mosquitoes

Mosquitoes Harry H, Stage

Mosquitoes have annoyed man and undermined his health for centuries. These voracious bloodsucking pests occasionally become sufficiently nu- merous to kill livestock. They have pre- vented industrial and agricultural development in many parts of the world. Merely as annoying pests they have kept large areas from becoming summer resorts. But all those losses are slight, compared to the damage done to human beings by mosquitoes as carriers of malaria, yellow fever, dengue, filariasis, and encephalitis. Practically every school child learns about the relationship of mosquitoes to malaria and yellow fever. There are few insects that have been studied more or about which more has been written, but only within the last 50 years have economical and effective control methods been developed. These methods are a result of extensive research by entomologists, engineers, malariologists, physicians, chemists, and others.

There are more than 2,000 different species of mosquitoes. All have different flight habits, food preferences, and climatológica! requirements. Mosqui- toes breed only in water, but great swarms can be produced in extremely small quantities of water, whether foul or clean, salt or fresh. They breed not only in extensive marshes but also in empty cans, abandoned automobile casings, tree holes, rain gutters, and the axils of some plants.

MOST OF THE present-day research on mosquitoes is concerned directly with methods of killing them. Before that can be economically accom- plished, however, we must first be able to tell the various species apart. The

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problems of classification have led to a great deal of biological work, with implications that go far beyond the im- mediate practical objectives. Perhaps the biological studies may seem to place an undue emphasis on morphology and taxonomy in entomological writing, but without it how could we refer with precision to a single mosquito species of a total of 2,000, around the world, having definite relationships? In all languages the name Culex pipieyis, for example, can refer to only one species of living thing.

Taxonomic research on mosquitoes began with Linnaeus, the father of systematic zoology. In 1735 he named the first genus of mosquitoes Culex, and his tenth edition of Systema Na- turae published in 1758 was the beginning of the systematic naming of animals. In that work we find the genus Culex with six included species, but only one of these—Culex pipiens—is recognized as a valid species of mosquito today. In 1818 J. W. Meigen described the genera Aedes and Anopheles, and in 1827 J- ^^- Rob- ineau-Desvoidy added the genera Sahethes, Psorophora, and Megarhi- nus. Late in the nineteenth century there were more attempts at classification, led by F. Lynch-Arribalzaga, who recognized all the old genera and proposed a number of new ones.

When the importance of mosquitoes as disease transmitters became known, late in the 1890's, the systematic study on the classification of mosquitoes was accelerated considerably. The leading research worker on classification of mosquitoes at this period was F. V. Theobald, who, from 1901 to 1910, published a five-volume monograph on the Gulicidae, a family which included all mosquitoes known to him. Many new genera and species were described and given scientific names, but their classification was based on superficial adult characters, which subsequent research has shown to be unreliable. Somewhat later, in 1912, H. G. Dyar, F. Knab, and D. W. Coquillett published larval characters and adopted

Mosquitoes

much sounder adult characters. Dyar continued his research on mosquito classification, and in 1929 the Carnegie Institution published his Mosquitoes of the Americas, which revised the pre- vious volumes by L. O. Howard, Dyar, and Knab, included many new species, and also increased the geographic scope of Dyar's knowledge to the entire Western Hemisphere.

The present classification of mosquitoes was rather firmly established by F. W. Edwards in Wytsman's Geii- era Insectorum in 1932. The family Gulicidae, as established by Edwards, consisted of 39 genera and some 1,400 species. New genera and a number of new species have been described since then, and many changes in names have been made. During the Second World War military entomologists discovered more than 200 new species of mosquitoes, mostly in the Pacific area.

The classification of mosquitoes now is probably in the best condition of any comparable group of insects, since it is based on intensive studies of the morphology of adults, eggs, larvae, and pupae, as well as on biological information. There have been major publica- tions on the mosquito faunas of Suri- nam, Australia, India, the Philippines, Ethiopia, Egypt, and the Americas, and more are in progress. Much remains to be done in describing species from out-of-the-way places, in describing presently unknown larvae and pupae, and particularly in clarifying the status of closely related species, such as the Culex pipiens and Anopheles maculi- pennis complexes.

