The Comparative Effectiveness of Certain Culicifuges under Laboratory Conditions

Abstract

For the purpose of testing culicifuges numbers of Stegomyia fasciata (the Yellow Fever mosquito), bred in incubators, were placed in cages kept in the laboratory. The efficiency of the preparations was tested by coating the forearm with a definite quantity of one or other of them and exposing in a cage, each test being controlled by inserting after a short interval the other and untreated arm in the same cage, the number of bites being compared in each case. By this procedure it was hoped to obtain a measure of the relative protection which the culicifuges might be expected to afford when used under practical conditions against Anopheles mosquitoes.In the first series of trials conducted within 15 minutes of application, eight preparations out of a total of 22 tested gave satisfactory results. Their active ingredients were (1) Oil of Cassia and Camphor, (2 a) Oil of Cassia and Peppermint, (5) Oil of Eucalyptus and Citronella with Phenol, (9 a) Crude Naphthalene (coke oven) and Camphor, (10) Crude “Parasitox,” (15 a) Light Wood Oil, (21) Oil of Turpentine, (22 a) “Lawson's Anti-mosquito Compound.” These preparations were then tested to ascertain for what period their protection could be depended upon. Preliminary trials indicated that this period was not likely to extend for more than two hours between treatment of the arm and exposure in the cage. None of the preparations gave complete protection in this series of trials; the most efficient were Nos. 1, 21. 2 a, 15 a and 9 a, over a two hours period.It appeared, however, that the two experimenters were unequally attractive to the mosquito. Making allowance for this, preparations 2 a. 1, 15 a, and 21 were almost on a parity, followed by 9 a, 10 and 5 in order. A series of experimental trials based on this element of preference on the part of the mosquito was confirmatory of the factor of preference and showed a ratio of attraction of about the order of G. T. 3, A. B. 2.Observations on the behaviour of the mosquitoes during the tests suggest that the protection afforded does not result from a dislike of the insects to the culicifuge but to its obscuring the attractiveness of the human odour.Regarding the make up of the essential ingredients it was found that fluid preparations were inconvenient and tended to be wasteful in application. Soap preparations, unless very soft, are apt to be difficult of economic use and in either case are more readily dissipated by perspiration than waxy or greasy ones. Culicifuges prepared with grease are more easily applied but are not so lasting as those put up with wax. Soft wax preparations, correctly adjusted to the temperature in which it is proposed to use them, are most suitable for out of doors use, especially in the case of troops. The retarding of volatilisation caused by the admixture of the active ingredients with wax or grease is a distinct advantage, but care must be exercised in respect of the relative proportion of active substance to the base. The golden rule is to use as much of the active constituent and as small a quantity of the inert base as is consistent with convenient application and the prolonging of the period of efficiency, not, as most proprietary compounders appear to think, as little as possible.In choosing a culicifuge for use. the human factor of choice in regard to pleasing and unpleasing odours should be as little disregarded as consistent with efficiency. Where large quantities are needed cost is an important consideration, but must not be allowed to endanger the willing co-operation of the individuals it is desired to protect.