A study of horn fly, Haematobia irritans (L.) (Diptera: Muscidae), target-site sensitivity, susceptibility, and resistance management at selected sites in Louisiana

Abstract

The purpose of the first study was to evaluate the effectiveness of the use of a mid-season avermectin pour-on to cattle for managing OP resistance in horn fly populations at the Rosepine Louisiana Agricultural Research Station. During an eight year study, ivermectin treatments were made three times (when the treatment threshold of less than five weeks of control was reached), and in the following years, the number of weeks of control increased to 9 to 10 weeks each time. The purpose of the second study was to test a two-year OP/one-year pyrethroid rotation strategy as a strategy for maintaining susceptibility in horn fly populations to both pyrethroids and organophosphates. A seven-year study was conducted at the Agricultural Research Stations at Iberia and Hill Farm, Louisiana. At Iberia, 6, 10 and 7 weeks of control were recorded when pyrethroid ear tags were used every third year. When OP tags were used, horn fly control was maintained at 9 to 7 weeks throughout the 4 years of use. At Hill Farm, when pyrethroid ear tags were used, weeks of control reduced from 11 to 6 to 2 weeks of control. When OP ear tags were used, weeks of control reduced from 12 to 3 in the 4 years of use. The objective of the third study was to determine the rate of change in kdr and skdr allele ratios and genotype proportions in horn fly populations in the absence of pyrethroid pressure at the Louisiana Agricultural Research Stations in Winnsboro and St. Joseph. Allele ratios decreased about every 45 days from July to September by an average 20% for skdr and 10% for kdr. The number of skdr homozygous susceptible horn flies increased significantly and the number homozygous resistant horn flies increased significantly. We showed for the first time that the allele ratio changes were related primarily to the RR kdr and skdr genotypes and that the heterozygote likely had no biotic fitness costs.