Popular Science Monthly/Volume 63/September 1903/Mosquitoes and Suggestions for their Extermination
|MOSQUITOES AND SUGGESTIONS FOR THEIR EXTERMINATION.|
By WILLIAM LYMAN UNDERWOOD,
LECTURER IN THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY.
THE statement has been frequently made of late that there is no more reason why we should suffer from mosquitoes than there is that we should allow rats and mice to continually annoy us, and this statement is in a measure true. Rats and mice are to a great extent effectively held in check; for we have become accustomed to them and their habits, and we know how to deal with them. Were it not for the fact that a constant warfare is being waged against them, they would soon overrun our houses and make life unbearable.
In order to fight the mosquitoes successfully it is important that every one should take an interest in the popular uprising against this insect pest. And now that it is known that, besides being a nuisance, mosquitoes may be a menace to the health of the community, it is equally necessary that every one should become familiar with all that pertains to their life history so that the war against them may be successfully and intelligently carried on. Notwithstanding all that has been written on the subject of mosquitoes, during the last year or two, the majority of people still know but little about them.
It is the purpose of this article to state, in as simple a manner a? possible, the facts that are now known regarding mosquitoes and how to deal with these pests, and it is hoped that this information may help to secure a more general cooperation in the work of mosquito extermination.
Few people realize that there are a great many different kinds of mosquitoes. Some three hundred species have already been described, and here in the United States we have about fifty species, belonging to nine different genera. The most common of these genera in the northern states are Anopheles, the malarial, and Culex, the ordinary, mosquito. Of the former there are two species and of the latter at least fifteen.
Only these two genera and the methods for their extermination will be especially considered, and as these methods may also be successfully applied to the other kinds of mosquitoes, no detailed description of the others need be given.
It is commonly and quite naturally thought that mosquitoes breed in wet grass, as they are often seen to rise from it in clouds when disturbed, particularly in the early morning and evening. They have not bred there, however, but have merely sought the shelter of the grass where they can be protected from the wind. The moisture of the dew upon the grass also furnishes an attraction for them and they always prefer damp rather than dry places.
Another popular theory is that mosquitoes will breed only in foul or stagnant water. This is also a mistaken idea though they often do breed in such water, not because it is impure or stagnant, however, but because these places are usually quiet and here the female can deposit her eggs undisturbed.
It is commonly supposed that mosquitoes do not breed in salt water, but the recent 'Mosquito Investigations' of Professor John B. Smith, of New Jersey, which were published in the New Jersey Agricultural College Experiment Station Report of November, 1902, show that the larvæ of Culex sollicitans the 'Salt marsh mosquito,' not only prefer salt or brackish water, but are seldom found in pools where the water is strictly fresh, and, contrary to the usual custom, this mosquito lays its eggs upon the soil of marsh or meadow land. There the eggs remain until the advent of an unusually high tide. Then after a few hours when the water has covered them, the infant larvæ make their appearance.
It is very generally believed that mosquitoes bite but once and then die. This is sometimes so; but, unless they are killed in the act of biting, they usually live to bite again. The female mosquito (for it is only the female that attacks human beings) bites many times. It is owing to this fact that Anopheles is able to convey the germs of malarial fever from person to person. When biting any one who is afflicted with malaria, the insect drawls in with the blood the germs of the disease, which it afterwards carries on into the blood of another victim. The vast majority of mosquitoes never get human blood for food. In its absence they live upon the blood of birds and other animals, and when these are not to be found, upon the juices of young and tender plants.
It is not known just how long mosquitoes can live, but their average life is much longer than is ordinarily supposed. Thousands of them live through winter hibernating or asleep in dark places in barns or house cellars. In sparsely settled localities, where they can not find such places for shelter, they live through the winter in hollow trees, in caves and holes under upturned trees; and, even though the temperature may fall far below freezing, they arc not winter-killed, but on the approach of warm weather become active again. mosquitos are frequently seen flying about in the woods before the snow has wholly left the ground.
