Survival of permethrin-susceptible, resistant, and F1 hybrid strains of Haematobia irritans (Diptera: Muscidae) on ear-tagged steers

The 24-h survival of susceptible, resistant, and F1 hybrid horn flies, Haematobia irritans (L.), was measured on caged steers with zero, one, two, or four pyrethriod-impregnated ear tags in place for up to 16 wk. With the susceptible strain, survival on one-and two-tagged steers was equal to 3% and 0.2%, respectively, of the survival on untreated steers. With the resistant strain, survival on one- and two-tagged steers was equal to 97% and 107%, respectively, of the survival on untreated steers. With the hybrid (heterozygote resistant) flies, survival on one-, two-, and four-tagged steers was equal to 61%, 23%, and 11%, respectively, of the survival on untreated steers. Heterozygote survival did not change during the 16 wk of treatment for two-tagged steers. These findings suggest that the survival of heterozygous-resistant flies from the onset of tag use has been responsible for the rapid evolution of pyrethroid resistance among U.S. horn fly populations