Reduction of horse and deer flies on the Cottonwood Range and Livestock Experiment Station as a result of grazing.

When tabanid populations were sampled in south-western South Dakota in 1977, 1978 and 1980, Tabanus similis Macq. and Chrysops aestuans Wulp were the commonest species collected by blackball and sticky-drum traps. T. similis was commonly observed as a pest of beef cattle from June to early August, whereas C. aestuans was attracted more commonly to man during this period. Fly populations were lower in 1980 when compared with numbers found in the earlier years. It is suggested that weather conditions and grazing by beef cattle in 1979 reduced the amount of vegetation on which flies could oviposit, resulting in fewer adults caught during the 1980 season. C. sequax Will. and C. carbonarius Wlk. were also attracted to man, while collections of T. marginalis F., T. orbicallus Philip and Silvius pollinosus Will. represent new records for the state.