Mosquitoes can not develop or come to maturity without water in which to live during the first weeks of their 'wiggler' existence.
A mosquito's life is divided into four stages—the egg, the larva, the pupa and the adult insect. In the larval and pupal stages, mosquitoes are more commonly known as 'wigglers' (see Fig. 1). Both
Anopheles, the malarial, and Culex, the common, mosquito larvæ are present in this picture. Mosquito 'wigglers' may frequently be found in rain-water barrels in as large numbers as are seen in this photograph. The female mosquito lays from one hundred and fifty to four hundred eggs upon the surface of some quiet water, and in a day or two these eggs develop into the larval or second stage (see Fig. 2).
It will be noticed that Culex hangs with its head down, and from its tail upward to the surface of the water extends a small tube. Through this tube it breathes. Anopheles rests just beneath and parallel to the surface of the water, and its breathing tube is much shorter than that of Culex. These resting positions are quite different, and each is characteristic of its kind. Except when disturbed. Anopheles is generally to be found at the surface, breathing and feeding in this position. Culex, on the other hand, comes to the surface only occasionally to breathe. It stays below the water for the greater part of the time, and is often found feeding from the bottom.
At the end of a few days the larvæ change into the pupal or third stage (see Fig. 3). To the left is seen the larval skin out of which this pupa has just come. The difference between Culex and Anopheles in this, the final stage of 'wiggler' existence, is very slight. Both now
live at the surface of the water, and they breathe through two funnel shaped tubes situated one on each side of the thorax, or 'head.' Unless disturbed, they remain motionless in this position at the surface until the time comes when, as adult mosquitoes, they leave the water (see Fig. 4). This is the critical period of a mosquito's life; for, should the surface of the water be disturbed at this time, the insect would be upset and drowned. It takes about seven minutes from the time when the skin along the back of the pupa begins to split until the full-grown mosquito comes forth and in a few minutes is ready to fly away. A mosquito never grows any larger after this change.
The length of time required to pass from the egg to the adult insect varies from ten days to three weeks, according to the temperature. Warm weather hastens their development, while low temperature checks it. The 'wigglers' of some species of mosquitoes live through the coldest weather of our northern winters unharmed, ready, when the first warm days of spring have come, to complete their natural changes.
Mosquitoes' eggs are so very small that ordinarily they remain unnoticed, but nearly every one who lives in the country is familiar with the little 'wigglers' that are often seen squirming up and down in rain-water barrels. Few people know that these little fellows are connected in any way with mosquitoes, but it is a very easy matter to prove that they are. Let any one who doubts this fact dip up a few in a glass jar or tumbler and place them in the house, where they can be frequently looked at. Seeing is believing; and after a full-grown mosquito has once been seen to come forth from a pupa (which is the last stage of the 'wiggler'), there can not any longer be any question as to what these 'wigglers' really are.
Most of the mosquitoes that annoy us are bred near by, often, though unknown to us, in our own dooryards. Any water that is accessible to mosquitoes and whose surface is undisturbed by winds or rapid currents furnishes a breeding-place for them, and 'wigglers' may often be found in water standing in old tin cans or bottles, in rain-water barrels, in pools in the rocks, in roof or street gutters that are not properly drained, in cesspools or in catch-basins, in fact, in any place that will hold water for a week or two, no matter how small the quantity, even if only a few teaspoonfuls.
Since we know that without water mosquitoes in their first stages can not exist, it naturally follows that all standing water should be done away with or treated in such a manner that 'wigglers' can not live in it nor mosquitoes get to it to lay their eggs. To this end all cans, bottles and every discarded utensil that will hold water should be removed. All stagnant pools, where it is possible to do so, should be drained or filled up. Cisterns, rain-water barrels and cesspools should be screened or otherwise covered to prevent the adult insects from having access to them. Where it is not practicable to fill, drain, or screen the places that are suitable for mosquitoes to breed in, the surface of the water may be covered with kerosene oil. This oil, when spread over the water, prevents the 'wigglers' from getting air when they come to the surface to breathe, and so kills them (see Figs. 5 and 6).