RESEARCH ON THE BIOLOGY and life history of mosquitoes was started about 1670 by Jan Swammerdam, of Hol- land. In 1691 P. Bonanni of Italy studied and described the life history of the common European mosquito, Culex pipiens. About 25 years later René de Reaumur of France studied the same mosquito, and his account of the development of the species remained valid until 1886, when the De- partment of Agriculture published

477 Howard's first full life history of an American Culex.

W. Raschkc, a German, studied the larva of a European Culex in 1887, and in 1890 an Englishman, C. H. Hurst, wrote on the pupal stage of Culex. Both Raschke and Hurst included observations on the physiology of the respiratory tubes, the gill flaps, and the tracheae, by which mosquito larvae breathe.

By 1892 Howard had worked out the life history of the southern house mosquito. In 1896 he published illustra- tions of the ^g^, larva, pupa, and adult of the northern house mosquito, the common mosquito around Washing- ton early in the summer and therefore a ready subject for Howard's interest. His research was followed shortly thereafter by the first complete history of the common malaria mosquito.

The Department of Agriculture published Howard's Notes on the Mosqui« toes of the United States in 1900. In it he described the anatomy and biology and suggested practical controls, which served as background and guide for W. C. Gorgas and J. A. Le Prince in their clean-up of mosquitoes in Ha- vana. The following year Howard published Mosquitoes—How They Live; How They Carry Disease; How They Are Classified; How They May Be De- stroyed, a book of 241 pages containing a chapter on the taxonomic characters of several of the common mosquito genera and an extensive account of the remedies suggested against mosquitoes. Howard stated that the results of this research appeared "at the psy- chological moment," and the book was widely distributed to members of the A^rmy Medical Corps. Its recommendations were soon put to use by the au- thorities responsible for the construc- tion of the Panama Canal.

G. M. Giles, an English naturalist for the Indian Marine Survey, in 1900 published a handbook on mosquitoes, which contained the results of a great amount of research on the life history and on the conditions affecting their abundance. In 1902 he published an

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enlarged edition of more than 500 pages.

A few years later Howard, stressing the lack of information which his papers and books disclosed, obtained a grant from the Carnegie Institution of Washington to finance the preparation of an extensive monograph, Mos- quitoes of North and Central America and the West Indies¡ by himself, Dyar, and Knab. Much study and research went into the preparation of two volumes, which appeared in 1912, followed by another in 1915 and one in 1917. The four volumes were an out- standing contribution to research on the biology of mosquitoes in the West- ern Hemisphere.

John B. Smith, of the New Jersey Agricultural Experiment Station, in 1904 published a monumental report on his investigations on the habits and life history of New Jersey mosquitoes. This report was revised by Thomas J. Hcadlee in 1915, 1921, and 1945. In his third revision Headlee stated that most of the chapter on biology first published in 1904 was used in the last edition because the research work by Smith and his assistants was so funda- mentally sound that it still served as the principal basis for modern mosquito-control procedure.

Although biological research on mosquitoes has been somewhat hampered by the complex problems of classification, a voluminous literature has been accumulated on the life history and biology of these insects, albeit the orig- inal reports are scattered through a wide range of scientific periodicals in several languages.

It was not until 1949 that a detailed volume appeared on mosquito biology, or The Natural History of Mosquitoes, as Marston Bates, the author, preferred to call it. Reporting on his own research, and compiling thousands of notes and reports by other scientists in all parts of the world. Bates prepared one of the very few volumes on the biology of a family of insects. His work and endless research provide a detailed summary of the knov/n behavior of the

479 adult stage, the process of cg^ laying, the time and place of flight, longevity, seasonal distribution, sexual behavior, food preferences, distribution, egg development, larval reactions to physical and chemical environment, the habitat of the larvae, and the classification of the multitudinous number of larval habitats. Despite all this knowledge on the biology of mosquitoes, the author concluded that the detailed and minute information necessary to make clear- cut definitions of habitat characteristics for some 2,000 species of mosquitoes was still lacking.

