38 PEEVENTIVE AND KEMEDIAL WORK AGAINST MOSQUITOES. 



laboratory of the board of health, Isthmian Canal Commission, 

 Panama, 1906. Bulletin No. 2 of the same service (1906) contains an 

 account of experiments in practical culicidal fumigation, also by 

 Doctor Kendall. 



The burning of dried orange peel has been recommended as a deter- 

 rent against mosquitoes, but there seem to be no records of conclusive 

 experiments, although the writer has been assured of its efficacy by a 

 Japanese physician visiting the United States. 



In the course of his experiments with different disinfectants against 

 mosquitoes, Kosenau, of the U. S. Public Health and Marine-Hospital 

 Service,^ did his principal work with formaldehyde and sulphur dioxid. 

 We have mentioned his conclusions with regard to the latter substance 

 in a previous paragraph. Formaldehyde gas, on account of its ger- 

 micidal use, was early suggested against mosquitoes when their 

 importance in the role of carriers of disease was ascertained, so that 

 exact experimentation was necessary. Rosenau's results were as 

 follows : 



Formaldehyde gas is a feeble insecticide. Mosquitoes may live in a very weak 

 atmosphere of the gas overnight. It will kill them, however, if it is brought in direct 

 contact in the strength and time prescribed for bacterial disinfection. For this pur- 

 pose any of the accepted methods for evolving the gas is applicable, but the methods 

 which liberate a large volume in a short time are more certain than the slower ones. 



Direct contact between the insects and the gas is much more difficult to obtain in 

 ordinary room disinfection against mosquitoes than against germs, because the sense 

 of self-protection helps the former to escape from the effects of the irritating gas. They 

 hide in the folds of towels, bedding, clothing, hangings, fabrics, and out of the way 

 places, where the formaldehyde gas does not penetrate in sufficient strength to kill 

 them. The gas is polymerized and deposited as paraform in the meshes of fabrics, 

 which prevents its penetration, and large quantities are lost by being absorbed by the 

 organic matter of fabrics, especially woolens. In our tests whenever the insects 

 were given favorable hiding places, such as in crumpled paper or in toweling, they 

 quickly took advantage of the best place for themselves and thus escaped destruction. 



There is a striking analogy between the strength of the gas and the time of exposure 

 necessary to penetrate the fabrics in order to kill mosquitoes and the strength and time 

 necessary to penetrate in order to kill the spores of bacteria. 



Mosquitoes have a lively instinct in finding cracks or chinks where fresh air may be 

 entering the room, or where the gas is so diluted that they escape destruction. They 

 are able to escape through incredibly small openings. Some of the smaller varieties, 

 such as the Stegomyia fasciata can get through a wire screen having 12 meshes to the 

 inch. Therefore, formaldehyde gas can not be trusted to kill all the mosquitoes in a 

 room which can not be tightly sealed. 



It was concluded that to succeed in killing all the mosquitoes in a closed space 

 with formaldehyde gas the following definite requirements are essential: A very large 

 volume of the gas must be liberated quickly, so that it may diffuse to all portions of 

 the space in sufficient concentration. The room must have all the cracks and chinks 

 where the insects may breathe the fresh air carefully sealed by pasting strips of paper 

 over them. The room must not contain heavy folds of drapery, clothing, bedding, 

 or fabrics in heaps or so disposed that the insects may hide away from the full effects 

 of this gas. 



a Bui. No. 6 of the Hygienic Laboratory, September, 1901. 



