|SF 593 T5 H83 [Copy 1 U. S. DEPARTMENT OF AGRICULTURE, BUREAU OF ENTOMOLOGY— BULLETIN No. 105. L. O. HOWARD. Entomoloeist and Chief of Bureau. THE ROCKY MOUNTAIN SPOTTED FEVER TICK. WITH SPECIAL REFERENCE TO THE PROBLEM OF ITS CONTROL IN THE BITTER ROOT VALLEY IN MONTANA. BY W. D. HUNTER, In Charge of Southern Firld Crop Insect Investigations, AND F. C. BISHOPP, Entomological Assistant. tin Cooperation with the Biological Survpy .tikI tlip Montana Agricultural College.] Issued November 17, 191 1. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1911. Qass S^:S43 Book Tg Hg 3 r U. S. DEPARTMENT OF AGRICULTURE, BUREAU OF ENTOMOLOGY— BULLETIN No. 105. L. O. HOWARD, Entomologist and Chief of Bureau. ,^L— ^ "^Lm THE ROCKY MOUNTAIN SPOTTED FEVER TICK. WITH SPECIAL REFERENCE TO THE PROBLEM ^.^X OF ITS CONTROL IN THE BITTER ROOT fS ^ VALLEY IN MONTANA. wf^= TD.'TIjUNTER, In Charge of Southern Field Crop Insect Investigations, F. C. BISHOPP, Entomological Assistant. [In Cooperation with tlie Biological Survey and the Montana Agricultural College.] Issued November 17, 1911. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1911. CxTJ^ ^ ^ ^ BUREAU OF ENTOMOLOGY. L. O. Howard, Entomologist and Chief of Bureau. C. L. Marlatt, Entomologist and Acting Chief in Absence of Chief. R. S. Clifton, Executive Assistant. W. F. Tastet, Chief Clerk. F. IT. Chittenden, i/i charge of truck crop and stored product insect investigations. A. D. Hopkins, in charge of forest insect investigations. W. D. Hunter, in charge of southern field crop insect investigations. F. M. Webster, in charge of cereal and forage insect investigations. A. L. Quaintance, in charge of deciduous fruit insect investigations. E. F. Phillips, in charge of bee culture. D. M. Rogers, in charge of preventing spread of moths, field work. Rolla p. Currie, in charge of editorial work. Mabel Colcord, in charge of library. Southern Field Crop Insect Investigations. W. D. Hunter, in charge. F. C. BiSHOPP, H. P. Wood, W. V. King, G. N. Wolcott, engaged in tick investi- gations. W. D. Pierce, J. D. Mitchell, E. S. Tucker, T. E. Holloway, G. D. Smith, E. A. McGregor, Harry Pinkus, W. A. Thomas, Thomas Lucas, engaged in cotton-boll weevil investigations. A. C. Morgan, G. A. Runner, S. E. Crumb, engaged in tobacco insect investi- gations. T. C. Barber, C. E. Hood, engaged in sugar cane and rice insect investigations. R. A. Cooley, D. L. Van Dine, Wilmon Newell, A. F. Conradi, C. C. Krumbhaar, collaborators. 2 i^r nf a LETTER OF TRANSMITTAL United States Department of Agriculture, Bureau or Entomology, Washington, D. C, July 20, 1911. Sir: I have the honor to transmit herewith a manuscript entitled " The Eocl^y Mountain Spotted Fever Tick, with Special Keference to the Problem of its Control in the Bitter Root Valley in Montana," prepared by Messrs. W. D. Hunter and F. C. Bishopp, of this bureau. The work of this bureau on the spotted- fever-tick problem began in 1909. It has been conducted in cooperation with the Biological Survey of this department and the Montana Agricultural Experiment Station. The investigation of the life history and habits of the tick which transmits spotted fever has revealed certain feasible and eco- nomical methods of control. These methods render it possible to reduce the numbers of the ticks to such an extent that the cases of spotted fever in the Bitter Root Valley will be very few in number, if, indeed, the disease is not eliminated altogether. The plans for this work are outlined in this manuscript. It is recommended that the accompanying manuscript be published as Bulletin No. 105 of this bureau. Respectfully, L. O. IIow^\rd, Chief of Bureau. Hon. James Wilson, Secretary of Agriculture. 3 CONTENTS. Page. Introduction 7 Work upon which this bulletin is based 9 Possibility of increase of area of spotted fever 11 Importance of the control of the spotted-fever tick 14 Summary of facts bearing on importance of tick control 14 Distribution of the spotted-fever tick 15 Factors governing abundance 17 Summary of life history of the spotted -fever tick 19 The egg and larva 19 The nymph 19 The adult 19 Seasonal history and habits 21 The host animals of the spotted-fever tick 26 Other species of ticks found in regions where Rocky Mountain spotted fever occurs 31 Species of ticks which may play an important part in the dissemination of the disease should it be introduced into new regions 31 Practical control or eradication of -the spotted-fever tick 32 Conditions favoring control 32 Importance of control throughout the Bitter Root Valley 36 Methods of destroying ticks 36 Dipping 37 Construction of vats 38 Handwork in the destruction of the spotted-fever tick 41 Definite recommendations for control or eradication of the spotted-fever tick in the Bitter Root Valley 41 Estimated cost of work 42 Supplementary means of control 43 Necessity for expert supervision 44 Bibliography of the more important writings on the spotted-fever tick 45 5 ILLUSTRATIONS. PLATES. Page. Plate I. Ticks and spotted fever in the Bitter Root Valley, Montana. Fig. 1 . — View in Lo Lo Canyon, which leads into the Bitter Root Valley, showing conditions under which ticks thrive. Fig. 2. — Camp laboratory near Florence, Mont., in one of the most heavily in- fested localities known 8 II. The Rocky Mountain spotted-fever tick {Dermacentor venustus). Fig. 1. — Adult female unengorged, dorsal view. Fig. 2. — Adult male, dorsal view. Fig. 3. — Adult female unengorged, ventral view. Fig. 4. — Adult male, ventral view. Fig. 5.— Adult female in act of depositing eggs 20 III. Spotted-fever tick {Dermacentor venustus) and Dermacentor albipictus. Fig. 1. — Adult spotted-fever tick which has deposited eggs. Fig. 2. — Larva of spotted-fever tick. Fig. 3. — Engorged nymph of spotted-fever tick. Fig. 4. — Same, ventral view. Fig. 5. — Adult male of Dermacentor albipictus. Fig. 6.— Adult female of Derma- centor albipictus, unengorged 20 TEXT FIGURES. Fig. 1. Map showing area in which the spotted-fever tick occurs 16 2. Diagram showing possible seasonal history of spotted-fever tick 27 3. Dipping vat 38 6 THE ROCKY MOUNTAIN SPOTTED-FEVER TICK, WITH SPECIAL REFERENCE TO THE PROBLEM OF ITS CONTROL IN THE BITTER ROOT VALLEY IN MONTANA. INTRODUCTIOlSr. For many years a disease of human beings, known as spotted fever, has been known to occur in certain localities in the Eockj' Mountain region of the United States. In fact the evidence is rather conclu- sive that the disease existed before the settlement of the country by white men. At any rate old residents of the Bitter Root Valley in Montana have informed us that the first white settlers were warned bj^ the Indians of the danger of contracting a very serious disease if they visited certain localities. From what has been learned in recent years it is evident that these dangerous localities are the very ones in which spotted fever is now most prevalent. The States in which the disease occurs most frecjuently are Mon- tana and Idaho. There is no doubt, however, that it occurs in at least portions of other States, such as Oregon, Washington, Nevada, Utah, Wyoming, and Colorado. Definite work on the nature and method of transmission of spotted fever was not begun until 1902. In that j^ear Drs. Wilson and Chowning announced the theory that the "wood tick ' ' is the natural agency through which the malady is transmitted from one human being to another. This hypothesis was based upon three observa- tions: First, that the majority of cases of spotted fever showed histories of tick bites ; second, that the localities in which the disease was most frequently contracted were those where ticks were most abundant ; and, third, that the season of spotted fever coincided with the period when the ticks were most frequently observed. Drs. Wilson and Chowning had no facilities for proving their hypothesis in a scientific manner, but such proof was soon obtained. Accord- ing to the late Dr. H. T. Ricketts ^ the first experiments which re- sulted in proof of the transmission of spotted fever by the tick were conducted b}^ Drs. McCalla and Brereton, of Boise, Idaho, in 1905. In these experiments a tick which was found attached to a spotted- fever patient was removed and allowed to bite a healthy person. In 1 Fourth Biennial Report, Montana State Board of Health, p. 3 06. 