THREE HIGHLY SIGNIFICANT pieces of research on the biology of mosquitoes have appeared since the volumes by Howard, Dyar, and Knab. S. B. Freeborn and R. F. Atsatt determined in 1918 that mosquito larvae for the most part are killed by toxic properties in petroleum oils that penetrate tra- chéal tissue, rather than by suffocation brought about by the layer of oil on the water.

N. H. Swellcngrebel and A. de Buck in Holland in the early 1930's found that two Dutch malaria-carrying mosquitoes belonged to two distinct species; that is, the adult feeding preferences, mating habits, and larval habitat were completely different, although the morphological structures of the two species were almost identical. The thorough research performed by these scientists showed that the species were definitely separated by sterility barriers.

The necessity of a low oxygen con- centration for the hatching of Aedes mosquito eggs was reported by C. M. Gjullin, C. P. Hegarty, and W. B. Bol- len in 1941. They found that any method, chemical or physical, caused hatching when the oxygen content of the water was reduced. They concluded that bacteria or other organ- isms stimulated Aedes eggs to hatch by reducing the oxygen content of the water flooding Aedes eggs, which arc normally laid on "dry" ground. This contradicts the earlier belief that heat, cold, or drying, by causing a so-called

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conditioning or incubation period^ was a necessary prelude to hatching.

The great importance of a thorough knowledge of the biology or life history of mosquitoes may be illustrated best in actual mosquito- or malaria-control operations. For example, it would be useless to attempt malaria control in Trinidad by destroying malaria-carrying mosquito larvae in ground pools, streams, and puddles, because the larvae of the principal malaria vector on Trinidad are found only in water in the axils of certain air plants growing in high trees. Mosquito annoyance within 20 or 30 miles of a salt marsh cannot be reduced by the simple operation of destroying larvae appearing in rain barrels, discarded tin cans, and other containers around the home. Many other facts based on differences in the biology of mosquitoes must be known before a successful control program can be conducted.

DISEASE TRANSMISSION by mosquitoes has furnished the incentive for intensive research in many parts of the world, and the unfolding of these stories of disease prevention by the control of the mosquito carriers, little by little, down through the years, has been a drama in medical entomology.

Two medical men, Josiah Nott of Mobile, Ala., and Louis D. Beau- perthuy of the West Indies, 100 years ago argued that mosquitoes were in- strumental in carrying yellow fever. Somewhat later Dr. Carlos Finlay of Havana conceived the idea independ- ently that mosquitoes carried yellow fever. In 1877, however, when Sir Patrick Manson's research proved that the southern house mosquito developed filarial worms within its stomach after biting a human patient in w^hose blood there were embryo filarial worms, evidence was given of the mosquito's part in disease. Sir Patrick later observed these worms developing within the stomachs of test mosquitoes, and finally he traced their migration through the stomach walls into the abdominal cavity and then up into the thoracic

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muscles. G. G. Low, of London, in 1899 found the worms in the mosquitoes' proboscises, where they were ready to flow with the saliva into the victim's blood and thus complete the cycle. Here, then, were the first proofs of a mosquito sheltering in its stomach a parasite of man's blood without harm- ing the parasite or being harmed by it.

Notwithstanding Sir Patrick's research, little attention was paid to his next theory that mosquitoes might also suck out malaria parasites, as well as filarial worms, in man's blood. But a Scots army surgeon, Ronald Ross, began his own research on malaria in 1895. Apparently Ross, who was working in India at that time, was the only person actively conducting research on the mosquito-malaria relationship. After more than 2 years of energetic research involving many mosquito dis- sections, Ross on August 20, 1897, found a malaria parasite growing within the stomach wall of an Anopheles mosquito. That discovery was confirmed by B. Grassi in 1898 and by R. Koch in 1899.

A year later Ross added an even more significant discovery, when he demonstrated the life cycle of bird malaria, which is transmitted by Culex mosquitoes. To Ross then must go the credit for proving the complete cycle of malaria—from an infected bird to a Culex mosquito and back to another bird. Although this tremendous and fundamental research should have been carried forward by Ross, he was soon ordered to another assignment, where he was able to continue his human malaria research only on his own time. His work was duly recognized, however, for he was awarded the Nobel prize in 1902 and was knighted somewhat later for his important discoveries.

