E-520 January 1941 United States Department of Agriculture Bureau of Entomology and Plant Quarantine RANCH MANAGEMENT FOR SCREWWORM PREVENTION AND ERADICATION IN TEXAS AND ADJOINING STATES By Daniel C. Parman and William L.. Barrett, Jr., Division of Insects Affecting Man and Animals INTRODUCTION Prevention of screwv/orra losses in livestock and valuable v/ildlife at low cost has been one of the investigational goals of the Bureau of Ento- mology and Plant Quarantine during the last 25 years. Through research rather complete information has been obtained on the identities, life, habits, adaptations, and regional and seasonal distribu- tion and abundance of most of the blowflies of known economic importance in the United States. Also, the more important factors affecting distribution and abundance of blowflies have been determined and grouped into such classes as climate-weather, animals attacked, natural enemies, method and rate of dissemination, type of terrain, land usage, and ranch-management practices , Analysis of information obtained has led to recommendations for ranch practices that will, if followed, materially reduce attacks of these flies on livestock and game animals. The indicated ranch practices are considered feasible by many officials and livestockmen and entail less expense than present methods. The investigations have disclosed that 90 percent or more of all screwworm cases are initially infested by one kind of fly, commonly called the primary screwworm fly, i in all areas where the fly is established and that very few wounds are infested in any areas where that fly is not present ^ 1 Known scientifically since 1933 as Cochliomyia americana Cushing and Patton. Hereafter in this circular the fly is referred to simply as the screwworm fly. 2 Several groups of common blowflies rarely initiate worm infesta- tions in wounds, but they may be destructive to animals as secondary invaders after an attack has been started by the screwworm. The common blowflies (Phormia regina Meig. . Lucilia spp. , Sarcophaga spp. , and Cochliomyia macellaria (F.)), unlike the screwworm. develop in carcasses and infest wool in sheep and goats. These common blowflies have short life cycles and build up to great numbers in short periods. This circular describes and recommends a plan of ranch management for reducing attacks of the screwv/orm fly on livestock and game animals, and presents the more pertinent facts upon which the recommendations are based. ESSENTIAL FACTS ABOUT THE SCREWWORM FLY Life Cycle and Breeding Habits The screwworm passes through several distinct stages. Eggs are laid by the adult fly; the eggs hatch into worms; the worms develop and when mature they drop from the wound and enter the ground and lie immobile in their hardened outer coverings. From the ground the flies of the new generation emerge and lay more eggs. The young worms of the screwworm fly require a wounded warm-blooded animal for their first 3 days of development. The early stages of the worms have never been known to develop in dead animals. Should an infested animal die after the worms are 3 or more days old, the worms will continue to develop in the carcass the remaining 1 to 7 days required to attain full growth. The developmental period from eggs to mature adult flies capable of laying eggs is about 3 weeks under most favorable conditions, or about twice the time required for development of the most common carcass-breeding blow- flies. Under less favorable conditions the developmental period in the ground is lengthened; but the fly has no true resting or hibernating stage, and the flies must emerge from the soil usually in less than 60 days or die. The percentage of survival and emergence from the soil is usually low if cool weather retards development for more than 40 days. No adult flies are likely to survive 2 months. Cold weather (below approximately 55° F.) prevents adult fly activity, and temperatures below approximately 20° F. kill adults in a few hours. Because there is nc true hibernating or dormant period in any; stage of the life cycle of the screwworm fly^, it cannot survive any period of 4 months without producing at least one generation. Factors Influencing Screwworm Attack Screwworm fly attacks and the resultant loss are proportional to the number of flies present. If the flies are limited in number, the wounds are not all infested, and infested animals usually survive the initial attack and a few subsequent reinfestations without treatments. As the numbers of the fly increase to the highest populations encountered, all open unprotected wounds become infested. Even slight scratches and fly and tick bites may become infested, and the larger wounds so grossly attacked that animals, unless treated, may die in 2 or 3 days. Even under favorable weather conditions, and in the presence of large numbers of untreated wounds, the fly requires from 2 to 3 months to build up high populations. Populations of the screwworm fly build up and diminish, other factors being constant, in proportion to the continuity and number of wouiided aniinala available for develupiug woiiufc. The aooooipaayiug chart (fig. 2) depicts the typical annual and seasonal population cycle of the adult fly on the western Balcoues Escarpment of the Edwards Plateau in south- western Texas. This also applies reasonably well to the remainder of the winter-survival area in southern Texas. Ill the different regions climatic conditions produce annual variations in the population cycle from year to year, and the weather conditions