29 

 MOSQUITO BREEDING IN SALINE WATERS. 



By Andrew Balfour, C.B., C.M.G., M.D. 



Director-in-Chicf, Wellcome Bureau of Scientific Research. 



In the " Bulletin of Entomological Research " for December 1920 Dr. J. M. Dalziel 

 deals in a most interesting and instructive way with the various, and sometimes curious 

 breeding-places of mosquitos in Lagos. He mentions the occurrence of mosquito 

 larvae in saline waters, and in a foot-note refers to a number of salt-water species. 



The subject of mosquitos breeding in salt and brackish waters possesses not only 

 considerable scientific interest, but is of practical importance in anti-malarial work, 

 for the suggestion has frequently been made to abolish ordinary mosquito breeding- 

 places by the introduction of salt or of sea- water, and in some instances this procedure 

 has actually been carried into effect. As will be seen, in the case of certain species of 

 mosquitos it is useless or worse than useless. 



Moreover, there is the question of the efficiency of larvicides in saline waters, a 

 matter to which attention has been directed, but on which, so far as I know, our 

 information is still defective. 



It may therefore be of interest to refer a little more fully to the subject than 

 Dr. Dalziel has done, although at the present time I am unable to deal exhaustively 

 with the matter, and, after a few remarks, propose merely to supply some annotated 

 references supplementary to those furnished by Dr. Dalziel, and dealing almost entirely 

 with mosquitos known to be vectors of disease. 



I hope that Mr. MacGregor, in charge of our Entomological Field Laboratory at 

 Wisley, in Surrey, may be able to carry out some research on the question in the 

 ensuing spring and summer, so far as it concerns indigenous species of mosquitos. 



My attention was first specially directed to the subject when Mr. Harold King, 

 the Entomologist of the Wellcome Tropical Research Laboratories at Khartoum, 

 found Culex sitiens, Wiedemann {sains, Theobald), larvae in sea- water at Port Sudan, a 

 fact to which Dr. Dalziel refers in his paper, and when, at Khartoum, I found Anopheles 

 (Pyretophorus) costalis, Loew, breeding freely in brackish pools formed by seepage 

 through the " weeping soil " of irrigation channels in Khartoum North. I had the 

 water of these pools analysed, but unfortunately cannot find the record. I think, 

 however, I am correct in saying that it contained at least 2 per cent, of common salt. 



Hence this Anopheline can breed in waters similar to those in which, as Dr. Dalziel 

 mentions, Willcocks in Egypt found the larvae of Anopheles multicolor, Camboulin, 

 or, as it used to be called, Pyretophorus cleopatrac. This is of interest in view of 

 the fact that Graham recommended the salting of water containing the larvae of 

 A. costalis. He added common salt in the proportion of 3 per cent., and found that it 

 caused disintegration and precipitation of the motile algae upon which the larvae 

 feed. The latter, being thus deprived of their natural food, become cannibalistic. 

 Salt, he says, in lesser concentration appears to inhibit the growth of young larvae, 

 probably by diminishing their food supply, but seems to hasten the fully-grown larvae, 

 which become pupae more rapidly than usual. 



As a matter of fact Dutton in 1902 had already shown that in the Gambia 

 A. costalis could breed in salt-water pools. Its larvae were found along with those of 

 Cnlex thalassitis, Theobald. 



Apparently Grassi in Italy was the first to direct attention to Anophelines breeding 

 in sea-water. Nuttall, Cobbet and Strangeways-Pigg took the larvae of Anopheles 

 maculipennis, Meigen, in brackish water in England — twice in ditches, four times in 

 pools — and Christophers and Stephens discovered Anopheline larvae in brackish pools 

 at Accra containing 0-6 per cent. salt. 