Ross's research on bird malaria was soon confirmed by several other scientists. In 1898 A. Bignami, an Italian, succeeded in experimentally infecting a man with malaria by the bite of an Anopheles mosquito. Dramatic con- firmation of malaria transmission was furnished by Sir Patrick Manson in

Mosquitoes I goo, when his son in London deliber- ately allowed himself to be bitten by infected Anopheles mosquitoes sent up from Italy by Professor G. Bastianelli. Fifteen days later Manson's son developed malaria.

Although Dr. Finlay advanced the mosquito-yellow fever relationship as early as 1881, his research was dis- credited until 1900, when the Army Yellow Fever Commission, headed by Walter Reed, and his associates James Carroll, Jesse W. Lazear, and A. Agra- monte, announced the results of their experiments in which yellow fever had been caused by the bite of an infected Aedes aegypti mosquito. Carroll and Lazear permitted themselves to be bitten by mosquitoes that had previously fed on a yellow fever patient. Both men suffered attacks of yellow fever and Lazear died, a martyr to scientific research.

T. L. Bancroft, working in Aus- tralia, published evidence in 1906 that infected Aedes aegypti carried dengue. Aedes albopictus and Armigeres obtur- bans mosquitoes have since been found capable of transmitting the virus of dengue fever. Research on this virus, however, has been greatly limited because no very suitable experimental animal is known with w^hich experiments can be conducted.

Mosquitoes were first associated wdth animal viruses in 1900 through research by Reed, Carroll, Agramonte, and Lazear, but the causative agent of yellow fever was not positively known as a virus until W. A. Sawyer, S. F. Kitchen, and their associates in the Rockefeller Foundation published a report of experiments in 1930. Since that time research has added much information on several mammalian viruses that are transmitted by mosquitoes. R. A. Reiser's experiments, the results of which were published in 1933, first proved that mosquitoes could transmit western equine en- cephalomyelitis. After results of extensive laboratory and field investigations by W. M. Hammon, W. C. Reeves, and others were published in 1940, Culex

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tar salis was first named as the culprit that carries sleeping sickness in horses. Additional cncephalitides and other species of mosquito carriers have since been incriminated.

The few pieces of research that I have mentioned are examples of many discoveries in which mosquitoes had a major part in diseases affecting man and animals. Until 30 or 40 years ago, practically all such discoveries were made by medical men, but they illustrated the value of entomological knowledge. A new concept of this sci- ence was thus created and greatly accelerated research in medical entomology. The field is a fertile one, and entomologists with broad training in the biological sciences are greatly needed. Research on mosquitoes and disease challenges the keenest intel- lects.

OIL APPARENTLY WAS the first material used to control mosquito larvae. Before 1800 its use on water was recommended in Europe and America. Dr. Howard, then a boy 10 years old, in 1867 tested the use of coal oil against mosquito wrigglers in a horse trough. Mrs. C. B. Aaron, in her Lamborn prize essay, published in 1890, gave the results of her research on killing mosquito larvae with kerosene. Between 1892 and 1896 Howard put this information to practical use in ridding two localities of the mosquito nuisance.

Howard in 1892 and Ross in 1900 recommended the use of kerosene and paraffin oil on infested water and suggested that men be employed to drain, fill, and oil mosquito-breeding puddles. These practices were shortly carried out as part of a malaria-control program in western Africa.

A. Celli and O. Casagrandi in 1899 published a pamphlet, On the Destruc- tion of Mosquitoes—A Contribution to the Study of Culicidal Substances. A wide variety of available substances was tested by these Italians, but only two—paraffin and an aniline product which they called Larycith—acted as poisons. Used at the rate of i part to

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7*000 parts of water, Larycith killed all larvae within 24 hours and was considered harmless to man, animals, and plants. It was cheap and more permanent than paraffin.