In Fig. 5 a 'wiggler' is seen trying to get air, vainly thrusting its breathing tube up into the film of kerosene.
In Fig. 6 the upper 'wiggler' is grasping its breathing tube in its mouth, apparently trying to pull off the small particles of kerosene
with which the tube has been clogged. The 'wigglers' upon the bottom have been suffocated and have given up the fight.
An ounce (two tablespoonfuls) of kerosene will spread over fifteen square feet of water surface, forming a film thick enough to kill all the 'wigglers' that arc beneath it. Kerosene of a cheap quality, known as high test light fuel oil, is preferable for this purpose. It can usually be bought at eight cents a gallon. If oil of this quality is not available, ordinary kerosene will answer the purpose. It should be applied as often as once in iwo weeks, for by that time the previous application
will have evaporated. A sufficient quantity should be used, in the proportions named, to cover completely any place that may need treatment.
Any one who is ill with malaria or yellow fever should be carefully protected from mosquitoes, for, should a person be bitten by an Anopheles, the malarial mosquito or Stegomyia fasciata, the yellow fever mosquito, at this time, there would be great danger that the insects might fly away and bite some one else and thus spread these diseases. Screens for both doors and windows form the best protection against mosquitoes at all times; but it often happens that the insects get into our houses, even though they are thoroughly screened, generally through some door or window that has been left open by mistake, or they may gain an entrance by coming down an unused chimney if the flue is allowed to remain open during the summer time. A house or a room may be cleared of mosquitoes by burning pyrethrum powder and allowing the smoke, which is not at all offensive to most people, thoroughly to fill the room that is under treatment. This smoke kills or so stupefies the insects that they will not bite. Pyrethrum powder is a preparation of the plant Pyrethrum roseum, and is sometimes sold as Persian Insect Powder or Dalmation Powder; it can be bought at any drug store for about thirty-five cents a pound. It is a very fine, light powder; and an ounce of it will go a long way, making a large volume of smoke. A pyrethrum smudge or smoke may be started by covering a live coal, taken from the kitchen stove, with the powder, first placing the coal upon a small shovel, so that it may be moved about conveniently without danger of setting anything on fire. The pyrethrum will
|Fig. 8. Profile of Anopheles punctipennis (Female). Three times larger than life. Showing the characteristic resting position of this mosquito.||Fig. 9. Profile of a Culex Mosquito (Female). Three times larger than life.|
quickly begin to smoulder and give off a dense smoke. All that is now necessary is to add from time to time a pinch of the powder as occasion requires, merely keeping the smouldering ashes covered so that they will give off a smoke. People are frequently annoyed and sometimes driven into their houses on summer evenings by the persistent attacks of mosquitoes. On such occasions, pyrethrum powder can often be used to advantage; and the smoke from a small quantity of the powder kept smouldering upon the piazza will drive away most, if not all, of the pests, thus making it possible to enjoy an evening out doors in comfort, when otherwise life would be unbearable except behind the protection of screens.
The Anopheles, or malarial mosquitoes, though not very common (see Figs. 7 and 8), are breeding quite abundantly in many parts of this country; and by referring to the accompanying photographs, particularly the ones in profile, it will be seen that there is quite a difference between the malarial and the common, or Culex, mosquitoes.
They may easily be distinguished from the common or Culex family of mosquitoes by the spots upon their wings, and also by the position which they take when at rest (see Fig. 8).
Notice the angle at which the insect shown in Fig. 8 stands out from the wall. Compare this with Fig. 9. It will also be seen that the proboscis, or 'stinger,' and the body of Anopheles form a straight line, while the Culex is rather humpbacked. The other Anopheles, maculipennis, does not stand out from the wall at quite such an angle as does punctipennis; but like the latter its proboscis and body form a straight line, and the angle formed by the insect when at rest is much greater than that of the Culex.
