Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Lane D. Foil


Studies on resistance of the horn fly, Haematobia irritans irritans (L.), to insecticides were conducted in Louisiana from 1989 to 1998. These studies included monitoring resistance and resistance dynamics under different strategies of insecticide use, monitoring the efficacy of insecticide products, establishing a relationship between bioassay findings and product efficacy, and evaluation of a new insecticide class. In four years (1989--1992), efficacy of 20% diazinon-impregnated car tags used yearly was reduced from greater than twenty to just one week of control, and tag failure was observed with resistance ratios (RR) from 1.8 to 5.7. Diazinon resistance developed more slowly and to a lower magnitude than that reported for pyrethroids. A strong correlation was found between RR and the frequency of flies (RF) surviving a 1.72 mug/cm2 discriminating concentration of diazinon. A high risk of diazinon tag failure was associated with a 5% RF in pre-season bioassays. Resistance among several organophosphate (OP) insecticides including fenthion, ethion, pirimiphos-methyl, and tetrachlorvinphos was observed following the development of resistance to diazinon. Esterase activity was significantly higher in OP-resistant flies than in susceptible flies from both laboratory colony and field population. From 1991 to 1997, the yearly rotation of lambda-cyhalothrin + piperonyl butoxide and pirimiphos-methyl ear tags was evaluated for control of pyrethroid-resistant horn flies in two locations. Control efficacy was reduced to a maximum of two and seven weeks for the synergized pyrethroid and OP tags in their last year of use, respectively. The rotation did not improve pyrethroid efficacy or prevent further development of resistance to the pyrethroid or the OP. During 1996 and 1997, the efficacy of experimental chlorfenapyr ear tags for horn fly control was evaluated under field conditions at one location, and the susceptibility of flies to chlorfenapyr was measured at seven locations in 1997. Ear tags containing either 30 or 40% chlorfenapyr were effective for fly control, and the number of weeks with >90% fly reduction ranged from eight to eighteen. All field populations of horn flies, including those resistant to pyrethroids and OPs, were more susceptible to chlorfenapyr than were flies from a reference susceptible colony.