In 1900 several communities had begun to use oils as mosquito larvicides.

The beginning of a long and per- sistent fight against mosquitoes in New Jersey was made by John B. Smith in 1900. Mosquito-control experts in New Jersey, led first by Smith and later by T. J. Headlee, have since done a monumental piece of research on perfecting control methods against the salt-water species in that State.

Howard's first public address on mosquito extermination was made in New Jersey in May 1901. This talk was the real beginning of a wide-scale con- certed effort against mosquitoes in the United States. As the New Jersey Mos- quito Extermination Association got under way in 1912, the managers of the abatement districts began to em- phasize the need for an effective larvicide that could be safely used where fish, warm-blooded animals^ and plants were involved.

D. L. Van Dine in 1903 outlined 21 simple rules, based on Howard's research, to be followed in controlling mosquitoes in Hawaii. The rules were published on placards in five languages and included practical suggestions on the use of coal oil on water containing mosquito wrigglers.

The next important killer for mosquito larvae was the so-called Panama Canal Larvicide, developed in 1904. It was made by mixing 150 gallons of crude carbolic wood resin (grade E) and 30 pounds of commercial sodium hydroxide in 6 gallons of water. A spray w^as made by diluting this con- centrate, I part to 5 parts of water.

M. A. Barber's and T. Haynes' discovery of the value of paris green against Anopheles larvae in 1921 revolutionized malaria control. Within 5 or 6 years practically all malaria-control organizations in the world were using this relatively cheap but effective anopheline larvicide. In 1923 W. V.

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King and G. W. Bradley tested its ap- plication by means of an airplane. From that time until the developmc^nt of DDT in 1942, paris green was the standard larvicide used against malaria mosquitoes. Scientists around the world tried to incorporate paris green with other materials so as to make it effective against other species of mosquitoes, but they met with little success.

J. M. Ginsburg, a biochemist of the New Jersey Agricultural Experiment Station, started to conduct research with mosquito larvicides in 1926. Annual papers reporting the results of his studies have added greatly to our knowledge. In various parts of the world researchers slowly began to study oil combinations for control of mosquito larvae. In 1924 A. S. Hurwood, of Australia, combined 70 parts of kerosene and 30 parts of residual oil to make a durable spreading film. In 1927 Ginsburg found that these desirable properties of a mosquito oil could be increased considerably by adding crude cresylic acid. In 1930 Ginsburg first published a report on the use of pyrethrum-oil emulsion as a mosquito-control agent. This well-known New Jersey mosquito larvicide has remained the one safe material for use on fishponds and ornamental pools.

The addition of a 4-percent emulsi- fying agent to Diesel oil in 1943 by E. F. Knipling, C. M. Gjullin, and W. W. Yates not only increased the efficiency of the oil when it was applied as an emulsion against the larvae, and particularly against the pupae, but also greatly reduced operating costs. Only 6 gallons of oil in emulsion form per acre of water surface was required, in- stead of the 20 to 60 gallons per acre previously recommended.

Research in medical entomology was greatly accelerated during the Second World War, and much of it was di- rected against the disease-carrying species of mosquitoes. For the protec- tion of the Armed Forces, the military agencies needed a mosquito larvicide more effective than the arsenicals and petroleum oils that were in wide use.

Mosquitoes

At the request of the U. S. Army, the Bureau of Entomology and Plant Quarantine organized a laboratory at Orlando, Fla., early in 1942 to conduct research on better methods for controlling disease-carrying mosquitoes and a more satisfactory repellent for preventing mosquito bites. The entomologists and chemists assigned to the project contributed all of the early recommendations to the military on the use of DDT as a mosquito larvicide. From 1942 until the research at the laboratory was reduced in 1945. more than 6,000 chemicals were evaluated as mosquito lar\âcides.

