Submitted to: Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 2, 2008
Publication Date: March 31, 2009
Citation: Li, A.Y., Lohmeyer, K.H., Miller, J.A. 2009. Dynamics and mechanisms of permethrin resistance in a field population of the horn fly, Haematobia irritans irritans (L.). Insect Science. 16:175-184. Interpretive Summary: Resistance to insecticides continues to be a major problem for the successful control of horn flies on pastured cattle. In order to understand resistance dynamics and to develop a better resistance management strategy, a study was conducted at a ranch in Texas between 2002 and 2006. A toxicity bioassay and a molecular technique were used to determine the levels of resistance and monitor the changes of resistance gene mutation frequency. The control efficacy on animals was also monitored before and after the pour-on formulation of a pyrethroid insecticide (cyfluthrin) was replaced with an organophosphate (diazinon) ear tags. High levels (up to 294-fold) resistance to the pyrethroid were found in the horn fly population in 2002, and a significant decrease in the resistance level was observed when the insecticide used was switched to diazinon ear tags to treat animals in 2003. There was a significant positive correlation between the frequencies of the gene mutations and levels of permethrin resistance, suggesting that these gene mutations are responsible for resistance in this horn fly population. Due to the high level of resistance to pyrethroid insecticides, the pour-on formulation of cyfluthrin failed to control horn flies at the ranch in 2002. Treatment with diazinon ear tags in the subsequent years led to reduced resistance to the pyrethroid insecticide and sustained control of horn flies on animals. This study validated the usefulness of diazinon ear tags in the control of pyrethroid-resistant horn flies on cattle.
Technical Abstract: A study was conducted at the Pressler ranch, near Kerrville, Texas, between 2002 and 2006 to determine the dynamics and mechanisms of resistance to permethrin in a field population of the horn fly, Haematobia irritans irritans (L.). Changes of resistance to pyrethroid insecticide associated with use of a pour-on formulation of cyfluthrin in 2002 and use of diazinon ear tags in subsequent years were studied using a filter paper bioassay technique and a PCR assay that detects two sodium channel mutations, kdr and super-kdr resistance alleles. A maximum of 294-fold resistance to permethrin was observed in the summer of 2002. A significant decrease in the resistance level was observed in spring, 2003, and resistance continued to decline after animals were treated with diazinon ear tags. In response to pyrethroid treatments, the allelic kdr and super-kdr frequency increased from 56.3% to 93.8% and from 7.5% to 43.8%, respectively in 2002, and decreased significantly in 2003 when the pyrethroid insecticide was no longer used to treat animals. Females were found to have a higher allelic super-kdr frequency than males in 2002, while no difference was detected between males and females in the allelic kdr frequency. There was a significant positive correlation between frequencies of the sodium channel mutations and levels of permethrin resistance, suggesting that the sodium channel mutations, kdr and super-kdr, are the major mechanisms of resistance to pyrethroids in this horn fly population. Results of synergist bioassays also indicated possible contributions of two metabolic detoxification mechanisms, the mixed function oxidases (MFO) and glutathione S-transfereases (GST). Compared to a horn fly infestation of an untreated herd, treatments with the pyrethroid pour-on formulation failed to control horn flies at the Pressler ranch in 2002. Sustained control of horn flies was achieved with the use of diazinon ear tags in 2003 and subsequent years.