J O H N E . L L O Y D A N D M I C H A E L J . B R E W E R 1

B-966 • April 1992

Cooperative Extension ServiceDepartment of Plant, Soil, and Insect SciencesCollege of Agriculture

IntroductionThe larva of the sheep bot fly, Oestrus ovis L., is aparasite that lives on mucous surfaces of thenasal passages and sinuses of sheep and goats. Ithas been estimated that sheep bot fly larvaecause a 4 percent decrease in weight gain.Drummond in 1981 estimated annual losses insheep production in the United States due tosheep bot fly to be $13.5 million. An externalsign of infestation is the appearance of a slightlyto moderate runny nose (Figure 1); thiscondition increases in severity as the infestationdevelops.

1Former Assistant Professor and Cooperative Extension Service Entomologist, Department of RenewableResources

Sheep Bot Fly BiologyAdult Oviposition Behavior

The adult sheep bot fly, which has a bee-likeappearance, is 10 to 12 millimeters(approximately 3/8 to ½ inch) in length (Figure2). Adults do not feed and the females do not layeggs directly on the host. Instead, fertile eggshatch within the female fly, and she depositsnewly hatched larvae in the nostrils of the host.Sheep react to the larva-laying attempts of theflies by running or walking with their noses closeto the ground or huddling in groups.

Figure 1. An external sign of sheep bot flyinfestation is the appearance of a runny nose.

Figure 2 – The adult stage of the sheep bot fly isshown.

Larval Life History

Like other flies, the sheep bot fly has fourstages: egg, larva, pupa, and adult. Becausethe egg stage hatches within the female fly, itis not seen. Larvae pass through three stagesor instars of increasing size (Figure 3). Thestages are seldom seen because they developexclusively within the nasal passage and sinusesof the host.

The first instar, which is only a little morethan a millimeter (approximately 1/32 inch)in length, is found on the mucosa lining ofthe nasal passages. As this stage moves fromthe nasal passages toward the openings of the

sinuses, it molts to the second instar. Thethird stage (Figure 4) grows within the sinusesup to 20 millimeters (approximately ¾ inch)in length until it is ready to leave the host.Fully developed larvae leave the sinuses andnasal passage, drop to the ground, and pupatein the soil.

The timing of the sheep bot fly life cycle isdependent upon climate. In southern areas ofthe United States where winters are moderate,adult flies are active during all but the verycoldest months, and larvae continue todevelop within the host during winter. Innorthern areas, adult flies are active onlyduring the summer and early fall. In late falland winter, first instar larvae are either in asuspended state of development or aregrowing slowly. In the warm days of summer,larval development time may require only 25to 35 days. Larvae that overwinter, however,may require up to 10 months to completetheir development. The duration of the pupalstage varies between one and two monthsdepending on weather conditions.

Sheep Bot Fly ManagementFrom 1983 to 1987, more than 400slaughtered sheep were examined at theUniversity of Wyoming to determine theextent of sheep bot fly infestation in the RockyMountain region on sheep originatingprimarily from Wyoming. A few sheep fromColorado, Idaho, and Nebraska were includedin the study. More than 90 percent of thesheep were infested with larvae during 11-month periods (Figure 5).

In Wyoming and neighboring states, adult flyactivity is greatest in the summer and fall, andincreasing numbers of first instar larvae appearin the nasal passages of sheep at that time ofyear. Second and third instar larvae begin toincrease in abundance in early winter, andtheir numbers increase steadily until June(Figure 6). The numbers then decline rapidlyas larvae leave their hosts to pupate.

Figure 3 – These are the larval instars of thesheep bot fly. The third instar is at the top of thephotograph, followed clockwise by the first andsecond instars.

Figure 4 – A cross section through a sheep skullexposes a third instar in one of the sinuses.

Management Options

Control of the sheep bot fly is difficultbecause of the location of the parasite in thehost. Currently, the most effective method isthe use of an approved parasiticide on thelarvae. (See the appendix for specificparasiticides registered for management of thesheep bot fly in the United States.) Thefavored time for treatment is during late fall orearly winters after one or more killing frosts

Figure 5 – This shows the seasonal incidence ofsheep bot fly larvae in sheep heads fromWyoming and surrounding states during the1983-1987 time periods.

