Determination of resistance mechanisms within field populations of Haematobia irritans through toxicological and biochemical techniques

The horn fly is a damaging blood feeding ectoparasite of pastured beef cattle.  Horn fly control is required in many areas to maintain profitable cattle production.  The longer fly season in the Southern areas of the US has required longer insecticide treatments; this has aided in selection for resistant individuals in field populations.  The current status of insecticide resistance was evaluated by glass jar contact assays, PCR for genetic point mutations kdr and G262A, and enzyme assays that evaluated inhibition of diazoxon (active form of diazinon) to AChE, general esterase activity, and Cytochrome P450 activity.  Horn flies were sampled from south (Labelle), Gulf coast (Ona), and north central (BTU) Florida.  Resistance ratios in glass jar exposure assays ranged from 3.0-66.6 for permethrin; >0.15-1.4 for beta-cyfluthrin and 3-14.7 for diazinon.  PCR evaluations of each population found RR kdr was present in 5-45% of flies, and RR G262A in 0-12% of flies sampled.  AChE inhibition by diazoxon was not different between collection sites.  P450 and general esterase activity was not statistically different in populations tested.  Resistance to permethrin and diazinon was found in one fly collection.  Multiple insecticide class resistance in horn flies is of major concern due to the limited chemistries available to cattle producers for fly control.