Notice how different is the resting position of the mosquito in Fig. 9 from that of Anopheles in Fig. 8.
The male mosquito (see Fig. 10) never bites. He may be easily distinguished by his large and feathered antennæ and palpi, which are very much more prominent than those of the female.
There is another mosquito, Stegomyia fasciata, which in form and habits closely resembles Culex, in which genus, until quite recently, it was classed. Stegomyia fasciata is the yellow fever mosquito, and it only inhabits the warmer portions of this country. It is common in most of our southern states and is seldom seen north of the Carolinas. It is easily distinguished from other mosquitoes by the conspicuous silvery white stripes upon its thorax and abdomen, and by the white bands upon its legs.
Fortunately for mankind, nature herself provides many energetic workers who are constantly doing their part towards holding in check these insect pests. Foremost among these natural enemies are many of the insectivorous birds, which daily destroy many thousands of mosquitoes. The swallows, the fly-catchers, the night hawks and the whip-poor-wills, all are insect exterminators, whose good work in this connection is seldom taken into account. The bat is also an efficient mosquito hunter; so too are the dragon flies which frequent the shores of ponds and pools where mosquitoes breed.
Besides these enemies of the adult mosquito, which may properly be called their 'foes of the air,' mosquitoes have other adversaries which destroy them in their early stages. These may be termed their 'foes of the water.'
It often happens that we can find no 'wigglers' in small ponds in which we would naturally expect to find mosquitoes breeding. In such ponds the presence of fish may account for the absence of mosquitoes. Their larvæ furnish food for many species of our smaller fishes, and by them myriads of mosquitoes are annually destroyed. Goldfish are particularly fond of mosquito 'wigglers,' and the pair of fish in the illustration (see Fig. 11) were seen to eat ninety-eight 'wigglers' in four minutes. Goldfish will live and multiply in almost any small and shallow pond in this vicinity, where the water is warm. They are perfectly hardy and will thrive just as well and perhaps better in stagnant water than they will in fresh.
The 'top minnow' the wach, the sunfish or 'pumpkin seed' and even the sluggish horn pout all play an important part in reducing the numbers of mosquito 'wigglers.' Besides the fishes, there are other 'foes of the water' that prey upon mosquito larvæ. Many of the predatory water bugs feed upon them. Professor J. B. Smith, in the report previously referred to, says that "among these predatory insects which abound in shallow permanent bodies of water wherever there is vegetation, the water boatman (Corisa and Notonecta), the water striders or 'skate bugs' (Hydrobatidæ) and the water scorpions Nepidæ, Belostomatidæ) deserve mention." He also speaks of the 'water tiger,' the larva of the large water beetle (Dytiscus), and tells of its ability to clear Culex larvæ from pools of water.
In this connection a brief description of a newly discovered mosquito, to which has been given the name Eucorethra underwoodi, should be of interest, since it has been found that their larvæ devour the wigglers of other mosquitoes, and unlike other mosquitoes, the adult female insect does not bite. As the proboscis of this insect is so formed that it can not puncture the skin, it should not perhaps be called a true mosquito, though it has been classed as one, since it belongs to the family Culicidæ.
The larvæ of this insect were found by the author on January 27. 1903, in the Maine woods in the eastern section of Penobscot County, and were discovered in a spring of water from which a crew of lumbermen
were getting their water supply. A few days later, other larvæ of the same species were found in a similar spring about eight miles distant, though in this case, as the spring was not in use, its surface was covered with a coating of ice an inch thick. The temperature of the water at the bottom (it was about two feet deep) was 42° F.
At first sight this larva would be taken for an Anopheles of extraordinary size, as it is of the same general shape, and when the water was cleared of ice, it lay just beneath and parallel to the surface, breathing through a short respiratory siphon, as is characteristic of the larvæ of Anopheles. In this spring a barrel had been sunk and in the fifty gallons, or thereabouts, of water which it contained there were twenty-five larvæ. They were all of about the same size—13 to 14 mm. long —and almost black in color. All were secured and taken into camp for further investigation.
