DYNAMICS OF PYRETHROID RESISTANCE IN HORN FLY, HAEMATOBIA-IRRITANS (L) (DIPTERA, MUSCIDAE), POPULATIONS ON TAGGED AND UNTAGGED CATTLE IN MANITOBA

Initially, fenvalerate and permethrin tags at two tags per animal and one tag for every other animal nearly eliminated horn flies [Haematobi a irritans (L.)] at Glenlea Research Station, Manitoba. However, fly n umbers increased 8 weeks post-treatment in 1987 and 6 weeks post-treat ment in 1988. LC50 values of the flies to fenvalerate and permethrin i ncreased toward the end of each season but the slope of regression lin es decreased in 1987. In 1988, initial LC50 values were lower compared with those observed at the end of the 1987 season. When tags were app lied in 1988, LC50 values increased from 0.42 to 3.14 mug per cm2 for fenvalerate and from 2.06 to 10.76 mug per cm2 for permethrin. Mortali ty observed at discriminating concentrations of fenvalerate (0.625 mug per cm2) and permethrin (5 mug per cm2) during the season decreased f rom 67.9 to 2.8% and from 66.8 to 5.7%, respectively. Cattle at Glenle a and in a second herd at Libau were not treated in 1989; susceptibili ty of flies resistant to pyrethroids did not change significantly duri ng the season in these herds. Discontinuing use of pyrethroid-impregna ted ear tags for one season was not long enough for substantial reduct ion in resistance to occur.