G. G. Deonier, R. W. Burrell, E. Not- tingham, T. D. Maple, J. H. Gockran. H. A. Jones, P. M. Eide, G. B. Wisecup^ and,others in 1945 published reports on their preliminary field studies with DDT as a mosquito larvicide. Properly formulated DDT was found to be at least 25 times more toxic to anopheline mosquito larvae than paris green. When an oil solution containing 5 percent of DDT v/as applied at the recommended dosage of o. i to 0.4 pound of DDT per acre of water surface, it was not hazardous to plants, fish, or warm- blooded animals. As soon as DDT in oils, emulsions, or dusts was found to be effective against mosquito larvae in the laboratory and under field conditions, entomologists at the Orlando laboratory cooperated with the Ten- nessee Valley Authority in the summer of 1943 to test its value when dispersed by aircraft against Anopheles larvae. Because of their physical properties, the DDT dusts were difficult to apply with available equipment; in those tests DDT showed very little practical superiority over paris green. In the fall of 1943 the Orlando entomologists tested DDT sprays from aircraft against salt-marsh mosquito larvae. Special equipment to disperse sprays was designed by G. N. Husman, of the laboratory. In the spring of 1944 A. W. liindquist and Husman first tested the use of small aircraft against mosquito larvae in the jungles of Panama. These tests were followed by others in Janu-

483 ary 1945, ^^ which Army and Orlando entomologists worked together. Large airplanes Vv'cre used. Excellent results were obtained with dosages as small as I to 2 quarts per acre.

Sprays containing DDT now are widely used as mosquito larvicides. Of the new hydrocarbon insecticides, it is considered the most effective and the safest.

Research on the control of mosquito larvae by fish, drainage, the building of dikes, and the filling in of low places has been carried on by engineers, entomologists, and others in many parts of the world. In some places those methods have given permanent control, although first costs have been high.

Managers of mosquito-abatement districts in California, Illinois, Florida, Virginia, New York, New Jersey, and other States have made significant con- tributions in modifying existing power shovels, cranes, trench excavators, marsh plows, and hand tools. Prob- ably nowhere else in the world has so much study and time been spent on developing mechanized equipment for use on salt marshes as in New Jersey.

Mosquito control is big business. The mosquito-abatement district managers across the United States have encountered difficult problems involving areas of enormous extent. They have met these problems with amphib- ious full-track, %-ton "weasels," small and large aircraft, insecticidal fog ap- plicators, hydraulic spray equipment, power shovels, "swamp angels," and "skeeter-eaters."

Extensive research has been carried out on the manipulation of water levels as a means of reducing mosquito popu- lations and thereby preventing malaria. In 1882 C. V. Ghapin, of Rhode Island, wrote about the malaria outbreaks adjacent to impounded water along railroad embankments. The building of the Hales Bar Dam for the development of hydroelectric power on the Tennessee River in 1912 created a malaria epidemic focus which lasted for more than 35 years. Malaria epi- demics followed other impoundments.

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and some resulted in lawsuits insti- tuted by residents who had contracted the disease.

The program of regional development undertaken by the Tennessee Valley Authority in 1933 afforded a unique opportunity for the study of water manipulation as applied to the control of Anopheles mosquitoes. More than 10,000 miles of shore line was involved in an area of wddely varying topography. This research was combined with that of the Public Health Service in 1947 in a manual, Malaria Control of Impounded Water. A sus- tained increase in the efficiency and economy of control practices on impounded water has been aided greatly by the combination of knowledge, experience, and research effort ex- pressed in the manual.

BEFORE WE HAD DDT we knew of no effective and economical way of killing adult mosquitoes in large areas out of doors. In 1893 it was reported that the Chinese burned pine sawdust, brimstone, and arsenic in incense pots to kill mosquitoes indoors. Late in the 1890's the Italians, Celli and Gasa- grandi, recommended dinitrocresol, known to them as Larycith III. By 1900 the burning of pyrethrum pow- der was generally known and effective within doors. General Gorgas banished yellow fever and greatly reduced malaria within a year in Havana by destroying adult Aedes aegypti and Ano- pheles mosquitoes. A considerable part of this accomplishment was ef- fected by burning pyrethrum and sul- fur within homes.