8 THE EOCKY MOUNTAIN SPOTTED FEVER TICK. eight days this person developed a typical case of spotted fever. The experiment was continued by allowing the same tick to bite a second person. In this case again a typical case of spotted fever resulted. The results of the important experiments of Drs. McCalla and Brereton were not published by them. In 1906 Dr. H. T. Ricketts, then connected with the University of Chicago, began a series of investigations which must always be considered classic. Not being aware of the exj^eriments of Drs. McCalla and Brereton, Dr. Ricketts started with the hypothesis of Drs. Wilson and Chowning. His first work was devoted to determin- ing whether guinea pigs and rabbits are susceptible to the disease and consequently suitable for inoculation experiments. The original exj)eriments with rabbits were somewhat inconclusive, but it was found that the injection of blood from a human being suffering with spotted fever invariably brought about the disease in guinea pigs. In fact in these animals the disease was found to run a course very similar to that in human beings. It was thus determined that guinea pigs were suitable subjects for experiments to determine whether ticks could transmit the disease. On August 4, 1906, Dr. Ricketts announced the results of the first experiment in the tick transmission of the disease. A small female tick was placed on a guinea pig which had been inoculated with the blood of a patient who died of spotted fever. The tick was allowed to feed on this inoculated guinea pig for two days. It was then removed and placed in a pill box for two days. At the end of that time it was allowed to attach to the base of the ear of another guinea pig which had not been inoculated with spotted fever. After three and one-half days the temperature of this guinea pig rose and remained above normal for more than seven days. The pig also showed practically all of the other symptoms of spotted fever. In fact, there was no doubt whatever that the guinea pig contracted spotted fever from the bite of the single tick. As a control on the experiment Dr. Ricketts placed two other guinea pigs m the cage occupied by the animal upon which the tick had been placed. They remained there for two weeks. These two pigs showed no indications whatever of fever. Thus the possibility of infection by contact or by feces was eliminated. The only difference between the conditions surrounding the pig Avhich contracted fever and those surrounding the others was that the former was bitten hyfx fever tick. During the following year (1907) Dr. Ricketts succeeded in trans- mitting the disease by ticks in a number of additional cases. In one experiment he found that the male tick as well as the female is capable of transmitting the disease. In other experiments it was determined that the larval or nymphal tick may acquire the disease and retain it through the molting period, and transmit the infection in the following stage to another host. The most interesting experi- Bui. 105, Bureau of Entomology, U. S. Dept. of Agricultun Plate I. Fig. 1.— View in Lo Lo Canyon, which Leads Into the Bitter Root Valley, Showing Conditions Under which Ticks Thrive. Fig. 2.— Camp Laboratory near Florence, Mont., in one of the most Heavily Tick-Infested Regions Known. TICKS AND SPOTTED FEVER IN THE BITTER ROOT VALLEY, MONT. WORK UPON WHICH BULLETIN IS BASED. 9 merits, however, were with adult ticks. It was found that when an adult becomes infected with the disease, the infection passes through the eggs developed in the tick, so that the young of the next genera- tion may transmit the disease.^ The main points determined by Dr. Ricketts are as follows : (1) Guinea pigs and certain other animals, as monkeys, are sus- ceptible to spotted fever. (2) Larval ticks applied to an infected animal contract the infec- tion and are able to transmit it to the following or nymphal stage. (3) Nymphal ticks feeding upon infected animals acquire the power of transmitting the disease as adults. (4) Adult ticks are able to acquire the disease by feeding upon an infected animal and to transmit it through the egg stage to the suc- ceeding generation. (5) Infective ticks are to be found in nature. The transmission of disease organisms through the egg stage of ticks is known in a number of other instances. It is the case with the tick Margaropus annulatus Say, which transmits splenetic fever of cattle in the soutJiern portion of the United States. The causative organism of splenetic fever has actually been found in the eggs of this tick. Dr. Ricketts recently made a tentative announcement of the finding of the spotted-fever organism in the eggs of Dermacentor venustus Banks. Future investigation will undoubtedly result in certainty regarding this point. Some of the main points determined by Dr. Ricketts were corrobo- rated about the same time by Dr. W. W. King, of the Public Health and Marine-Hospital Service, whose results were published in the Public Health Reports of July 27, 1906. WORK UPON WHICH THIS BULLETIN" IS BASED. The work of the Bureau of Entomology on the spotted-fever tick began in 1908, when the investigation of the life history and habits of the species was undertaken. Plans were made for deter- mining the distribution of the tick and for the exhaustive life-his- tory investigations necessary in the formulation of plans of con- trol. Following the plan for determining the distribution of the tick, two men were selected, one to travel through the southern Rocky Mountain region and the other through the northern. The late Mr. F. C. Pratt made investigations in New Mexico, Arizona, southern California, and Colorado. Mr. W. V. King, whose work as an agent of the bureau began July 1, 1909, made the investiga- iThe Rocky Mountain spotted-fever tick, like a number of other species, exists in four distinct stages, namely, egj;, larva, nymph, and adult. The eggs are invariably deposited on the ground in large masses. The larvse which emerge from the eggs are minute six- legged animals. After- feeding upon a suitable host, they drop to the ground and molt, becoming nymphs. In this stage they have eight legs. The nymph waits until it can attach to a host, engorges blood, drops, molts its skin, and becomes adult. 8464°— Bull. 105^11 2 10 THE EOCKY MOUNTAIN SPOTTED FEVER TICK. tions in the northern Rocky Mountain region. He explored Wyo- ming, Idaho, portions of Utah, and Oregon and Washington. Prof. R. A. Cooley, of the Montana Agricultural College, consented to co- operate with the bureau by directing the work of Mr. King and by submitting specimens from many localities in Montana. During 1909, Mr. J. D. Mitchell, of the Bureau of Etomology, visited New Mexico, and succeeded in determining the southernmost locality in which the fever tick is at present known to occur. The life-history work upon the tick was conducted at Dallas, Tex., by Messrs. H. P. Wood, G. N. Wolcott, and the junior author. This began early in 1909 and has continued without interruption. In February, 1910, a conference Avas held in Washington, D. C., with Prof. R. A. Cooley and Dr. C. Hart Merriam, then Chief of the Biological Survey, for the purpose of formulating definite plans for the continuation of the Avork. It was agreed that the determina- tion of the range of the tick should be continued by correspondence rather than by sending men into the field and that the local aspects of the problem in the Bitter Root Valle}^ should be investigated