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Volume 50, 1918
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Art. XIX.—On Mosquito Larvicides.

[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 investigate 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 Government 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 Gorgas, 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 complete: it is very fatal to insects, and a larva thrusting the breathing-siphon

[Footnote] * “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° C. The two higher fractions are known as “medium oil” and “heavy oil” respectively.

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into the film is paralysed and seldom comes again to the surface. Dishes of equal size, 2 ft. by 1 ft. 3 in., containing the same quantity of water and the same number of larvae at the same stage, have been treated, one with light oil, the other with crude petroleum in like amount. In all experiments—and I have repeated them over a dozen times—the larvae under the film of light oil have been dead or helpless on the bottom within fifteen minutes, while in the petroleum dish some have been active after an hour or more. In view of the possible breaking of films, comparative rapidity of action is a matter of great importance. The experiments referred to have been repeated on the large scale on pools in various parts of the North Island, and the laboratory results have been amply confirmed.

The film is best produced by spraying the pool, but the oil may be sprinkled from a bottle or other vessel, or a leafy twig may be dipped in it and shaken over the water. In choosing a spraying instrument for light oil it is necessary to choose one without rubber tubing, as some constituents of light oil are solvents of rubber.

Experiments with regard to the toughening of films to render them less easily broken are now being made. Up to the present I have found nothing better than raw linseed-oil. It should be shaken up well with the light oil before being applied. I am not yet sure that the advantage gained is sufficient to justify a strong advocacy of its use; but it certainly does make a film more resistant.

In testing the killing-power of crude carbolic acid I have taken the formula for the emulsion from the report of the Wellcome Laboratory at Khartoum for 1911, p. 109, where directions sent from Panama are quoted: “Crude carbolic acid* containing about 15 per cent. phenol is heated to 212° F., finely pulverized resin is added, and the mixture kept boiling until the resin is all dissolved. Caustic soda is then added, and the mixture kept at 212° F., for about ten minutes, or until a perfectly dark emulsion without sediment is obtained. The mixture is thoroughly stirred from the time the resin is added until the end.” It is stated that 1 part of this mixture in 5,000 parts of water containing mosquito-larvae will kill all the larvae within five minutes. If it is used in the proportion of 1 to 8,000 the larvae are killed in thirty minutes. In my experiments I was unable to obtain results as good as these. I obtained, however, much better results when using an emulsion of light oil.

The experiments tabulated below are only a few of a very long series, and all have been verified by actual work at normally infested pools in the open. With regard to various entries in the table I may make the following explanation:—

In the column headed “Twitching” is noted the time at which the larvae were first observed to be all motionless or twitching helplessly at the bottom of the vessel. This is for all practical purposes the time of death, as the larvae do not recover from this condition unless removed to fresh water. Time of actual death is, however, of importance in view of the fact that mosquitoes sometimes breed in slowly flowing water.

In the column “Apparently dead” is entered the time at which response could not be obtained to weak induction shocks.

[Footnote] * A fine account of the efficacy of crude carbolic acid and other larvicides is given by Howard, Dyar, and Knab in The Mosquitoes of North and Central America and the West Indies, vol. i, pp. 379 et seq., Carnegie Inst., Washington, 1912.

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In the column “Dead” is noted the time at which removal to abundant fresh water was made in cases where this removal did not bring about at least temporary recovery. For this purpose I regarded proof of death as sufficient if no movement of any kind took place within twenty-four hours. No very young larvae were used in these experiments.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Larvae observed to be
Emulsion. Strength. Number of Larvae. Time. Active. Twitching. Apparently Dead. Dead. Remarks.
Carbolic ¼,000 3 5 p.m. 6 p.m. 7.30 p.m. 9 p.m. Recover in fresh water.
" ⅛,000 3 6.25 p.m. 7.25 p.m. 11.30 p.m. "
" 1/16,000 3 6.30 p.m. 3 days later 4 days later One recovers in fresh water.
Cresolia ¼,000 3 3.40 p.m. 4.20 p.m. 4.20 p.m. Dead in 40 minutes.
" ⅛,000 2 3.50 p.m. 4.30 p.m. 6.20 p.m. 6.20 p.m. Dead in 2 hours 30 minutes.
Light oil ¼,000 * 2 4.50 p.m. 4.52 p.m. 4.52 p.m. Dead in 2 minutes.
" ⅛,000 6 3.15 p.m. 3.22 p.m. 3.55 p.m. 3.55 p.m. Dead in 40 minutes.
" 1/16,000 * 6 2.50 p.m. 2.52 p.m. 3 p.m. 7 p.m. 7 p.m. No observation between 3 p.m. and 7 p.m. Dead within 4 hours 10 minutes.
" ⅓2,000 6 2.50 p.m. 3 p.m. 7 p.m. 2 p.m. next day No observation between 7 p.m. and 2.52 p.m. next day. Dead within 24 hrs.

Many other substances, including well-known disinfectants and plantsprays, were used, but with no results worth publishing. Sulphates of iron and of copper, potassium ferrocyanide, and other well-known substances gave, in the dilution of 1 in 4,000, negligible results.

From the above table it will be seen that when there is no access of fresh water an emulsion of light oil may be used in the proportion of 1 in 32,000. The emulsion that will give this result must, of course, be one that contains nearly all the light oil that the emulsionizing agent can carry and that has no needless water. The formulae here given, chosen from a number that have been arrived at, may be relied upon:—

(1.) Soft-soap 100 parts.
Light oil 440 "
Water 100 "
Caustic soda 80 "

It is best to add the light oil after the other substances have been heated together to a temperature of 100° C.

This is a thick jelly, and may be diluted with water to liquefy it.

(2.) Soft-soap 20 parts.
Light oil 50 "

A thick jelly-like soap.

Where transport was not an important consideration the desired amount of water to make these emulsions liquid would usually be added when they were being made.

(3.) Castor-oil 50 parts.
Caustic soda (sat. solution of 98 per cent. caustic soda) 15 "
Water 20 "
Light oil 170 "

[Footnote] * One of these was a pupa.

[Footnote] * One of these was a pupa.

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It is best first to make a soap by boiling the castor-oil and the causticsoda solution. When an even yellow-green soap is formed the light oil may be added. Constant stirring is, of course, necessary whichever formula is used.

As is well known, potash is generally more suitable than soda, but its greater cost makes it unsuitable for this purpose. Owing to a shortage of potash, soft-soap is becoming costly, and therefore other emulsionizing agents are being experimented with. Up to the present good results have been got with resin, neatsfoot-oil and whale-oil. The last-named is the cheapest, and will be used for work in military camps. Unfortunately, it is sometimes difficult to saponify it by the means always available.

The castor-oil emulsion referred to above is a clear liquid emulsion, and keeps well.