In 1910 Howard compiled a list of materials for use as smudges and fumi- gants. Somewhat later a number of household sprays for kilHng mosquitoes and other insects were advocated. Dur- ing the early 1930's several workers in New Jersey tested the use of pyrethrum sprays against mosquito annoyance at out-of-door gatherings. These pyrethrum and kerosene sprays were fairly effective for several hours.

B. DeMeillon in 1936 and P. F. Rus-

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sell and F. W. Knipe in 1939 reported their experiments in Africa and India, respectively, where the number of malaria cases was reduced by killing adult mosquitoes with sprays. In 1940 F. L. Soper and D. B. Wilson showed how they had been able to exterminate Anopheles gambiae in Brazil, partly at least, by using a spray containing pyrethrum, carbon tetrachloridc, and kerosene. This eradication of an insect from a country is one of the most interesting chapters in medical history.

In 1940 W. N. Sullivan, L. D. Good- hue, and J. H. Fales published their first report on a ncAv method for dis- persing insecticides in air. This method suggested to Goodhue an entirely new and revolutionary approach to insecticide dispersal. The aerosol bomb they developed provided in a small pack- age the most effective means known for killing adult mosquitoes within en- closures—the release of the aerosol for a few seconds in 1,000 cubic feet of air space will kill all the mosquitoes in it.

When in June 1943 it had been shown that very small quantities of DDT sprays applied from the ground were effective against adult mosquitoes, the Orlando entomologists immediately started tests with aircraft. Air- craft had not previously been employed for controlling adult mosquitoes. If usable, this could be the means for reducing the adult mosquito popu- lation within a matter of hours, thereby immediately checking the spread of malaria and dengue among our military forces in various places. Special spray equipment was designed by Hus- man and the initial tests made against salt-marsh adult mosquitoes on the Florida Keys in November 1943. The results indicated that practically all adult mosquitoes could be killed by spraying 2 or 3 quarts of a 5 percent DDT oil solution per acre over dense vegetation. In December of the same year, the first tests were made with large combat planes, using Chemical Warfare Services M-io tanks. Similar tests were followed in Panama in April 1944 where Lindquist and Husman

Mosquitoes practically eliminated the adults of Anopheles alhimanus in jungle forest at 0.4 pound of DDT per acre with a 10 percent DDT spray.

Their early research established the value of DDT aerial sprays for controlling adult mosquitoes and stimulated world-wide investigations by the Allied Forces. Airplanes became one of the most effective ways of controlling mosquitoes and mosquito-borne diseases wherever our troops were lo- cated.

Early in 1943, ^' ^- Lindquist, J. B. Gahan, B. V. Travis, F. A. Morton, Wisecup, Eide, and others at the Orlando laboratory found that the new DDT insecticide possessed unusual residual properties. Following intensive laboratory tests, their first field tests were made near Tallahassee, Fla., in August 1943. DDT applied in sprays on the interior sufaces at the rate of about 200 milligrams per square foot protected buildings for at least 70 days against infestations of the common malaria mosquito, the most important vector of malaria in the Southeastern States. Here was born the most important method yet devised for controlling malaria. The Orlando entomologists recommended this method of mosquito control to the Armed Forces who were experiencing many cases of malaria in various parts of the world.

In 1944 the Orlando entomologists tested two formulations under practical conditions near Stuttgart, Ark., a rice- growing area heavily infested with Anopheles mosquitoes. In two areas, each 9 miles square, the interior of every outbuilding was sprayed with an oil solution containing 5 percent of DDT. The experiments confirmed the Tallahassee tests and recommendations were made for using 5 percent DDT sprays at the rate of 200 milligrams per square foot. The information was sent to our allied countries. Further large- scale tests were started by C. B. Symes, A. B. Hadaway, and G. Giglioli, in Jan- uary 1945 in British Guiana, and by Gahan in New Mexico in April. The experiments were aimed at two very

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efficient but different vectors of malaria and were successful.

This research has been of great bene- fit to practically all peoples of the globe. In 1950 more than 800,000 homes in the Southeastern States were treated by Federal and State public health agencies employing DDT residual sprays. Elsewhere the World Health Organization reports a total of about 50 million persons as now being protected against malaria by this residual insecticide alone.

