Effective poisons are such good exterminators of flies, that the main consideration is how to safely and most suitably employ them. There are some people who have an invincible aversion to the mere thought of poison purposely administered by way of food or drink, though possibly they do not have an equally strong repugnance to the use of insecticides used for stifling. However, one cannot help running counter to much misplaced sentimental humanitarianism in some people on some subjects; reasons and arguments will not move them, for they do not wish to think otherwise than as their prejudices influence them. The house-fly is itself a poisoner of our food, and it, or rather she (the offender being nearly always female), is a more dangerous and a more subtilely baneful enemy than, for instance, the human flea, which even the Brahmans or the disciples of Buddha may kill.
Fumigatory insecticides, though occasionally useful, may be left out of consideration in discussing the rival merits of means in a warfare against the house-fly.
A liquid or moist food poison employed in a safe and effective way will excel every other weapon of warfare within the house. One of the newest recommended substances is formalin, which has the advantages of being a disinfectant, a strong fly-poison not avoided by flies, and not dangerous or attractive to domestic animals. A tablespoonful of (40 per cent.) formalin should be mixed with one half pint of milk and water; this, when exposed in saucers or shallow dishes, is said to be an attractive and a fatal bait. It would be evidently dangerous and objectionable to use some other commoner poisons in the same way; but it is the opinion of some users that formalin and milk is not sufficiently alluring.
Contrary to a generally prevalent idea a powerful odour is not required as an indoor allurement for the common house-fly; again, a saucer or shallow dish with liquid contents is not a good method of presenting the fatal bait. Placed on a table, or on a window sill, or on a shelf, a saucer is liable to be tipped up and its contents spilt; moreover, the form of such receptacles is radically faulty by reason of the strenuous walking habits of the fly on the level. Out of many flies walking over a table ten or twelve may pass by, or round, an overhanging saucer's rim to one fly that will mount the same and sip inside. However, let such a saucer or plate be placed on a table upside-down, and let a slightly moist substance be placed in the shallow central depression, which ordinarily is the base, then the said ten or twelve flies will all mount and sample the moist substance, even though it be not apparently attractive in smell; an inclined plane is ever an irresistible invitation to mount and prospect the summit.
It is another great mistake to suppose that an extra tasty food material is desirable as a bait. The same kind of mistake is made by people baiting a mouse trap with toasted cheese, whilst a bit of dry bread, or better still, a green pea, would much better entice a common mouse. Strong smelling and saccharine foods immensely attract the blue-bottle and the wasp, which are thus enticed indoors and induced to become occasional visitors to our tables; but the house-fly requires little of such lures; indoors she is an inquisitive prospector, who will never pass by any moist material without testing its quality. Moreover, the use of poisoned milk, or even jam, should be rather avoided for fear of injury to dog or cat; furthermore, it is dangerous to place a piece of bread in a saucer of liquid fly-poison, as is sometimes done, to serve as a sop and as a standing stage. However, there is one good lure well worth mentioning; it is beer-dregs with or without a little sugar; moistened yeast is good, and the advantage of beer-dregs with just a little sugar is due to the mild yeast-like odour of slow fermentation, which may fail if formalin be the insecticide ingredient.
The handiest and safest preparation of fly-poison is that sold in the form of a dry flypaper, which is said to contain arsenic as the deadly ingredient. A very small piece of one of the sheets ordinarily sold should be placed on the summit of an inverted saucer; a mere spoonful of water now and then will suffice to moisten the same; there is little or no advantage in sprinkling a little sugar thereon, unless beer-dregs are added. This plan of using moistened poison paper is clean; it is safer than using a more fluid bait, and the ingredient is certainly efficient; the slightest taste thereof by an inquisitive fly ensures its speedy death. Another poison which has been recommended is a strong decoction of tea-leaves, to which a little sugar and beer may perhaps be added.
It is said that the smell of geraniums is odious to the house-fly, and so pots of these plants may be grown beneficially on window-sills. Certain other odours and scents are believed to be likewise more or less fly deterrent, but their use is not effective warfare against fly propagation. Paraffin painted on window-sills is said to be very efficient.
Flies may be easily prevented from entering the open windows of any room, which has windows only on one side, by the use of venetian or louvre blinds or shutters, or of many kinds of screens, although the apertures thereof may allow of ample room for flies to pass to and fro. If, however, there be windows on two sides of a room, then venetian blinds and the like will be useless, and window screens must have very close meshes to be effective. The house-fly will pass through netting only when there is light shining on the further side. A knowledge of this fact is very important in the planning of hospital wards. In a sick room, if there be windows on two sides, one in summer time should be darkened when the other is open for ventilation.
The protection of the larder and the screening of food should never be neglected, but what is of even greater importance is the prevention of access by flies to fæcal matter, or to purulent and all unhealthy discharges from the sick room.