produce seasonal variations in the numbers of the fly in all areas. Where the continuity of fly activity and survival is not broken at some time of the year, high peak populations occur in the spring and the fall months, Fewer flies are present and much less fly activity occurs in the late summer and in the late winter months. In any locality the total annual population of the screwworm fly has rarely varied more than 50 percent from the average. The most destructive populations have usually built up over 2 or more years with favorable weather conditions. In almost every year there are short periods in which the fly becoaies persistent in attacking livestock, when weather is most favorable for activity of the fly and is such that wounds are kept attractive for longer intervals than at other times. During those periods even a few flies may cause more than ordinary losses. Overwintering of the Pest The fly is exterminated by climatic conditions every winter in that part of Texas north of the solid line shown on the accompanying map (fig. 1), and it rarely survives the winters in the area north of the broken line. The fly survives during normal winters in the dotted area on the map. The density of the dots indicates the winter abundance of the fly. Although the numbers of flies that survive in all of the overwintering area are small, two areas of higher winter populations have been found. One of the areas of higher winter population is in the densely brushy section of the southern Gulf Plains; the other is on the western Balcones Escarpment. There is less abundance in the intervening region. To the present time there has been observed no relation of winter activity on the southern Gulf Plains area to spring, and summer activity in the region to the north. Under present conditions a uicderateiy high pvpuiation of the fly builds up in the southern Gulf Plains area late in the winter and early in the spring as a result of attack on young calves, spreads to the Gulf coast area to the northeast, attains highest populations usually in April or May, decreases to slight activity during the summer, and builds up again in the fall in Gulf coast tick infestations in cattle. In the area intervening between the Gulf coast and the Balcones Escarpment a relatively low population survives the winter, chiefly by being propagated in surgical wounds and in connection with the birth of young in cattle, In the spring there is a slow build-up of flies until a peak of fly abundance is reached in April and May, after which the fly population diminishes rapidly as the result of hot, dry weather. - 4 - Along the western Balcones Escarpment there has been each year a close relation between the amount of winter activity and survival of the fly and the spring and summer activity and population of the fly in that area. Also, the spread of the fly northward in Texas and to States as far away as Iowa, Illinois, and Mississippi is affected by the amount of survival on the escarpment. The escarpment area from Terrell to Kerr Counties has con- sistently had populations of the fly that were from 2 to 20 times as great as in any other area in the United States. The winter activity diminishes more from fall to spring in the escarpment area than in the southernmost Gulf Plains area. Screwworm flies living in November are dead in February; only their descendants are alive. Under present methods of ranch management a few cases of screwworms occur in December and January and these enable the pest to survive the winter. During the winter months there are fewer natural wounds in livestock, such as those caused by boils, prickly pear, and needle grass, and virtually no wounds in wild animals other than a few produced by traps and bullets. No wild animals that are common hosts of the screwworm have young during the winter. More than 90 percent of the December and January screwworm cases have been found io k§ in wounds caused by birth of young domestic animals and b^ surgical operations . Even in the presence of these man-made wounds, from each 100 flies present in November there results a much smaller number in the mid- winter generation and only 3 survive until March. From these 3 an average of 500 flies develop and become active in June. These 500 flies are produced by successive generations developing in wounds resulting from shearing, surgical operations, birth of young livestock and deer, and other miscel- laneous wounds in animals. Some rather high populations of the fly have apparently developed from deer during the fawning season in some few sections where deer are abundant, chiefly in the eastern Balcones Escarpment and on the southeastern Edwards Plateau. The dates of fawning in the two last- mentioned areas are such that if the arrival of the screwworm there could be retarded 1 or 2 months, most of the fawns would be saved, and the development 0^ large numbers of screwworm flies that later attack domestic animals would be prevented. Migration The screwworm fly is migratory and may spread at a rate of approxi- mately 35 miles a week. Mass migrations are not apparent, but there is a dispersion of some flies in all directions. The rate of spread appears to be about the same from high or low fly population centers, but the number of flies reaching adjoining uninfested territory is proportional to the size of the infested area and to the number of flies it contains. The first dis- persion in the spring appears to take place in the most northern areas where overwintering occurs. These include river valleys in