Military pickets, night laborers, ex- plorers, and other personnel working in areas heavily infested with mosquitoes have sought constantly for some means of protecting their persons from mosquito bites. In 1910 Howard listed several essential oils for this purpose. Of these, the oil of citronella was the most widely known repellent. It was effective for a few minutes to an hour, but some people preferred the stings of mosquito bites to the odor of citronella.

The first extensive research on mosquito repellents was begun by Phillip Granett of New Jersey in 1935. In 1940 he had developed a repellent that was greatly superior to all others. Unfor- tunately, however, this material, after being tested on experimental animals, was declared unsafe for use on humans.

When the United States entered the Second World War the need for a mosquito repellent became acute. The military services were particularly in- sistent on knowing of a good protective agent against mosquito bites. With funds allotted by the Office of Scien- tific Research and Development, the Department of Agriculture initiated an extensive research program in April 1942 at the Orlando laboratory. Many industrial companies and manufac- turers cooperated in the investigations. Subsequently more than 10,000 syn- thetic organic chemicals in various formulations were tested against hun- gry mosquitoes by Travis, Morton, and their associates on the skin and cloth- ing of men. Promising materials were also tested for their toxicological effects

by the Food and Drug Administration. Three materials and combinations of

them were recommended to the miH- tary services—dimethyl ph thai a te, Rutgers 6-12 (2-ethyl-i,3-hexane- diol), and indalonc (rz-butyl mesityl oxide oxalate). These and a mixture of them, commonly known as 6-2-2, were standard insect repellents in 1952.

HEALTH CONDITIONS in many parts of the world have been revolutionized by mosquito control. Vast regions are now healthier, safer habitations for men as a result of the tremendous amount of research that has been carried on during the past 75 years. Today more than 25 million people of the United States live in areas where mosquito-control programs are in progress.

One of the world's most effective malaria-carrying species of mosquito was eradicated from Brazil between the time of its discovery in March 1930 and November 1941. Other eradication projects have been undertaken.

Still, mosquito research is far from finished. Each year, workers all over the world add hundreds of articles on mosquito investigations to a considerable bibliography already available. All this information is immediately put to practical use by the 250 separate local, city, county, State, and Federal mosquito-control agencies.

HARRY H. STAGE is assistant leader of the division of insects affecting man and animals. Bureau of Entomology and Plant Quarantine. After complet- ing work at Syracuse University in igiy and subsequent mosquito control work in the Navy in the First World War, he was employed as entomologist of the St. Louis Southwestern Railway Lines and actively participated for 10 years in malaria control in the area served by that railroad. From ig^i to igdo he was in charge of the Bureau's northwestern laboratory on mosquito investigations. Since ig^o he has traveled extensively in the Arctic and the Tropics on various kinds of mosquito-control research.

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The Control of Insects Affecting Man E. F. Knipling

We have known for half a century that insects, ticks, and mites are the transmitting agents for malaria, typhus, bubonic plague^ yellow fever^ tick fevers, dysenteries, typhoid, and many other dangerous diseases. The role of insects as carriers of human diseases is discussed by F. C. Bishopp and C. B. Philip in another chapter of this book.

As scientists gathered more and more information on the relation of insects and diseases during the last dccadi^ of the nineteenth century and the early years of the twentieth century, entomologists, parasitologists, and medical doctors investigated ways to control insects that attack man, but the world did not seem concerned enough to sup- port research adequately, and progress was slow.

As late as the First World War, louse-borne typhus caused millions of illnesses and deaths among Europeans. Nevertheless no intensive effort was made to develop eflPective and practical ways to control the lice. By the time the Second World War began no substantial improvements had been made in the methods.

Malaria, the most important of the diseases of man, could be controlled if enough manpower, materials, and equipment were used. But th(^ control methods were costly and weeks or months of intensive cfTort were required to achieve substantial reduction in the incidence of the diseases.

Such slow methods could not protect troops when they invaded malarious areas and moved quickly from one region to another for combat or training. Nor did we have positive and practical ways to kill flies, fleas, ticks, mites,

mosquitoes

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