b}" placing an agent there. The Bureau of Entomolog}" provided the necessary funds and established a laboratory near Florence, Mont. (See PL I, fig. 2.) Prof. Cooley agreed to supervise the work in Montana, and was appointed a collaborator in the bureau on March 1, 1910. At the same time Mr. W. V. King was appointed to work under the direction of Prof. Cooley in the Bitter Root Valley. This plan of cooperation has continued doAvn to the present time. The results obtained have been due, to a large extent, to the energy and acumen of Prof. Cooley and to the high grade of Mr. King's work. But a special word must be said about Mr. King. Unde- terred by the possibility of contracting spotted fcA'er, he located on an abandoned farm in the most dangerous locality known. In the immediate vicinity a number of deaths from spotted fever had oc- cift-red within a short time. He remained there throughout the sea- son of 1910, subject to the risk of contracting the fcA'cr on his dailv trips into the field or from the ticks used in the experiments at the camp laboratory. His devotion to the investigation outweighed all considerations of personal safety. Great credit must also be given Mr. C. Birdse.ye and Mr. A. H. Howell, of the Biological Survey, for assuming the risk of residence at the laboratory during a por- tion of the season of 1910. Mr. Birdseye continued the investiga- tion of the mammals of the valley in 1911. In addition to the work in cooperation with the Montana Agri- cultural College, in 1910, the bureau undertook to obtain informa- tion regarding the exact extent of the area in which the spotted- fever tick occurs. By means of a system of circulars and the gen- erous cooperation of many physicians and other persons throughout POSSIBILITY OF INCREASED AREA OF FEVER. H the Rocky Mountain region, a very large amount of information ■was obtained. In fact the correspondents sent in altogether 1,400 lots of ticks, 850 of which were of the fever species. These repre- sented 225 localities in California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. Very many of the ticks received during the course of this work were in immature stages. Unfortunately our present knoAvledge of ticks is not sufficient to enable us to determine the species to which immature forms belong. This necessitates very special care in rear- ing to maturity the immature forms received. This work was done at Dallas, Tex., and naturally involved a large amount of skilled attention. The information now in hand regarding the spotted-fever tick was greatly increased through the cooperation of the Biological Survey of this department. In 1910 two agents of this survey, Messrs. A. H. Howell and C. Birdseye, were located at the camp laboratory of the Bureau of Entomology near Florence, Mont. These agents were engaged in the collection of wild mammals upon which one stage or another of the spotted-fever tick occurs. This work resulted in showing the relative importance of the different mammals found in the Bitter Root Valley and adjacent mountains as carriers of the spotted-fever tick. It also revealed many points having a bearing on the original source of the disease in nature and on other impor- tant matters. The Biological Survey has also studied carefully the possibility of the eradication or control of all the wild mammals which carry the fever ticks. In September, 1910, Prof. H. A. Morgan, director of the Tennessee Experiment Station, consented to make a trip to the Bitter Root Valley and to advise the forces cooperating regarding the sufficiency of the data obtained and the feasibility of plans of eradication based thereon. Of course the authors have made full use of the available literature on the investigations that have been conducted by other persons. Most useful have they found the first and second spotted-fever re- ports of Dr. H. T. Ricketts, published in the Fourth Biennial Report of the State Board of Health of Montana. POSSIBILITY OF INCREASE OF AREA OF SPOTTED FEVER. The approximate area in which spotted fever occurs has been indi- cated in a pi-evious paragraph. Since it has been shown, however, that a certain tick {Dermacentor venustus Banks) is the onl}'' known agent of transmission of the disease in nature, it follows that the possible area in which spotted fever may occur is at least coin- cident with the range of the tick, exactly as the possible range of jelioAv fever is as extensive as is the area in which the mosquito 12 THE KOCKY MOUNTAIN SPOTTED FEVER TICK. which transmits it is to be found. Extensive work conducted by the Bureau of Entomology has shown with considerable accuracy the area in which spotted fever may be thus propagated. The map (fig. 1, p. IG) shows the area in which the necessary agent for trans- mission occurs, and consequently the possible geographical distribu- tion of the disease. This map is based upon the examination of 850 lots of spotted-fever ticks received from 230 localities during the seasons of 1909, 1910, and 1911. One of the most remarkable features of spotted fever is the fact that strains of different degrees of virulence exist in different locali- ties. In Idaho the death rate is from 5 to 7 per cent. In the Bitter Root Valley in Montana, however, the death rate is about 70 per cent. One consideration which has caused the Bureau of Entomology to concentrate its efforts in the Bitter Root Valley is the possibility that the virulent form of the disease, now restricted to that valley, may eventually be carried into other regions where the presence of the tick would make transmission possible. There are several ways by which the virulent strain of the disease might be carried out of the Bitter Root Vallej-. It could be taken either by ticks or in the blood of human beings. Carriage by ticks might occur when these animals are transported on men, horses, or cattle. Moreover, tick eggs or other stages of the tick which have been shoAvn to contain the disease organism might be transported in hay or other commodities. There is also a chance that ticks in various stages might be transported on the hides of domestic or wild animals; As regards carriage of infection in the blood of human beings, our conclusions are largely theoretical. It is not known how long the blood of a person who is attacked with spotted fever remains in- fective. It is probable, however, that it is infective for some days before the height of the fever and for some time thereafter. During the period either preceding or following the climax of the disease a person might leave the Bitter Root Valley. If in another locality he should be bitten by the fever tick and the specimen should escape, the establishment of the virulent form of the disease would be accom- plished. In certain diseases similar to spotted fever, such as splenetic fever of cattle, the organism of the disease remains in the blood for many years without causing an acute or noticeable attack. Nevertheless, all ticks which feed upon these apparently immune animals become infected and can transmit the disease in acute form to other animals. Although nothing is known as to the persistency of the organism of spotted fever in the blood of persons who have ajoparently recovered, there is a possibility that it may remain for some months or even years. In this way there is a probability of POSSIBILITY OF INCREASED AREA OF FEVER, 13 considerable extension of the territory in which the virulent form of the disease occurs, by migration out of the valley. Naturally the chances of spread will increase with the develop- ment of the Bitter Root Valley and the growth of shipments of cattle or movements of people to other regions. These considera- tions are sufficient to justify very energetic means for control where the virulent form of the disease now occurs and where, as will be shown in this bulletin, the practical eradication of the tick, and, consequently, of