Popular Science Monthly/Volume 37/September 1890/Can the Mosquito Pest Be Mitigated?
THE annoyances caused by flies and mosquitoes have invited the special attention of Dr. Robert H. Lamborn, and prompted him to efforts to secure such study of their life histories and of their natural enemies as might lead to the discovery of some practicable means of mitigating their depredations. In 1889 he addressed a circular letter to the working entomologists of the country, offering prizes for essays containing original investigations regarding methods of destroying these pests. He had especially in view the utilization of the dragon-fly—a harmless insect, but at the same time exceedingly voracious and very fond of mosquitoes—and the possibility of propagating it artificially in places where mosquitoes abound. The results of the correspondence he had on the subject are published in this interesting book of studies, which, while it fails to verify the hopes which Dr. Lamborn entertained respecting the dragon-flies, does not fail but is encouragingly successful in pointing out some methods of considerable probable efficiency for reducing the numbers of the hosts of these enemies of mankind. The prizes were awarded by a committee consisting of Drs. H. C. McCook and J. S. Newberry—the first prize to Mrs. Eugene Aaron, of Philadelphia, for an essay on The Dipterous Enemies of Man; and the second prize equally divided between Messrs. Archibald C. Weeks and William Beutenmuller, of New York, for papers on The Utility of Dragon-Flies as Destroyers of Mosquitoes and on the Destruction of the Mosquito. These, with other contributed papers, are embodied in the volume. From Mrs. Aaron's essay we learn that the Culicidæ, or mosquitoes, breed in stagnant water, and have been observed living, in all stages of growth, in the most insignificant puddles—as "in a puddle of water, eight inches square and one inch deep, made by the rain in an iron pulley in a foundry-yard. They are also to be observed teeming to overcrowding in the hoof-holes in boggy cow-pastures. But the shallows occasionally overflowed and replenished by rivulets in swamps, the stagnant pools formed by ditches without outlets, and the vastly more numerous murky pools made by the joining of tufts of grass in marshes, are the usual breeding-places in the rural districts. In village and urban localities rain-tanks, undrained gutters, badly paved, damp byways, and garden ditches are the most fruitful places for recruiting their numbers. These surroundings are selected by the female with a view to the fact that from three to four weeks will be required to perfect the changes from the egg to the imago; and they must be situated so as to receive sufficient water from rain or outside overflow to replenish the evaporation or soaking into the ground. In this selection the female shows the usual instinct which is so noticeable in insect economies." When hatched, they hug the sides of pools and shallow margins, and, spending most of the time at the surface with the orifice of the air-tube just in contact with the air, are not usually found at great depths. They are easily frightened by any stir or motion from above, but pay little attention to any dangers that may menace them from the water. Very little is known of their feeding habits; but the statement that they are scavengers, feeding on decaying substances in stagnant water, has not been confirmed or disproved. They have been observed to feed on minute animals, and to destroy young trout. They go through several transformations, and reach a curious shape in the pupa, which—the head, thorax, legs, and wings, all being folded in one mass, and the abdominal segments being left free for the purpose of navigation—has a top-heavy and clumsy appearance, although it is quite as active as the larva. After the insect has matured and has begun its flight, the principal objects in its remaining brief existence "are the search for the desired mate and the duties of reproduction. To suppose that the tormenting of man occupies any considerable time in the mosquito economy is certainly a mistake. It is only the female which can thus make our lives miserable" They are local in their range, and the supposition that they can be carried long distances by the wind is declared a mistake. House-flies are omnipresent with us, while mosquitoes appear only in spots. According to Packard, "fresh horse-manure, with plenty of heat and moisture, furnishes the best food for the young maggot. From a hundred to a hundred and fifty eggs are deposited in irregular, loose sacs, usually within eighteen hours, and hatching in twenty-four hours or less. The maggots molt twice; the three stages of larval development being of the following periods: first stage, one day; second stage, from twenty-four to thirty-six hours; third stage, three or four days. To this maximum period of seven days is to be added the same length of time for the pupal life; thus it will be seen that fifteen or sixteen days are required for the entire development from egg to imago."
The expediency of trying to exterminate them is more than doubtful, for, according to the same author, "it should be remembered that flies have an infancy as maggots, and the loathsome life they lead as scavengers cleanses and purifies the August air, and lowers the death-rate of our cities and towns. Thus the young of the house-fly, the flesh-fly, and the blow-fly, with their thousand allies, are doing something toward purifying the pestilential air and averting the summer brood of cholera, diphtheria, and typhoid fevers which descend like harpies upon the towns and cities. It is a useful species, to which man owes more than he can readily estimate, and with which he can dispense only when the health of our cities and towns is looked after with greater vigilance and intelligence than is perhaps likely to be the case for several centuries to come."
Mosquitoes, therefore, are entitled to exclusive attention in the exterminating effort.
Mrs. Aaron has a poor opinion of the efficiency of the dragonflies, or Odonats, as mosquito-destroyers. They become rarer about the time that the mosquitoes are most numerous. In the matter of flight they are very local, and it seems impossible to conceive that they could ever be brought to frequent deep woods or city streets where mosquitoes abound. The author's observations of their feeding habits lead her to believe that they prefer robust, meaty insects, and that studies of their appetites in confinement are misleading. The habit of migration among them will also militate against their efficiency as mosquito-destroyers.
