Pyrethroid Resistance in the Horn Fly., Haematobia irritans (L.) (Diptera: Muscidae)

Haematobia irritans (L.) populations were sampled from different locations in Louisiana and assayed with two pyrethroids, fenvalerate and permethrin. At the LD50, resistance to fenvalerate relative to an unexposed horn fly population was 36-, 32-, and 10-fold greater in the Red River, Rosepine, and Ben Hur populations, respectively. Resistance to permethrin at the LD50, level was 24-, 27-, and 10-fold greater in the respective populations. Thus, these horn fly populations were resistant to both fenvalerate and permethrin. The inherent irritancy/repellency of the pyrethroids for horn flies suggest that behavior may be a contributing factor to the pyrethroid resistance in these insects.