spotted fever, is entirely feasible. It has been shown by experiments conducted in the Institute of Infectious Diseases in Chicago that several species of ticks other than the form which occurs commonl}^ in the Bitter Eoot Valley are capable of transmitting spotted fever. A very hopeful feature of the situation, however, is that in the valley there is but one tick species which attacks man. Therefore the other species are of no practical importance as regards spotted fever. Even among the species which feed upon the lower animals there are many thousands of specimens of Dermacentoi' venustus to every one of all other varieties. Moreover, means of control of this one species, such as will be described in this bulletin, will serve greatly to lessen the number of the other forms. For these reasons, in formulating plans for practical eradication it is necessary to consider only the one dominant tick in the valley. There is one respect, however, in which the discovery that species other than Dermacentor venustu8 can transmit the disease may be of importance. The other forms occur over wide areas in the east- ern and southern portions of the United States. It is conceivable that if the disease were once introduced in the blood of a human being or otherwise, the other ticks might propagate it and transmit it in regions far outside of the territory in which the fever is now known to occur. But the danger on this score is not so great as might be thought. In the first place, in no localities in the United States are any species of ticks as numerous as is the fever species in the Bitter Boot Valley and elsewhere in the Rocky Mountain region. Consequently, the occurrence of anything like an epidemic of the disease would be impossible. Only occasional or rare cases could be expected. In the second place, it can not be foretold whether spotted fever would find general conditions suitable for propagation in localities outside of the Rocky Mountain region. Nevertheless the degree of danger from this source, while perhaps slight, emphasizes the importance of eradication of the spotted-fever tick in the mountain region and also of the discovery of effective means of control for all species of ticks wherever the}^ occur. 14 THE EOCKY MOUNTAIN SPOTTED FEVEE TICK. IMPORTANCE OF THE CONTROL OF THE SPOTTED-FEVER TICIC. The most conspicuous loss from spotted fever is in human lives. In the Bitter Root Valley it was estimated in 1904 that 200 cases of the severe type of the disease had occurred up to that year. A con- servative estimate of the mortality there, as has been stated, is 70 per cent. This means a loss of about 140 lives in this small valley. At the present time, with an increase in the population of the valley, it is estimated that about 20 cases of the disease occur annually. This means a loss of about 15 lives each year and this loss is certain to increase as the population of the valley becomes larger. In Idaho it was estimated in 1908 by Dr. E. E. Maxey that the annual average of cases of spotted fever was 375. Undoubtedly, as Dr. Maxey pointed out, this estimate is very conservative. In all probability 500 would be a small estimate. The comparatively small mortality in Idaho would give a loss of human lives each year of about 35. Taking into consideration the whole area over which spotted fever is more or less prevalent, it is conservative to estimate 750 cases each year with probably 75 deaths. A great indirect injury the tick does in the Bitter Root Valley is in preventing the proper development of a region favored by a rich soil and by remarkable climatic advantages. As long as it is known that a dangerous disease exists there and that persons who farm or go into the country are especially subject to it, the valley can not prosper as it should. Relief from the tick would imme- diately result in increased land values and larger immigrations into the valley. In a larger way the possibility of the spread of the virulent form of the disease outside of the valley must be considered. This alone would warrant a much larger expenditure than is actually required for extermination or control in the valley. SUMMARY OF FACTS BEARING ON IMPORTANCE OF TICK CONTROL. It has been proved beyond peradventure by the investigations of Dr. Ricketts and others that spotted fever is transmitted in nature only by certain ticks. In the region where the disease now occurs it is transmitted to man by a single species of tick. Therefore the rational method of eradicating the disease is to attack this tick. In this way the proper procedure is exactly analogous to that being fol- lowed in the eradication of splenetic fever of cattle from the United States, by the eradication of the tick which transmits it. In the case of splenetic fever, certain more or less effective means of combating the disease itself have been discovered. These are in the form of a method of preventive inoculation and the administration of certain DISTEIBUTION. 15 drugs. In spite of this it has been found that the only hope for the eradication of the disease, or even for practical control, is in the destruction of the ticks. Inasmuch as no means of preventing or curing spotted fever are known, the importance of attacking the ticks is much greater than in the case of splenetic fever. The situation is also analogous to that brought about by malaria and yellow fever, which, as is well known, are transmitted by certain mosquitoes. The control of these diseases in all parts of the world has practically re- solved itself into a warfare against the mosquitoes. These considerations seem to make it very evident that the logical course to follow in the eradication or control of spotted fever is the elimination of the tick. The problem becomes purely an entomologi- cal one. Under these circumstances, it is most fortunate that certain feasible and economical means of eradication, first outlined in a rather general way by Dr. Ricketts, have been placed upon an exact and certain basis by the recent investigations of the Bureau of Entomology. DISTRIBUTION OF THE SPOTTED-FEVER TICK. As is shown in the accompanying map (fig. 1) the range of the Rocky Mountain spotted-fever tick extends throughout the north- ern part of the Rocky Mountain region across the Great Basin to the eastern edge of the Cascade Range. The southernmost limit of the tick is in the northern edge of New Mexico. Although the dis- tribution of the species in Canada has not been determined, there is little doubt that it extends over the southern half of British Colum- bia and the western portion of Alberta. HoAvever, only one accurate record of the occurrence of this species in Canada has been made, namely, by Dr. H. G. Dyar, who captured two female specimens at Kaslo, British Columbia, in 1903. While infestation occurs throughout large portions of Montana, Idaho, Washington, Oregon, Xevada, Utah, Wyoming, and Colorado, comparatively small areas in New Mexico and California are infested. The tick probably occurs throughout the entire Black Hills region in South Dakota and Wyoming, although but one collection has been made in that region. Naturallj^ there is no uniformity in the abundance of the tick throughout the territory in which it occurs. Our knowledge of the local occurrence of the tick throughout the Western States is not sufficiently complete to enable us to make definite statements as to areas within the whole infested region in which comparatively few ticks are to be found. We do know, how- ever, that certain sections of the country which are unfavorable for the development of the species are only slightly or not at all infested. 16 THE EOCKY MOUNTAIN SPOTTED FEVER TICK. During the investigation about 850 lots of tlie fever species have been collected. The following is a list of the counties and the num- :;M;aNT. * . I • I t N. DAK S. DAK. NEB. ::;V;":\- • • • ' vV:"^ •.