the escarpment, and territory north and west of the escarpment. The migrating flies usually arrive in central Texas by May 1, in southern Oklahoma about June 1, in southern Kansas by the middle or last of July, and in Mississippi by Septem- ber. In favorable years the fly may reach Iowa by September. Few flies migrate into the northern areas, and serious outbreaks would not occur if - 5 - there were no multiplication of the inigratinji. flies. Aa the flies reach the Northern States weather conditions are favorable for fly breeding, and in the presence of wounds that are untreated large fly populations result. Later in the summer hot dry weather tends to retard increase in fly abundance. Many records have been made of shipments of screwworm-infested animals to distant points from the southern Texas area, and such shipments, made early in the season, have established infestations that have become destruc- tive. SUMMARY OF IMPORTANT FACTS UPON WHICH THIS PROGRAM OF SCREWWORM PREVENTION IS BASED Worms must develop more than 3 days in a wound before they can become mature enough to transform later into flies. If all worms in all wounds were killed be f o re they were 4 days old, the screwworm fly would be ex- terminated. It is almost impossible to find all cases of worms in livestock in the rugged brushy area of the Balcones Escarpment, and the high wild animal population aiakes complete wound treatment impractical during some seasons of the year. Since there are few wounds in wild animals in the fall and winter months, complete treatment of all infested wounds may be approached. In leafless brush, animals are more easily found; and as the weather is cooler, animals can be handled to better advantage. When a program of ranch management for screwworm prevention is con- sidered, the essential facts about the screwworm fly may be summarized as follows: 1. The young worms require a wounded warm-blooded animal for develop- ment and have never been observed to develop in carcasses. 2. The severity of attacks on livestock and wild animals is propor- tional to the number of flies in an area. 3. The fly has no true dormant stage in its life cycle and cannot survive a period of 4 months without producing at least one new generation. 4. The population of the fly rises or falls, other factors remaining constant, in proportion to the number and continuity of live, warm-blooded, wounded animals available . 5. The fly is exterminated each winter by low temperatures in all areas except the extreme southern portions of the United States. In Texas the area of continued winter activity and survival is generally confined to the southern part of the State shown on the accompanying map below the heavy broken line, and is usually only in the dotted area. 6. The breeding of the fly under the prevailing ranch-management conditions persists mainly because of wintertime surgical operations, the birth of young livestock, the wounding of game animals, and the lack of treatment. The winter season in the overwintering area is the critical - 6 period in the breeding cycle and subsequent population of the fly. Condi- tions are most critical for the pest in the northern portions of the winter- survival area. In this area the insect is most destructive and from it the early advance of the pest northward takes place. 7. By natural migration of the fly, which may be as rapid as approxi- mately 35 miles a week, and by the shipment of infested livestock, the pest spreads each year over the vast region to the north and east from the relatively small overwintering area in southern Texas. 7^hen this rate of dispersion continues for a number of months, sections as much as 1,000 miles from the overwintering area may be infested naturally. 8. The importation of infested animals into uninfested areas may establish early infestations that will have time to build up destructive populations before the beginning of cold weather. 9. Although southern Texas furnishes the flies that each year reinfest the remainder of Texas and adjoining States, each area suffers only if the insect is permitted to breed up after arrival of the few flies by migration or through introduction in infested livestock. RANCH-MANAGEMENT PROCEDURES RECOMMENDED TO AVOID SCREWWORM LOSSES The screwworm has originated mainly from an abnormal number of wounds in livestock during the winter months in southern Texas. The customary system of ranch management in that region provides wounds at the time of year when it is difficult for the fly to survive. If this system is changed so that the birth of young and all wound-creating operations are suspended during this critical period, much ground would be gained in keeping the initial population low. In some of the more northerly areas of Texas and in adjacent States the necessary wound-creating surgical operations could be performed and the birth of young could occur during periods of complete absence of the fly. Ranchmen in all areas outside of the winter-survival area should instruct shippers not to transport animals infested with screw- worms. Animals imported should be examined, and, if infested, treated immediately. On the accompanying map are presented the suggested dates for carrying out ranch-management procedures in each of the areas outlined. The chart gives further details of the practices recommended for the area below the heavy broken line on the map. Briefly, Texas has been divided into 4 zones, running roughly from west to east, in which are indicated periods when surgical operations, shearing, and