Close observation of the larvæ showed that besides being much larger (12-14 mm. long instead of 5-7 mm.) they differed in many other particulars from the larvæ of Anopheles (see Fig. 12). In proportion to the rest of its body, its head is larger than the head of Anopheles. It does not turn its head upside down when feeding as does Anopheles. Its mandibles are strikingly large and powerful and are prominently toothed. It lacks the frontal tufts or brushes which are conspicuously present in Anopheles, and its antennæ, which extend directly forward parallel with the sides of the head, are much longer and more slender, and are tipped each with three hairs of equal size. The thorax is broadly elliptical and is much wider in comparison with its abdominal segments than is the thorax of Anopheles. The sides of the thorax and the abdominal segments bear fan shaped tufts of hairs, not plumosed as in Anopheles. The tufts on the last segments, both dorsal and ventral (see Fig. 13), are more profuse in Eucorethra than in Anopheles, especially the ventral tuft which in Eucorethra occupies nearly the whole segment. Only two anal papillæ are present, while Anopheles has four.
A few days before the author returned to Boston, several larvæ died and three changed to pupæ. The pupa resembles that of Culex (see Fig. 14) rather than of Anopheles and its respiratory siphons are of the same shape as those of Culex. When stretched out at full length, the pupa measures ten mm.
On reaching home, the new wigglers, eighteen in number, were put into a quart jar which was placed near a window where it would receive the sunlight for two hours each morning. The temperature of the water now averaged about 70° F., and with this change the larvæ developed a new trait—they began to eat each other up. The act was witnessed on several occasions. The larva would grasp its adversary just forward of the respiratory siphon with its powerful mouth parts, and working the tail in first it would gradually swallow its victim, shaking it now and then as a terrier would shake a rat.
After losing many of the insects in this way, those that remained were separated, and each individual was placed in a small bottle by itself. Eventually, 1 succeeded in rearing a number of
males and females. The pupal stage of this insect varies from five days and nine hours to six days and ten hours. The adult (see Fig. 15) resembles Anopheles in having maculated or spotted wings, but is much larger and measures eleven millimeters in length. Its mouth parts, however, are not adapted for biting. A full description of the imago is soon to be recorded by Mr. D. W. Coquillett, of the National Museum, by whom the name above mentioned was given.
During a visit to Maine in June, a large number of larvæ of Eucorethra were taken from the spring where the barrel had been sunk. It was noticeable that larvæ of other kinds of mosquitoes were absent, although the adults were very numerous in the immediate vicinity.
The absence of other mosquito larvæ was accounted for when later it was discovered that the larvæ of Eucorethra fed upon the larvæ of other mosquitoes, eating them apparently with great relish. On several occasions fourteen Eucorethra larvæ ate, during the night, sixty Culex larvæ out of the seventy that had been placed in the water with them. When eating the larvæ of mosquitoes smaller than themselves, the victim is caught, shaken violently a few times, and swallowed in a few seconds in very much the same way that a pickerel would catch and swallow a smaller fish.
As yet no experiments have been made to see if this new species will devour the larvæ of Anopheles as readily as they will those of Culex. Whether or not this species will thrive in the climate of southern New England is as yet uncertain, but experiments are now being carried on to determine this point.
Although myriads of mosquitoes are destroyed by the natural enemies which have been mentioned, man should be the most destructive foe of these insects. There is no doubt that the mosquito pest may be very largely abated by the employment of scientific methods for causing its destruction in the early stages of its development.
While it is the duty of boards of health!to recognize mosquitoes as active agencies for the dissemination of certain diseases and to take such measures as are possible for their extermination, the work can never be effectively done until the people of each community are fully informed in regard to the life history of the mosquito so that all may cooperate intelligently to secure its destruction.
- Illustrated with photographs from life by the author. The article and the photographs are copyrighted.
- Under the title 'A New Mosquito' a description of this mosquito appeared in Science, August 7, New Series, Vol. XVIII., No. 449.