^ //I'-COL. KAN. ! ^. OKL. • Fig. 1. — Map showing region in tlie United States in wliich tlie Ilocky Mountain spotted- fever ticli occurs. Tlie degree of sliading indicates the relative abundance of the tick in different sections. (From Bishopp.) ber of localities within tho.se counties Avhere the .species has been taken by the bureau : NUMBER OF LOCALITIES, BY COUNTIES AND STATES. IN WHICH THE SPOTTED-FEVER TICK IS KNOWN TO OCCLTR. California. — Modoc County, 3; Lassen Comity, 1. Colorado. — Boulder County, 4 ; Clear Creek County. ] ; Eagle County, 1 ; Garfield County, 1 ; Gunnison County, 1 ; Jefferson County. 1 ; I.,orimer County. 3; Mesa County, 2; Pitkin County, 1; Summit County, 1. Idaho. — Bannock County, 7; BingLani County, 2; Blaine County, 3; Boise County, 1 ; Bonner County, 2 ; Canyon County, 1 ; Cassia County, 2 ; Elmore County, 3 ; Fremont County, 6 ; Kootenai County, 2 ; Lemhi County, 2 ; Lincoln County, 2 ; Oneida County, 4 ; Shoshone County, 1 ; Washington County, 1 ; Twin Falls County, 1, DISTEIBUTION. 17 Montana. — Beaver Head County, 3 ; Broadwater County, 2 ; Carbon County, 1; Custer County, 1; Flathead County, 4; Gallatin County, 5; Granite County, 5 ; Lewis and Clark County, 4 ; Lincoln County, 2 ; Madison County, 6 ; Meagher County, 3 ; Missoula County, 8 ; Park County, 2 ; Powell County, 3 ; Ravalli County, 7 ; Rosebud County, 4 ; Sanders County, 5 ; Silver Bow County, 1 ; Teton County, 2 ; Yellowstone County, 1. Nevada. — Eureka County, 1; Humboldt County, 2; Lander County, 2; Lin- coln County, 2 ; Nye County, 3. 'Neic Mexico. — Rio Arriba County, 1 ; San Miguel County, 1. Oregon. — Baker County, 1 ; Crook County, 3 ; Grant County, 1 ; Harney County, 3 ; Klamath County, 1 ; Lake County, 1 ; Malheur County, 2 ; Sherman County, 1; Umatilla County, 2; Union County, 1. Utah. — Boxelder County, 2 ; Cache County, 2 ; Iron County, 1 ; Uinta County, 1 ; Utah County, 3 ; Wasatch County, 3. 'Washington. — Asotin County, 2 ; Chelan County, 2 ; Douglas County, 1 ; Grant County, 1 ; Kittitas County, 1 ; Spokane County, 3 ; Stevens County, 14 ; Yakima County, 3. Wyoming. — Albany County, 3; Bighorn County, 6; Carbon County, 3; Fre- mont County, 6; Latrona County, 3; Park County, 2; Uinta County, 2; Weston County, 1. The aboA'e is far from being a complete list of those counties in which the spotted-fever tick occurs, yet it gives a definite idea of the territory infested. It should be understood that the number of lo- calities given for a county does not represent the relative abundance of the tick in that county. The table includes only the number of localities from which the tick has actually been received. Greater population or a larger number of collectors in some counties has given more localities than in others, while the actual abundance of the tick may be exactly the reverse. Further investigation through- out the Eocky Mountain region will undoubtedly show the tick to be present in the majority of the counties included in the area shown to be infested in figure 1. Dr. E. E. Maxey^ (1908, p. 4) reports that the tick has been found to occur in all of the counties of Idaho with the exception of Latah. As is pointed out in Circular No. 136 of the Bureau of Entomology, the fever tick is known to occur at various elevations from slightly over 500 feet to nearly 9,000 feet above sea level. The species occurs in greatest abundance in the area known as the transition zone. It is also commonly found in the Canadian and Upper Sonoran life zones. FACTORS INFLUENCING ABUNDANCE. The occurrence and abundance of this tick within a given locality are dependent, to a large extent, upon the presence of favorable con- ditions for development. These conditions are. first, the existence of the small mammals which serve as hosts for the immature stages; second, the presence of large mammals upon which the adults may 1 See Bibliography, p. 45. 8464°— Bull. 105—11 8 18 THE ROCKY MOUITTAIN SPOTTED FEVER TICK. engorge, and, third, the existence of a certain amount of protection for the development of the stages when not on hosts. As a rule the abundance of ticks is dependent upon the amount of vegetation. Lands upon which some fallen timber and undergrow^th occurs are usually found to harbor ticks in abundance, provided the hosts^-cer- tain small mammals and domestic animals — are also present. In the Bitter Root Valley the areas in which more or less heavy second groAvth has followed the removal of the original timber have been found to be most heavily infested wdth ticks. These areas are locally known as " slashings." (See PI. I, fig. 1.) It has been determined that the direct rays of the sun during the sunnner have a markedly injurious effect upon the early stages of the tick. This fact may be utilized to some extent, as will be shown later, in the control of the species by clearing the land of timber and under- brush. In small experiments it has been found that when the seed ticks are exposed to the sun during very hot weather they imme- diately crawl down the grass to the surface of the soil to seek pro- tection, and in the absence of an abundance of moisture death results in a very few days. The exposure of freshly deposited eggs to the sun at Dallas, Tex., has been found to cause them to shrivel and dry within less than a day's time. The relative abundance of rain, especially during the spring months, in different years has a marked effect upon the number of ticks occurring in a given locality. This factor is of little importance in the natural control of the adult stage of the tick, but is a potent factor in the destruction of the eggs and immature stages, particularly after the latter have become engorged and dropped from the animal. Several other natural means of control of minor importance are also operating to some extent to keep the species in check. In barn lots, chickens have been observed to destroy the females which drop to the ground after becoming filled with blood. Some wild birds are known to feed upon various species of ticks, and in one instance, at least, the}^ have been observed to destroy the engorged females of the spotted-fever tick. Certain species of ants are also thought to be im- portant enemies of the pest, particularly when the ticks are in the immature stages. Owing to the fact that the Rocky Mountain spotted-fever tick is primarily a northern form, and therefore accustomed to severe cold, it is doubtful whether severe winters are of much importance in its destruction. This is particularly' true where there is an abundance of protection provided by brush and litter on the ground. SUMMARY OF LIFE HISTORY. 19 SUMMARY OF LIFE HISTORY OF THE SPOTTED-FEVER TICK. As is the case with nearly all species of ticks, this one passes through four distinct stages, namely, the egg, the larva or seed tick, the nymph, and the adult, THE EGG AND IjARVA, The eggs (PL II, fig. 5) are small, ovoid, brownish objects, about one thirty-eighth of an inch long. These hatch into minute, light brown, active six-legged creatures known as larvae or seed ticks. (PL III, fig. 2.) Before further development takes place it is neces- sary for these seed ticks to feed upon the blood of some animal. They usually attach to small mammals, such as ground squirrels, and become filled with blood in from 3 to 8 days. They then drop off "the host and find a convenient protected place in which to continue their development. Before engorging the seed ticlv measures about one thirty-seventh of an inch in length, but during feeding the body is considerably distended, so that it measures about one-eighteenth of an inch in length by one thirty-first of an inch in width when en- gorgement is complete. The color of the larva? when engorged is slate-gray. Activitj' is greatly reduced on account of the weight of the blood imbibed. THE NYMPH. After a resting period of from 6 to 21 days the skin is shed from the body of the engorged