly essential oils and other products having a variable composition and to the methods used in testing being insufficiently precise and standardized. Particularly, standardization was lacking in that most important respect, viz. the condition of the mosquitoes used in the tests. The whole treatment of repellency on a scientific basis depends upon the suitability of the methods used in testing and for this reason a full and detailed account has been given in § IV of the methods used in the present enquiry in rearing standardized cultures ofAedes aegyptiand in § V of the methods of laboratory testing and of interpreting results.

3. Some remarks have been made on the proper sphere of field tests. Field tests, it is pointed out, are extremely valuable but less suitable than laboratory tests for determining with precision the properties of repellent substances owing to the impossibility of obtaining controlled conditions. The proper sphere of such tests would appear to be as an extension of the results of laboratory tests to cover the many qualifying circumstances which are introduced when repellents of known effectiveness are used in practice.

4. A description has been given in § III of the chief features of such repellents as have been in use, and in § VI are given the results of testing a considerable number of compounds and preparations, with data showing their comparative repellent power and duration of protection afforded. In this series three types of compound especially exhibit repellency, viz. (1) unsaturated long-chain alcohols, aldehydes and phenolic compounds such as are characteristic of most essential oils, oil of citronella, due to its active principle citronellal, being one of this type; (2) high boiling-point esters of the lower methyl series with various high molecular weight acids, such as phthalic, adipic, cinnamic, citric, etc., of which dimethyl phthalate is the best-known example, and (3) high boiling-point alcohols, especially diols, which need not necessarily be unsaturated or contain methyl series groups, of which Rutger's 612 or 2ethylhexane-l:3-diol is that best known. There are, however, undoubtedly many other groups which for some reason may possess this property as may be instanced the last entry on the list of compounds tested which belongs to none of these groups but was found as powerful as any compound tested.

5. Attention has been directed to the importance, in connexion with duration of effect, of the boiling-point. It is shown that to give protection for a period sufficient to qualify a substance to be considered a repellent, i.e. in the criterion adopted by the Unit a protection period of 2 hr., it must have the relatively high boiling-point of at least 250° C. at 760 mm. pressure. Compounds with lower boiling-point merely volatilize completely on the warm skin by the time this period is reached. To give protection for 6 hr. the boiling-point must be at least of the order of 280° C. Substances with boiling-point much over this, whilst they show lasting effect, are apt to exhibit a lower grade of repellency. In this connexion a method, sufficiently accurate for the purpose, has been quoted by which any boiling-point given as at a reduced pressure can be reduced to its theoretical boiling-point at 760 mm. The basis on which the necessary factors for this have been arrived at will be found in a note kindly furnished by Dr E. A. Moelwyn-Hughes given as an appendix.

6. The question of rub-resistant creams, of measurement of effect of repellency at a distance, of impregnation of clothing with repellents and of the testing of penetrability of clothing fabrics to mosquito bites are dealt with in their respective sections and any conclusions regarding these subjects there given. The last has been dealt with in some detail since no account of such testing has, so far as is known, been given in the literature. A note by Mr J. R. Whinfield, Ministry of Supply, giving technical methods of measuring structural features of fabrics has been given as an appendix and should be useful to anyone taking up the study of fabrics from this point of view.

7. It remains desirable, perhaps, to indicate in a general way what recent advances in regard to mosquito repellents amount to in practice, apart altogether from theoretical or scientific considerations. Results during the war have related almost entirely to the use of repellents for military purposes and the repellent almost universally employed in practice has been dimethyl phthalate. A number of reasons have combined to make this the repellent of choice for such a purpose. For the American Army the so-called 6: 2: 2 mixture, i.e. a mixture in the above proportions of dimethyl phthalate, Rutger's 612 and indalone, has been the official repellent. But Rutger's 612 has never been so freely available as dimethyl phthalate and neither it nor indalone has been in extensive use in this country. Dimethyl phthalate has very generally been found most suitably used ‘straight’ being carried in a suitable receptacle from which a little is shaken out into the palm of the hand and used to anoint exposed skin areas or even clothing. For impregnation of tropical uniforms it has usually been used suitably dissolved in a solvent or as an emulsion and the articles sprayed or dipped and wrung out. It is effective against many forms of biting insects and has been used against mites. In this last respect, however, it has been to some extent replaced by dibutyl phthalate, which when used for impregnating clothing is by reason of its higher boiling-point more resistant to washing.

