For larger collections of water, by far the best appliance is an ordinary gardener’s watering-pot. The oil should be applied mainly along the windward side of the pool by a coolie, who should, if possible, walk out some distance into the water, and should be trained to sprinkle the oil by rapid single sweeps of the rose of the can, a few steps being taken between each sweep, as there is no need to make the loop-shaped areas of water sprinkled in this way continuous, as the oil will spread laterally and join each of them together even if separated by several yards, and no advantage whatever appears to be gained by applying the oil thickly. In exceptional cases, such as the moats of fortifications, where the water is enclosed within high vertical walls, a garden syringe may be required in place of the watering can.
In situations such as the pools in canal beds, ditches, &c., the oiling must obviously be done after each flow of water through the channel; but in most ordinary situations, the effects of a careful oiling may be trusted to last for at least three weeks, as, though larvæ may be beginning to reappear by the end of that period, none of them will have had time to complete their metamorphoses. For the destruction of wintering larvæ, two oilings, one at the commencement, and one towards the end of the cold season, amply suffice; and in Continental hot climates it is needless to repeat the process during the dry hot season, as at that time only artificial breeding places, such as garden tanks, require attention, and these are better dealt with by periodical emptying.
At that season indeed, the undried-up breeding places are so few and far between that people have only themselves to thank if mosquitoes are in evidence at all; and yet in many places there is no season of the year when they are so numerous and tormenting. There are no remaining natural breeding-places, and the pains of a weekly stroll round one’s premises to ensure the emptying and cleaning out of all garden tanks, water-vessels, &c., are all that is required to secure complete immunity at that season of the year, but it is most difficult to induce people to take even this trifling trouble; and the mosquitoes are likely to flourish undisturbed until the existence of breeding places within the premises of any person is treated by the authorities in the same way as other dangerous nuisances; and yet these very people are the loudest in their condemnation of the inertia of the native in sanitary matters, and while they maintain malaria breweries on a scale in which the modest extent of his premises forbids him to compete, inveigh against him for starting a cholera factory on the most modest lines. The native who refuses to avail himself of the protection against plague of a health camp is, they admit, an impracticable fool for his pains; but the sahibs of the civil lines, whose gardens supply a large share of the harmful mosquitoes to the neighbouring city, are merely “common-sense” people who attach no importance to doctor’s fads and “scientific rot” of all sorts. The native, however, has at least usually the excuse that he is unable to read or write; and for the rest, does not pretend to be a very highly civilised person.
Once they have emerged from the pupa case, our means of attacking mosquitoes are comparatively feeble, and for those that habitually pass most of their time in the open air practically nil.
In the case however of domestic species, which habitually shelter in houses, a good deal can be done in this matter; and, just as in the repression of other troublesome insects, such as fleas and bugs, scrupulous tidiness and cleanliness is by far the most important of all agencies. Useless, rarely dusted draperies and curtains, and untidy collections of clothing hung about on nails and pegs, instead of being kept in properly closed wardrobes, are the things that are mainly responsible for attracting and sheltering mosquitoes within houses, for they will not, if they can avoid it, remain in a well-lighted room, with freshly colour-washed bare walls.
In the tropics, the Italian villa with its frescoed walls and minimum of useless furniture, is the ideal that should be aimed at, and not the elaborate lumber warehouse of an English drawing-room; which, though comparatively harmless in our own climate, is about as well adapted for imitation in hot countries as the coat of the Polar bear in the Zoo is suited to our summer.
Perhaps our most effective agency in dealing with adult mosquitoes depends on the intense objection all species of these insects entertain to smoke.
That it is quite possible to effectually protect animals from mosquito bites by the agency of smoke alone, has recently been conclusively shown in the course of Mr. Power’s experiments on the prevention of horse sickness in South Africa; but, though most semi-civilised people seem to live comfortably enough in a smoky atmosphere, Europeans would find such a state of things intolerable; and the plan can only be utilised to drive mosquitoes out of houses, other methods being relied on to prevent their re-entering. Smoke from almost any source will put mosquitoes to flight, and if sufficiently intense will stupefy them; but certain special materials must be burnt if it is desired to kill them outright, and our modus operandi must be varied according to the means at our disposal, for if we can only expect to annoy the insects sufficiently to drive them out of doors, the latter must be left open during the fumigation; whereas, if it is proposed to kill them, all openings must be closed as completely as possible, so that the fumes may reach the insects in as concentrated a form as possible.
For simply driving out mosquitoes, any fuel that produces a dense smoke, such as damp straw, will serve. Maize cobs are excellent for the purpose, but probably the least objectionable is the burning of a little incense (Hindustani, Lobán). To actually kill them, on the other hand, it is necessary to shut up all openings as closely as possible, and to burn certain special substances, such as sulphur, unopened chrysanthemum flowers, the leaves of the neem tree, tobacco, &c., and the degree of success attained will vary a good deal according to the character of the building in which it is attempted; it being obviously extremely difficult to secure an adequate concentration of the fumes in a very pervious structure, like a house with a thatched roof and walls of bamboo matting, such as is often met with in Burmah and the Malay Archipelago. Under such circumstances, the rapid burning of a large amount of the fumigating agent is the only way of meeting the difficulty; and under any conditions, plain sulphur, simply ignited as a powdered mass, or placed on the hot coals of a charcoal brazier, burns so slowly as to be quite useless.
A few instants’ exposure to an atmosphere containing a fairly high percentage of sulphurous acid will kill any insect, whereas they will be merely stupefied or unaffected by prolonged exposure to a weaker mixture of the poison. I am quite aware that sulphur has been decried as almost useless by several observers, but this is simply because they have not used it properly; for to secure success, something must be added to the sulphur which will make it burn quickly, and what is wanted is, in fact, a weak firework. In well-constructed buildings the sulphur pastiles prescribed in the second edition of my work on the “Gnats or Mosquitoes” answer sufficiently well, but subsequent experience has shown that the proportion of nitre is not enough to secure a sufficiently rapid combustion to instantly flood the air with sulphurous acid, and that nothing short of a slow burning firework, such as a Roman candle or Bengal light, will do this in a building roofed with tiles or thatch. Possibly it might be advantageous to raise the proportion of sulphur; and it is undoubtedly better to pack the material as a powder in paper cases, than to mould it into pastiles. Proper precautions against fire must of course be taken, but in fireworks of this sort the sparks do not fly far, and it is quite safe to use them in any ordinary room if the precaution be taken of placing the cases under a low shield, formed of a sheet of corrugated iron supported on four piles of bricks.