seed tick and an active eight-legged nymph appears. The extra pair of legs is gained during the resting stage. This character is sufficient to distinguish the nymphs from the pre- ceding or larval stage. In this stage it is necessary for the young tick again to find a host and fill with blood. This feeding period requires from 3 to 9 days. "VVlien engorgement is complete (see PL III, figs. 3, 4), the nymphs measure about one-sixth of an inch in length, while before engorgement the length is usually about one- seventeenth of an inch. The engorged njmiphs are bluish gray in color and not very active. THE ADULT. As in the case of the engorged larva?, the nymphs, after dropping, seek a protected place in which to transform, and there become com- pletely inactive. This resting stage requires a longer period than the preceding. During this lime the sexual organs of the ticks de- velop. "\"\lien the skins are shed the ticks appear as mature males and females. Shorth^ before the molting of the nymphs the light- colored shields on the back of the adult ticks can be seen through the thin skins which are soon to be shed. After the mature ticks es- cape from the nymphal skins they are rather soft and comparatively 20 THE ROCKY MOUNTAIN SPOTTED FEVER TICK. inactive. They soon become dried out and the external structures become thoroughly hardened. The color pattern becomes more pro- nounced and activity increases. This is the stage in which the ticks are ordinarily observed in the spring months. The males (PL II, figs. 2, 4) and females (PL II, figs. 1, 3) are nearly the same size, but the former have a hard plate or shield covering the entire back. Upon this shield is a somewhat complicated pattern formed by white bands or stripes. In the female the shield is much smaller, covering only the anterior portion of the body. Almost its entire surface is covered with wdiite. The portion of the body of the female behind the shield is rather soft and elastic. It is usually somewhat w^rinkled and of a dark reddishrbrown color. In this stage, as well as in the preceding, the ticks have eight legs, but the white markings on the backs of both sexes and the presence of a small genital opening on the underside near the " heads " of the ticks serve to distinguish them readily from the other stages. Of course the size of the adult ticks is considerably greater than that of either of the immature stages. Prior to feeding they usually measure about one-sixth of an inch in length by one-tenth of an inch in width. Before reproduction can begin it is necessary for both the males and females to feed upon the blood of some animal. They usually attach to the large domestic animals, and after feeding about 4 days or more the males start in search of mates. Fertilization takes place on the host, and in from 8 to 14 days after attachment the females, having become filled with blood, drop from the host and seek a protected place in which to deposit their eggs. During the course of feeding the portion of the body of the female behind the shield is greatly distended, so that the specimens now measure about one-half inch long by one-third inch wide by one-fourth inch thick. On account of the enormous distention of the back part of the body of the female, the legs and head are rendered inconspicuous. A close examination, however, wall show the white shield on the back just behind the "head." When the females are filled with blood the back part of the body is usually a bluish-gray color. Although the males imbibe a certain amount of blood when attached to an animal they never increase greatly in size as do the females. The females always drop from the host animal before beginning the deposition of eggs. Deposition continues for about 30 days, during Avhich time several thousand eggs are deposited. (See PL II, fig. 5.) During the process of deposition the female gradually shrinks in size. A'N'lien all of the eggs are expelled the tick is much shriveled (PL III, fig. 1) and has changed in color to a mottled yel- lowish. She dies within a few days after the last eggs are deposited. While depositing her eggs the female remains in the same place, so that all of the eggs are in one large mass. The eggs hatch into seed ticks in from 16 to 51 days and the life cycle is again repeated. jl. 105, Buri-au of Entomnlovy, U. S. Dept. of Agriculture. Plate II. The Rocky Mountain Spotted-Fever Tick (Dermacentor venustusl Fifi. 1.— Adult I'eiuale, uueimorRed, dorsal view. FIk- 2.— Adult male, dorsal view. ' Fig. 3.— .\didl female, unensor^wl, ventral view. Fig. 4.— .\dult male, ventral view. Fig. •'..— .\diiU leniale in act of depositing e.ggs. ({ )ri.ginal. ) B'jI. 105, Bureau of Erifc,rcol'.g/, U S, D*pt. li-f Agri«.ulfuf'». Pl/ite III, The SpottechFever Tick 'Dermacentor vemustus) and Dermacentor albipictus. Fig 1. — Afl ults^rttfcd- fever tick whifth ha.«i rlepr^itef] («sfi. Fij?.2, — ^Larva oi ^ji'^tferl-fever tick. Fi^. 3- — ErisroTged nymph of spotterl-fffrer tir-k, Kj)?, ♦. — Sam^r. Vfimtral riew. Fijf, 5.— AdnJl rnaleof htrmrvnerdtirriiBnpiHwi. Fig. 6. — Aflnlt female f/f /'. iiOnpiHiuf, nT>»rn(?r»Tjf»-'J, (f^r^nal.) SEASONAL, HISTOEY AND HABITS. 21 SEASONAL HISTORY AND HABITS. For convenience in tracing the life c^'cle of the Hocky Mountain spotted-fever tick we will begin with the appearance of the flat or unengorged females and males which appear with the first warm days of spring. It shoidd be remembered that these ticks have remained dormant throughout the winter months. When they are rendered active during the warm spring- days they are immediately ready to attach to an animal and engorge. Some of these ticks pass the winter in places where they are not readily reached by the warmth of the sun. Such specimens become active later than others. Emergence from winter quarters is therefore gradual, usually exteilding over a period of a few months, beginning about the 1st of March. The time of the beginning of activity in the spring is also depend- ent to a considerable extent upon the relative earliness of the season and upon the locality. In lesser altitudes, and at the southern limit of the range of the species, activity may begin as early as the middle of Februarj'^, while in the Bitter Eoot Valley it is probable that the ticks seldom become active in numbers before nearly the middle of March. After leaving their winter quarters the adult ticks begin crawling about and usually ascend brush to await a host. They may crawl upon trees or other objects so as to get several feet above the ground. In all ticks the anterior legs have well-developed sense organs located near their tips. These front legs are used as feelers. When the tick is disturbed it immediately begins to wave them in an endeavor to catch any passing object. Having found a host, the ticks crawl about upon it until a suitable place for attachment is found. On cattle they are usually found in numbers on the dewlap, between the fore and hind legs, and along the belly. On horses they are commonly found between the legs and sometimes in the mane. They may, however, attach to any part of the host. Attachment to the host is accomplished by means of a spiny beak, which has an opening in the end through which the blood of the animal is drawn. In from 4 to 8 days after attaching the males begin searching for mates. In order to fertilize the females they crawl beneath them, and after mating usually attach to the animal immediateh^ under their mates. When the females have become one-half engorged the blood is rapidly imljibed, and com- plete engorgement is reached in a very short time, after which they loosen their hold and drop to the ground. Table I shows the time required for the engorgement of females on different hosts and during diffeTent times of the year. 22 THE EOCKY MOUNTAIN SPOTTED FEVER TICK. Table I. — Time required for engorfjcment of females of Dermacentor venustus at