| Summary: | The effects of mid-season avermectin treatments on pyrethroid-resistant horn fly populations were examined between 1999-2002 at three separate Louisiana State University Agricultural Center research stations; Red River Research Station (Bossier City, LA), Macon Ridge Research Station (Winnsboro, LA) and St. Joseph (St. Joseph, LA). The cattle were treated with pyrethroid ear tags in all years at all farms, and each farm received a mid-season avermectin treatment in one year. With consecutive yearly use of pyrethroids the number of weeks of control decreased for all farms. The number of weeks of control rebounded at Red River from 2 to 13 weeks in the year following the mid-season treatment of avermectin. At Macon Ridge there was a minimal reversal of weeks of control from 0 to 2 weeks in the year following the mid-season treatment. No change was observed at St. Joseph. The concentration required to kill 50% of the flies tested (LC50s) for fly populations at Macon Ridge and St. Joseph increased for pyrethroids from the spring populations to the fall populations between 2000 and 2002. The LC50s for fly populations at Red River followed the same trends except in 2000, when the avermectin treatment was administered, a decrease was seen from spring to fall.
Flies from St. Joseph were assayed for two alleles (kdr and skdr) associated with target site resistance to pyrethroids. In every year, the frequency of RR-kdr increased significantly in flies collected in the fall compared to flies collected in the spring. In all comparisons, the frequency of RR-kdr was significantly higher in flies collected in the fall compared to flies collected in the spring in the following year. Also, the frequency of R-skdr alleles was significantly higher in fly populations tested in the fall compared to fly populations tested in the spring for 1999, 2000 and 2002. The frequency of RR-kdr resistant horn flies collected in the fall at St. Joseph increased each year. By fall of 2002, the frequency of RR-kdr at St. Joseph had reached 100%.
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