the dropping of young should be avoided or carried out. These zones are as follows: Zone 1 includes counties south of a line running north of Terrell, Kimble, Blanco, Lavaca, and Matagorda Counties. In this zone, surgical operations should be performed during the periods February 1 to April 30 and August 1 to September 30. All wounds should be avoided during the periods October 1 to January 31 and May 1 to July 31. Zone 2 lies north of zone 1 and south of a line running north of Pecos, Upton, Green, Lampasas, Cherokee, and Shelby Counties. In this zone surgical operations should be performed from December 1 to April 30, and all wounds should be avoided between May 1 and November 30. Fall shearing may cause some wounds during this period. These should be carefully treated. Zone 3 includes the extreme western part of Texas from Brewster to El Paso Counties and northward to a line running along the northern edge of Bailey, Floyd, and Hardeman Counties, thence eastward along the Red River to the Arkansas line. In this zone all surgical operations should be performed between October 1 and April 30. All wounds, except those incidental to fall shearing, should be avoided from May 1 to September 30. Zone 4 includes all of the region north of zone 3. In this zone all surgical operations should be performed between October 1 and May 31, and all wounds should be avoided between June 1 and September 30. Stock owners in adjoining States should make their ranch operations conform in general to the periods indicated by extending the zone boundary lines eastward or westward as the case may be. If the suggested practices are followed by all ranchmen, the fly should be eradicated in most or all of the present winter-survival area_^ If the fly were eradicated in the winter-survival area, the natural initial Infestations in more northerly areas would occur later or not at all . The treatment of the first spring and early summer cases in all areas prevents increase of screwworm fly abundance while weather is favorable for fly breeding. The individual or community acting alone could profit by following the procedures outlined. The individual ranchman could avoid screwworm cases in essential wounds and in birth of young by restricting such essen- tials to the most nearly fly-free period; and the community, by preventing the development of worms in essential wounds and birth of young, v/ould eliminate the development of flies that could later initially infest wounds or more grossly reinfest wounds. Where conditions are such that the wounding and birth-management practices outlined are not possible, the desired break in winter survival of the fly can be obtained by careful treatment of all wounds every 3 days. However, the elimination of wounds is simpler and more economical than wound treatment, and prevention of screwworm breeding is more certain in complete absence of wounds. By managing to have young born and by making all necessary wounds in certain short periods of time, long seasons of treating worm cases will be avoided and more uniform herds of animals will be produced. The greatest protection to livestock and game will be derived from a coordination of the program in large areas, and the results of such procedures will be cumulative from year to year as the annual population cycle of the fly is broken down. stat^ ^^BOABD - 8 - The elimination of any cause of screwworra infestations does more than prevent screwworms in those wounds, for by so doing the production of flies which would infest other wounds is stopped. Clearing of useless types of brush, killing cacti of various kinds, use of bloodless emasculators, removal of projecting nails or other sharp objects in pens, reduction of needle grass, and removal of any cause of accidental and unnecessary wounds will all contribute to screwworm control. The Bureau of Entomology and Plant Quarantine is developing wound protectors that give longer and more efficient protection. The purpose of this paper has been to present to the ranchmen the more significant results of the research work that has been done on the ranch-management phase of screwworm prevention. The information will enable ranchmen individually, or, better, on an area basis, to begin work toward abatement of the pest. The Bureau of Entomology and Plant Quarantine realizes, however, that the ranching business changes over a period of years; and as new changes become necessary and as new information is obtained from research work, additional suggestions for improved and more adequate control will be made available to livestock men. FIG. 1 ZONBS AND DATES RECOMMENDED FOR SGREWWORM PREVENTION TEXAS Perform surgical ' operations Oct. 1 - May 31 Avoid wDunds June 1 - Sept. 30 ZONE 3 I ::•► Perform surgical ope rati ons Oct. 1 - Aprij 30 Avoid wounds May 1 - Sept. 30 (Except fall , , -qhearing) MAimtrnoom •« bft!ffefe*TlMt^ cocfi HOCK .use. |r ecToff mDCn Bms. NAU cm. TE/rr. LYNN SA/OA HCNT DON. cocn MRTIN HOW. U/TOt NOLAh rAYLOf STOt. COnE \fiUHNlCOL£. ^■^i STOMC 'iASK '*''Cfi. KING yi^^t^j^xmm akh. SCUR.\riSHV\JONES SHACK STtm PALO. cJ? !»• wise OCNT% COLi Perform Surgical operatioa Dec. 1 - April 30 Avoid woiinds May I-N0V..3O ^ (Except fall shearing) ^^ '?■ Perform surgical operations Feb. 1 - April 30 Aug. 1 - Sept. 30 XON€ Avoid wounds Oct. 1 - Jan. 31 May 1 - July 31 iLZKtn UCN. V/ILiifiDC tASTCl COmARDS iKERRl mmmi jhh.Ulus scAU-swrvn itttcs 0 x 40 ao ao 100 Normal winter screwworm survival area dotted. ,N0 WOUNDS DURING THIS PCRIOD MEANS DEATH TO THIS CROP OF FLIES. #, .^. W^