Dallas, Tex. Adults attached. Dates of dropping as engorged females. Period of engorge- ment. Date. Host. First. Last. May 15,1908 Mar. 19,1910 Apr. 1,1910 May 4,1910 Mar. 29,1911 May 29,19111 Ox May 23 Mar. 28 Apr. 12 May 12 Apr. 7 June 3 June 1 Mar. 28 Apr. 13 May 17 Apr. 12 June 15 Days. 8-17 9 11-12 8-13 9-14 5-17 Guinea pig Ox Ox Ox.. . Goat ' The specimens in this lot were fertilized and slightly engorged when applied. After the dropping of the females the males usually remain on the host for some time. We have found that they crawl about over the animal, reattaching in different places and fertilizing a number of different females after one infestation of females has become en- gorged and dropped from the host. Immediately after leaving the host engorged females endeavor to find some protected place in which to deposit their eggs. As has been stated, deposition ninj begin as soon as the seventh day after dropping, and all of the eggs, which usually number about 4,000, are deposited within 30 days. During the process of egg laying the female gradually shrinlvs in size and death takes place within a few days after all of the eggs have been laid. The length of time before the beginning of egg laying depends largely upon the temperature. During cool w^eather a period of 41 daj^s has been loiown to pass after dropping before the first eggs were deposited. The development of the seed tick begins within the egg as soon as it is deposited. After the embryonic tick has grown for about two weeks, a small white spot appears on one side of the egg. The appearance of this spot enables one to determine whether the eggs will hatch. The time required for incubation is largely dependent upon temperature conditions. In the Bitter Root Valley Mr. W. V. King has determined that this period ranges from 34 to 51 days, the longer period occurring in the early spring months. At Dallas, Tex., we have observed eggs to hatch as early as 15 days after they were deposited, the longest incubation period observed in that locality being 41 days. After the small seed ticks hatch from the eggs they usually remain clustered upon the eggshells for a few days and then crawl upon any object in their immediate vicinity to await a host. In this .stage also the front legs are used as feelers, and when an animal comes into contact with the seed ticks, these immediately catch hold. Naturally during the larval stage, as well as during the adult stage, large numbers of the ticks starve before finding a suitable host upon which to engorge. The larvae die much sooner from starvation than do the other stages of the tick. SEASONAL HISTORY AND HABITS. 23 During the summer months we have found that all of the seed ticks hatching from a mass of eggs usually die within one month after the first eggs hatch. In one instance a period of 117 days elapsed between the beginning of hatching of the eggs and the death of the last seed tick. This is the greatest longevity which we have observed. Table II indicates the variations in the time required for the be- ginning of egg laying, incubation of the eggs, and length of time required for the starvation of the seed ticks : Table II. -Time required for beginning of deposition of rgg^^, hdtchlng. and starvation of seed ticks of Dermacentor vcnttf- 9 14-21 11-17 17-20 8-11 1908. Aug. 6 1909. Sept. 15 Oct. 3 1910. Mar. 23 Apr. 14 July 21 Aug. 1 Aug. 19 17 5 3 2 13 1 6 12 1908. Aug. 20 1909. Aug. 6 Oct. 21 1910. May 4 May 24 Aug. 1 Aug. 14 Aug. 31 1908. Aug. 22 1909. Aug. 7 1910. Mar. 22 May 23 May 30 Aug. 1 Aug. 16 Sept. 4 Days. 14- 16 21- 22 18-170 42- 61 40- 46 11 13- 15 12- 16 1 These records were made in the Bitter Root Valley, Mont.; all others were made at Dallas, Tex. Table IV. — Time required for engorgement of seed ticks and nymphs of Der- macentor venustus. Seed ticks applied. Date of dropping as engorged seed ticks. Period of en- gorge- ment. Nymphs applied. Date of dropping as engorged nymphs. Period of en- gorge- Date. Host. First. Last. Date. Host. First. Last. ment. 1908. Apr. 2 July 12 1909. July 28 Aug. 2 Aug. 27 1910. July 191 1911. May 18 Ox 1908. Apr. 5 July 15 1909. Aug. 2 Aug. 7 Aug. 29 1910. July 22 1911. May 21 1908. Apr. 10 July IS 1909. Aug. 4 Aug. 7 Sept. 3 1910. July 23 1911. May 24 Days. 3-8 3-0 5-7 5 2-7 3-4 3-6 1908. Apr. 1 1909. Aug. 13 Sept. 10 1910. May 24 Aug. 13 Aug. 19 Aug. 172 0.V 1908. Apr. 5 1909. Aug. 17 Sept. 14 1910. Mav 30 Aug. 18 Aug. 23 Aug. 22 1908. Apr. 8 1909. Aug. IS Sept. 15 1910. May 30 Aug. 20 Aug. 28 Aug. 27 Days. 4-7 do Guinea pig. Rabbit Guinea pig. Ground squirrel. Guinea pig. Guinea pig. Rabbit Bovine Rabbit Rabbit Ground squirrel. 4-5 4-5 6 5-7 4-9 5-9 1 This record was made by W. V. King in the Bitter Root Valley, Mont. Dropping probably began on July 21, or the second day after application. 2 This record was made iii the Bitter Root Valley, Mont. Records not referred to in footnotes were made at Dallas, Tex. Those lai*v?o which hatch from egg.s deposited by females Avhich do not find hosts until late in the spring become engorged during July and August and do not molt to nymphs until shortly before winter. It is thus necessary for the nymphs which appear late in the summer to pass the winter in that stage. These nymphs appear in the spring shortly after the emergence of the adult ticks ; that is, shortly after the middle of March. They continue to emerge from SEASONAL HISTOKY AND HABITS. 25 their winter quarters for some time, the hist individuals not securing hosts upon which to engorge until early in July. These individuals molt to adults during the hitter part of the summer, and the resulting adults pass the winter before feeding. In contrast to the short length of life as exhibited by the larvae, we find the vitality of the nymphs and adults to be remarkably great. It has been determined that adults collected on vegetation during the spring months may survive for a period of 413 days without food. These individuals undoubtedly passed the winter in the adult stage, and therefore the total length of life must have been approximately one and two-thirds years. However, in nature the gveat majority of the ticks with a vitality equal to this lot would probabhi- find hosts and become engorged. Unfed nymphs have been found to survive a period of more than 300 days. It is thus possible for ticks which pass the winter in the nymphal stage to live until at least Julj^ 15 of the following year. Under natural conditions this longevity is probably even greater. The following is a summary of the life cycle of the tick: The winter is passed as flat or unengorgecl males and females and as un- engorged nymphs. The former are present from about March 15 to July 15, during which time they find hosts and become engorged. It is during this period that the pest attacks man and communicates to him the germs of Rocky ^Mountain spotted fever. The eggs de- posited by the females which find hosts early in the spring hatch into larva?, Avhich may develop into adults b}^ the first or middle of September. The offspring of the females which become engorged late in the season succeed in developing only as far as the unengorged nymphal stage before cold weather begins. The overAvintered nymphs begin appearing from their winter quarters during the latter pait of March. They are to be found upon small wild mammals from that time until about the middle of July, at Avhich time the nymphs which have developed from the females engorged during that spring are also present. Overv.intered nymphs transform to adults during the summer and fall, and the majority of these adults pass the winter in the unfed condition. A few of the first nymphs to find hosts early in the spring may molt to adults sufficiently early in the summer to alhnv the adults to become engorged, deposit eggs, and the transformation lo proceed to the unfed nj^nphal stage by the approach of cold weather, thus. completing a life cycle in one year. However, the individuals which proceed with development beyond the unengorged adult stage during the same season must be very exceptional. AMien the mean temperature is low during the spring and early summer it is almost certain that none of the individuals which have passed the winter as unengorged nymphs develop further than unengorged adults during that season. 