Kirk. — On Mosquito Larvicides. 



Art. XIX. — On Mosquito Larvicides. 



By H. B. Kirk, M.A., Professor of Biology, Victoria University College. 



[Read before the Wellington Philosophical Society, 25th July, 1917 ; received by Editors, 

 31st December, 1917 ; issued separately, 10th June, 1918.\ 



In connection with work that has been entrusted to me in some of the 

 military camps in the matter of fly-control it has been necessary to investi- 

 gate the effectiveness of various substances as killing agents. Incidentally, 

 the matter of larvicides for mosquitoes came under investigation. This 

 year the New Zealand Institute has set aside a sum of £25 from the Govern- 

 ment research grant, which sum may be drawn upon in refund of actual 

 expenses in investigating and experimenting in this direction. The present 

 paper deals mainly with the relative value of certain mosquito larvicides. 

 The experiments have been made mainly with the larvae of various species 

 of Culex found in New Zealand and with the larvae of a culicine mosquito 

 found often in brackish water on the coast near Wellington. As there is 

 probably a real danger that Anopheles or other harmful forms may at 

 any time be introduced into New Zealand, it is advisable that methods 

 of extermination should be as effective as possible. I hope to be able 

 shortly to make some contribution to our knowledge of the best means of 

 dealing with adult mosquitoes. 



The work that has been done in the Panama Canal zone under Corgas, 

 at Khartoum under Balfour, and in other places where disease-bearing 

 mosquitoes occur is well known. Larvae are generally dealt with by means 

 of a film that prevents their breathing when they come to the surface, or 

 by use of a lethal agent that diffuses evenly throughout the water. The 

 substance used as a film is generally crude petroleum. One of the best- 

 known direct lethal agents is an emulsion of crude carbolic acid. 



In Notes on Fly-control in Military Camps, issued last year by the 

 Defence Department, I called attention to the value of light oil* as a 

 killing agent. It is sprayed in mixture or in emulsion with 3 or 4 parts of 

 water, and is very fatal to maggots and to adult flies. It has to be applied 

 in greater strength to kill fly-pupae. Experiments with light oil as a 

 mosquito larvicide show that it is a most valuable substance, whether used 

 as a film or as an emulsion. 



The question whether it is best to use a film or an emulsion depends 

 upon several considerations. Of these, the relation of volume of water to 

 surface may be important. This is, however, a consideration of economy 

 or of ease of treatment. A consideration of actual efficiency is the exposure 

 of the surface to wind. If a surface is wind-swept a film is broken very 

 quickly. Certain experiments in toughening the films will be referred to 

 later. In certain cases it may be best to use both film and emulsion, 

 especially if many pupae are present, these being less easily killed by 

 the emulsion than are the younger larvae. 



Light oil makes a film that spreads more rapidly than crude petroleum ; 

 its colour enables the operator to see at a glance whether the film is com- 

 plete : it is very fatal to insects, and a larva thrusting the breathing-siphon 



* " Light oil " is the lowest of the three great fractions into which the distillation 

 products of coal-tar are first broken, and it comprises those constituents that have 

 a boiling-point up to about 200° or 210 J C. The two higher fractions are known as 

 " medium oil " and " heavy oil " respectively. 



7— Trans. 