Other writers find that they are capable, in natural conditions, of working great havoc among mosquitoes, but doubt the utility of efforts to improve on nature in the matter. Captain C. B. N. Macauley, U. S. Army, relates that, in a mosquito-infested region of Montana, he was told how the mosquitoes had disappeared, as if by magic, on the sudden appearance of a brood of devil's-darning-needles or dragon-flies of rather large size. The agency of this insect in the matter was corroborated by the evidence of squaw-men and Indian traders, who said that the flies did not appear every mosquito year, but, when they did, they came in droves and cleared the mosquitoes out. They were called "mosquito-hawks." The captain himself afterward had an opportunity of observing them at work, and to determine that they were dragonflies. "I noticed," he says, "that they flew in an irregular kind of skirmish-line, moved slowly, and every now and then made what he described as short 'dabs' at apparently nothing. Mr. Heistand said that 'each one of these dabs means a mosquito.' I was curious to see how deliberate they were about it, and how fairly aligned the skirmish-line was. They appeared somewhere about 11 a. m., and when I went into the post later I crossed the parade-ground and saw detachments of about half a dozen flying slowly about. They stayed at about an average of three feet from the ground. I do not know how late they kept it up or how early they began. They stayed until all the mosquitoes appeared to be gone." Dr. Lamborn also tells how his own attention was drawn to the subject. It was while he was in the forests of Lake Superior, railroad-building. "Sitting in camp while supper was being prepared, I often, with a sentiment of gratitude, looked through my mosquito-veil at the dragon-flies that collected in the open spaces among the pine-trees. They darted from side to side like swallows in a meadow, but with amazing rapidity, and at every turn, the natives assured me, 'a mosquito ceased from troubling.' Afterward I happened to observe an entomologist feeding a dragon-fly that had eaten thirty house-flies in rapid succession without lessening his voracity. What thought could be more natural than the one that came to me, that an artificial multiplication of dragon-flies might accomplish a mitigation of the mosquito pest?"
Mr. Beutenmuller, of the Museum of Natural History, New York, avers that "the dragon-flies (Odonata), especially the Æschinus, Gomphina, and Libellulina, are the natural enemies of the mosquitoes; they are voracious—they sometimes appear in great numbers, and, as a matter of fact, the mosquito disappears before them, while their breeding-grounds are, in many respects, similar, so far as fresh and brackish water habitats are concerned; and, finally, in the metamorphosis of the dragon-fly we meet conditions which introduce it in antagonism to the mosquito at the same stages of development." The dragon-fly, however, prefers sunlit areas, and will not live in the woods.
These are the conditions in nature. But the attempts to subject dragon-fly life to the rules of art do not appear to have been successful. Mr. Weeks tried earnestly and most intelligently to raise the insects artificially on his father's farm on Long Island and in his house in Brooklyn, and failed to obtain any results worth boasting about. He finds that they are diurnal, working in the sun, and never present at night, when the mosquitoes are busiest; that they are short-lived, and frequently destroyed in large numbers by heavy showers and winds; that, with few exceptions, they confine themselves to the vicinity of their place of birth, and, if removed therefrom, quickly return—hence, can not be colonized; and he concludes that "an attempt to destroy flies and mosquitoes by the artificial propagation of dragon-flies or any other insect would be unprofitable, unadvisable, and impracticable."
Mr. Beutenmuller thinks that positive statements can not yet be made respecting the expediency of artificially breeding dragon-flies for use against the mosquito. Differences in the habits of the two insects are against the scheme. Dragon-flies seek open places and the sun, while the mosquito finds hiding-places in the woods and in tall grass. "Under these circumstances the dragon-fly will not find its prey. Great numbers will escape; only those encountered in its busy flight through the air will be captured, for the dragon-fly does not hunt for its booty nor scour the forbidden shadows of woods and forests, and at nightfall the mosquito will elude his pursuer and rise to his murderous intent." But the dragon-fly "may, in some genial locations, suit the elements of the question and be of practical service; it may, indeed, be more widely beneficial than we suspect."
Of other means of keeping down mosquitoes, Mrs. Aaron recommends flushing the breeding-places with water, draining swamps, creating active artificial currents, encouraging fish, and spraying their hiding-places with petroleum. Mr. Weeks has faith in the enforcement and observance of sanitary laws and the encouragement of birds. Mr. Beutenmuller advises the use of lanterns so arranged as to attract and destroy the mosquitoes, with pans of kerosene or other strong mixtures for their destruction, which may be placed around houses and hotels and in marshes, general and scientific drainage of swamps; encouragement of fish and waterfowl; and, where the conditions are favorable, the use of coal-oil in the waters of estuaries of rivers and on the rain-invaded areas of deep woods for destruction in the larval stages. Astringents, like logwood or alum, will also prevent the growth of the mosquito in its incipient stages. Dr. H. C. McCook thinks it might be well to call spiders into service.
The most generally effective of these remedies seem apparently to the authors in the book to be petroleum spraying and draining. A very little petroleum, spreading itself in a minute film over the water surface, will go a great way in destroying the larvæ. Drainage also promises to be very efficient. "It goes even farther back than the larval stage, for it precludes the incipient acts of the mosquito at propagation. It robs her of the congenial nidus for the development of her eggs." The question can, however, only be satisfactorily settled by a concerted movement over wide tracts of land. "The arrest of the plague in one portion of the country when the next section makes no effort to suppress its own contingent can only lead to discouragement and ridicule." Against the house-fly the most promising measure of offense is the encouragement of the fungus that destroys it, which is identical with the yeast-plant; but, as flies seem to do as much good as harm, it will probably be wisest to leave them alone.