26 * THE EOCKY MOUNTAIN SPOTTED FEVER TICK. It has been observed that even though the adults which transform from overwintered nymphs are kept confined with the host animal during the summer or fall following their maturity, they show no marked desire to feed, usually endeavoring to crawl away and be- come quiet. Thus the habit of the adults of attaching to hosts in the spring appears to be so well established that they can scarcely be induced to attach to a host after midsummer. From the foregoing statements it is evident that although a few of the ticks may complete their life cycle — that is, the transformation from unengorged adults to unengorged adults of the next generation, or from unengorged nymphs to unengorged nymphs of the next gen- eration — during one season, the majority require two years for this cycle. Should overwintered nymphs not find hosts until late in the season and thus not become adult until the approach of winter, the resulting adults, if unable to find hosts, may survive until the second spring following. Ticks which pass the winter in the adult stage may survive until the second spring following, then engorge and pro- duce offspring which develop to nymphs the second summer, pass the winter in the nymphal stage, and complete development to unen- gorged adults during the third season. Thus it is apparent that under certain conditions three years might be required for the com- pletion of the life cycle. This would necessitate the destruction of the adult ticks during three successive seasons in order to eradicate the species. Figure 2 shows several of the ways in which development may proceed. THE HOST ANIMALS OF THE SPOTTED-FEVER TICK. The investigations conducted by Dr. Ricketts indicated that the Rocky Mountain spotted fever tick is restricted in regard to its host relations. Our investigation has shown that this restriction of certain stages of the tick to certain classes of animals is very well marked. The examination during three seasons of nearly 800 wild Tnammals which are inhabitants of tJie Bitter Root Valley and numer- ous ohservations made elsewhere have shown that^ with few excep- tions^ only the immature stages of the tick are to he found on this class of hosts. On the other hand., the large domestic animals are the j>rincipal hosts of the adult ticks, and the immature stages are rarely, if ever, found upon them. This restriction of the adult stage to the larger mammals, now a firmly fixed habit of the tick, undoubt- edly arose from the fact that the adult ticks are so large that they can be easily removed by the smaller mammals. As will be pointed out in the discussion of remedial measures, this habit of the Rocky Mountain spotted-fever tick may be taken advantage of in the con- trol or eradication of the species. SEASONAL HISTORY AND HABITS. 27 2^ ^ j3 73 9 'I T hr ,t3 +^ tx (- ft a -"^ s^ e t4 ,=! o p U) H ■^ o II- 0-3^1 MS g « ^Sg-ft -, d S « £ O fl a; d £?ot-2 30 K CD yi '3 ta. ' a^^d^S: o * m curt mS g- !U - o j:: 'd ■ 5 m'*^ ■;; 5- d Q,.a c :3 . o £ o ^ oj r- ? a 5 ' o fi a E 1- „ c^ o o Ol o o : be fcij bJD W3 tL'7 ,d d J; d d , M ^ ' ■— o •SB ^ 03 o a) ^-< o o d d w)'3 9 >.i6 03 d .3 2 2 ■- 1 1 S£?SdS&S§"^S^» t-i b£^ P^d^ d ftdjs o« E-i H H Eh « ^1 ^ d » d d-M-S Oj dxl 01 i6 b£.d=:: o w£2 oooot~i-< »_i,_(000000 coeoo O O t-l o o o 000«0 OCCOOOOOCOOOOOOO MCOOOO-HOOO rl< O CO OS O C^ 00 -* CO ^00 r^o 1-" ■*-* t~0-HOOOOOO OOOOOOOOO „ - «-^ S S.L )-^MCC»-l ooocoo tDoor-«^co fill?" 5i ■ftg ■2 3 ■" s -^ 1^ o u:) N 00 lo r-^ c^ 00 00 COCOC^»0»0 COt^COf— t T-t -ic^oooot-^coocoooooo ooooocoooo r-t^^t^tx) oM* :oo 00 w CO 00 w loc^ -* coo cooc-^-* "-I .-(u:) lo CO t-i IN -< '-' OCO ^hC^ OC^ ^ COlft CO t^ ■*cooc-^-* cs'; 2 >■: n o ^ 1-1 — C3 b F o a 3 073 Q Ij „ 3 63 3 3 m ^„ „ - , ^ , .. «5^-p.,fc3-2&||-^-y' a.g' * ©"SiS 3v5 b3'«.^a^ '^ o biJ 'o2 ■2 5 'god ' tt 3 CD - ■2 g-S 3 „ ^ .. f,'ii3 3§5^|:2 C w5aj(DOc3^'"' "^ — -"S ^ w x-ii S S h3QOOoe-iPL|pi<> --?^;S'«eo'S.T3-ak;3 .-O c! 3 • . c3 R o_; , a> ^ p (a ' OJ g ^ C IH 4 i, £ ' a|! o a £9 "o t*. S ^ O O I- ^ ~fi y ■3 a iooa CO P< 0x2 a IK 30 THE EOCKY MOUNTAIN SPOTTED FEVEE TICK. fested. Among the other mammals which are of considerable impor- tance as hosts of the immature stages are the large chipmunk, the woodchuck, snowshoe rabbit, roclf squirrel, wood rat, white-footed mouse, and meadow mouse. Our knowledge of the tick hosts in the valley was greatly increased by the cooperation of the Biological Survey of this department. Messrs. Howell and Birdseye, of the Survey, were located at the camp laboratory and made extensive collections during 1910. This Avork was continued in 1911 by Mr. Birdseye. Table V furnishes a list of the wild mammal hosts of this tick. It includes all the records of the Biological Survey,^ as well as a number made independently by Mr. King, of the Bureau of Entomology. The mammals are listed in the table according to their relative importance as hosts of the im- mature stages. It is especially worthy of note that among the wild mammals which act as hosts for the adult stage the mountain goat and brown bear are the only ones wdiich were found to have ticks upon them which were engorged sufficiently to deposit eggs. One hundred specimens of the birds commonly found in the valley were examined and found to be free from ticks. Table YI. — Host animals on ivhlch Dcrmacentor vcnustus in the adult stage has been found. ON DOMESTIC ANIMALS AND MAN. Common names. Scientific names. Approxi- mate number of hosts exam- ined. Approximate number of ticks collected. State of engorgement Males. Females. Total. of females. Ox 200 800 5 9 75 100 5 10 900 1 2,000 2,500 9 14 22 IS 2 400 1 2,000 2,500 5 17 32 20 4 4 400 4,000 5,000 14 31 54 38 4 6 800 1 Unengorged to fully. Equuscaballus Equus asinus Equus asinusXcabal- liis. vis airies Do. Ass Do. Mule Do. Sheep Unengorged to one- Dog Canis familiaris Capra liircus . . . half. Unengorged to three- Goat fourths. Unengorged to Hog slightly. Unengorged to one- Man Homo sapiens Felis domesticus third. Unengorged to one- Domestic cat fourth . Unattached. ON WILD ANIMALS. Mountain goat. Coyote Brown bear Jackrabbit Woodchuck Snowshoe rabbit . . Wildcat Badger Oreamnos montanus , Canis lestes Ursus amerieanus Lepussp Marmota flavi\'enter , Lepus bairdi , Ivynx Uinta , Taxidea taxus 3 l.'-fl 150 300 1 15 10 31 1 1 9 13 15 3 9 51 2 or 3 2 or 3 5 4 2 1 3 1 1 1 4 21 1 Unengorged to fully. Unengorged to slightly. Slightly "to one-fourth. Slightly to one-sixlh. Slightly. One-seventh engorged. 1 See United States Department of Agriculture, Biological Survey, Cir. No. 82. 2 Dead. OTHER SPECIES OF TICKS. 31 OTHER SPECIES OF TICKS FOUND IN REGIONS WHERE ROCKY MOUNTAIN SPOTTED FEVER OCCURS. Five species of ticks other than Dermacentor venustus have been found to occur more or less commonly in the Bitter Root Valley of Montana. These are: Dermacentor albipictus Pack. (PI. Ill, figs. 5, 6), Ixodes angustus Neum., Ixodes texanus Banks, Ixodes hhigi Bishopp, and Haemaphy sails leporis-palustris Pack. On account of the host relations of these ticks it is impossible for them to play any important part in the dissemination of Pocky Mountain spotted fever. Dermacentor albipictus has been found to occur on practically no other animals than horses, cattle, and mountain goats. It never attacks man. Neither one of the three species of Ixodes has been found to occur on man, and they ver}^ seldom attack the domestic animals, being confined to certain of the small wild mammals. The las