Figure 6 – This shows the seasonal incidence ofthree larval instars of sheep bot fly found indissected sheep heads from Wyoming andsurrounding states during the 1983-1987 timeperiod.

Figure 7 – Oral drench is being applied to sheepto control sheep bot fly.

have eliminated the adult flies. At that timelarvae harbored by sheep are predominatelyfirst instars and are found mostly on the nasalmucous membranes. Based on a study of anisolated range in New Mexico, it would appearthat sheep bots can be controlled if all sheepin any given area are treated annually witheffective parasiticides.

An oral drench of parasiticide has been shownto be an effective application method (Figure7). See the label instructions to ensure properdosage and administration.

Cultural strategies to reduce the impact of thispest have not been developed nor haveeffective biological controls been identified.Cooperative Extension Service educators orother animal health professionals may havemore information about sheep bot fly inparticular locations.

AcknowledgementsWe thank R. Kumar, E. Sturgeon(Department of Plant, Soil, and InsectSciences), and M. Riley (Department ofAnimal Science) for their many contributionsto the development of this bulletin.

Issued in furtherance of cooperative extension work, acts of May 8 and June 30, 1914, in cooperation with theU.S. Department of Agriculture. Jim DeBree, director, Cooperative Extension Service, University of Wyoming,Laramie, Wyoming 82071.

Persons seeking admission, employment, or access to programs of the University of Wyoming shall be consideredwithout regard to race, color, religion, sex, national origin, disability, age, political belief, veteran status, sexualorientation, and marital or familial status. Persons with disabilities who require alternative means forcommunication or program information (Braille, large print, audiotape, etc.) should contact their local UWCES office. To file a complaint, write to the UW Employment Practices/Affirmative Action Office, University ofWyoming, 1000 E. University Ave., Department 3434, Laramie, WY 82071-3434.

Cooperative Extension Service

Selected ReferencesDrummond, R. O., G. Lambert, H. E.

Smalley, Jr. & C. E. Terrill. 1981.“Estimated Losses of Livestock to Pests.”Pimentel, D. (ed.), CRC Handbook of PestManagement in Agriculture, Volume 1,CRC Press, Boca Raton, Florida.

Kimberling, C. V. 1988. Chapter 7 “Diseasesof the respiratory system.” Jensen andSwift’s Diseases of Sheep. 3rd edition, Leaand Febiger, Philadelphia.

Lloyd, J. E. 1985. Chapter 15: “ArthropodPests of Sheep.” Williams R. E. et al.(eds.), Livestock Entomology. Wiley, NewYork.

Meleney, W. P. and S. A. Apodaca. 1969.“Regeneration of a Population of OestrusOvis in Sheep on an Isolated Range.”Journal of American Veterinary MedicalAssociation 155:136-138.

Meleney, W. P., N. G. Cobbett and H. O.Peterson. 1963. “Control of Oestrus Ovisin sheep on an Isolated Range.” Journal ofAmerican Veterinary Medical Association143:986-989.

Appendix AParasiticide registered for control ofsheep bot fly in the United States

Ivomec (ivermectin) Sheep Drench, 0.08percent solution, is currently registered forcontrol of the sheep bot fly Oestrus ovis L. inthe United States. It can be used asparasiticide as outlined in this bulletin andmust be used according to label instructions.IVOMEC provides effective control of thesheep bot fly. IVOMEC Sheep Drench alsoprovides effective control of gastrointestinalroundworms and lungworms. IVOMECSheep Drench may be applied orally with anystandard drenching equipment at a dose of3.0 milliliters per 26 pounds of body weight.Do not treat sheep within 11 days ofslaughter. Observe all label precautions.

(release date: 1992)

The information given herein is for educationalpurpose only. References to commercial productsor trade names are made with theunderstanding that no discrimination isintended and no endorsement by theCooperative Extension Service is implied.