8. For civilian use there would appear to be a wider field. One paramount requirement for military use has always been that the repellent should give very prolonged protection, i.e. that it should retain its effects for such periods as overnight, or as often specified for 6 hr. This has limited the choice of repellents excluding many substances which might be very effective for shorter periods. For the civilian such emphasis on duration of effect does not arise, for circumstances must be rare in which the application could not be renewed, say after 2 hr. if necessary. Again should it only be a matter of giving comfort against moderate numbers of mosquitoes something less drastic than anointing all bare skin with dimethyl phthalate might well suffice. For military purposes smell is a serious, indeed fatal, drawback. For the civilian a pleasant smell might even be an advantage. The civilian too would probably prefer a light pleasant cream to the repellent in liquid form. There is much scope here for the chemist and the cosmetician.

What then at present should the civilian be advised to use? This is dependent very largely on what he can now readily purchase, and there is some difficulty in saying under present circumstances what this might be. If a repellent is required to prevent attack where mosquitoes are a serious nuisance, and the same applies to midges, probably the best advice would be to try dimethyl phthalate straight as in military use. This might be obtained by order through a chemist to some wholesale firm such as Boake Roberts and Co. or Stafford Allan and Co. These or other firms would probably supply a moderate quantity, say 4 oz. on such an order. This could be used as stock from which to fill a small corked bottle for use. A corked bottle is better than a stoppered one as dimethyl phthalate is very liable to leak. It should be applied by pouring a little in the palm of the hand, rubbing the hands together and anointing any exposed parts such as the neck, behind the ears and even the face, but avoiding neighbourhood of the eyes. It might be more pleasant, especially if conditions were not so severe, to use a dimethyl phthalate cream such as Dr Hamil's very pleasant wax and arachis oil cream or the 70% magnesium stearate Porton cream as given in § VI(2). This could probably be made up to the formula given by a dispensing chemist. If it were possible to obtain it, a good Java citronella oil would probably be even better, for the great advantage here is that a good citronella oil will keep mosquitoes at a distance and complete covering of skin with the repellent is not so necessary as with dimethyl phthalate. Or if such an oil is not to be at present obtained it might be possible to get citronellal, its active principle, or even citronellol the corresponding alcohol. A little experimentation and careful preliminary trial is always desirable as to what can be done and as to how effective the substance seems to be. This applies especially to the case of midges which are not always the same species and might not in some cases be repelled. It should be realized that for civilian use there is still much that is experimental. For those with a liking for experiment it might be well worth while to try out that very pleasant smelling substance hydroxycitronellal, a note on which will be found in § III.

Mention should also be made of certain proprietary products issued by certain American firms which might at any time be obtainable in this country, such as ‘Skat’ or ‘Eveready repellent’. Skat (Skol Company Inc., New York) is issued in two forms, viz. as containing dimethyl phthalate and as containing indalone (in both cases practically these substances), and Eveready Insecticide (National Carbon Co. Inc., New York) as ‘formula 612’, in this case Rutger's 612. Very likely in time good repellents will eventually be put on the market by various firms in this country.

9. As regards the future no limit can probably be placed upon what may result from organized research which is likely to be taken up more seriously now than was previously thought worth while. Research in this field may be said to have reached a point where methods of testing will enable precise results to be obtained and if work by the synthetic chemist is linked with that of the insect biologist further progress in the discovery of still more effective repellents is extremely likely. Research should not, however, stop here for much still remains to be done in the direction of ascertaining methods of using such repellents to the best advantage to meet different requirements and even of ascertaining exactly what such requirements are.