ARS | Publication request: Bioassay improvements for assessing pyrethroid resistance in horn flies

Submitted to: Workshop Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: November 21, 2012
Publication Date: N/A

Technical Abstract: The horn fly (Haematobia irritans) is a major cattle pest in the American continent, causing yearly economic losses of over US $860 million in Brazil. Frequent use of chemical insecticides has led to the development of insecticide resistance, leading to significant economic losses and environmental issues. Monitoring, and management of resistance primarily rely on insecticide bioassays; however, the current technique requires to test multiple concentrations that limited its use as the large number of flies are needed. Although synergists are powerful tools in resistance mechanism studies, little information is available regarding the effect of the oxidase inhibitor piperonyl butoxide (PBO) on horn flies. The present study is aimed to develop a diagnostic concentration (DC) bioassay for detecting cypermethrin resistance as well as to evaluate PBO toxicity and synergism to cypermethrin in horn flies. Dose- and time-response bioassays of horn flies to impregnated filter papers were conducted to establish a cypermethrin DC bioassay using horn flies from a susceptible colony (ARS-KBUSLIRL, Kerrville, TX). Complementary molecular studies (PCR) were performed to look for knocking down resistance in surviving flies from bioassays. The DC bioassay was later tested in field populations at the LSU Experimental Stations. Studies on PBO were conducted at LSU with susceptible colony flies (ARS-KBUSLIRL) and resistant field populations. Toxicity of PBO concentrations (0.5, 1, 2.5, 5, 10 and 20%) to horn flies and respective synergism to cypermethrin were evaluated at 1, 2 and 4-hr dose-response bioassays. Cypermethrin at a concentration of 3.19 µg/cm2 caused 99% mortality of susceptible flies in 1hr, thus was regarded as a reliable DC for detection of cypermethrin resistance. Suitability of this DC was validated under field conditions. Even high PBO concentrations showed low toxicity to horn flies in bioassays; however, the highest cypermethrin+PBO synergism was achieved by the lowest concentration (0.5%) of PBO. The single concentration bioassay makes the technique cheaper, faster, and more practical, expanding its use in resistance research and monitoring programs. Results from the PBO toxicity and synergism studies contribute to improving research on mechanisms of metabolic resistance in horn flies. Ultimately, such study may also help improve horn fly control and resistance management. The future of scientific cooperation between Embrapa, ARS-KBUSLIRL, and LSU, mainly on the control and resistance of livestock pests is also discussed. USDA is an equal opportunity employer.