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Title: High levels of insecticide resistance in introduced horn fly (Diptera: Muscidae) populations and implications for management

item OYARZUN, M - Southern University Of Chile
item Li, Andrew
item FIGUEROA, C - Southern University Of Chile

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2010
Publication Date: 1/20/2011
Citation: Oyarzun, M.P., Li, A.Y., Figueroa, C.C. 2011. High levels of insecticide resistance in introduced horn fly (Diptera: Muscidae) populations and implications for management. Journal of Economic Entomology. 104(1):258-265.

Interpretive Summary: The horn fly is a major pest of cattle in the Americas. The use of chemical insecticides continues to be the most effective way for horn fly control. However, insecticide resistance impedes control efforts. In constrast to the situation in the United States where there is a history of horn fly management programs, the horn fly problem in Chile started in the 1990s when it was introduced from neighboring countries. Given the insecticide resistance problems we observed in horn flies in the US, Mexico, and Brazil, resistance is likely to develop in the Chilean horn fly populations after sustained insecticide use. A study was conducted to assess horn fly resistance in Chile using a combination of available techniques. Insecticide bioassays were conducted to determine the level of resistance to both pyrethroids and organophosphate insecticides. A PCR assay was used to detect the sodium channel mutation, which is known to be one of the mechanisms of resistance to pyrethroids. The levels of esterase activity were also determined in fly samples to evaluate its role in resistance. Results from this study revealed high levels of resistance to cypermethrin, a widely used pyrethroid insecticide in Chile, and high allelic frequencies of the sodium channel mutation gene in these resistant populations. This study provides much needed information in order to develop an effective resistance management strategy for successful control of the horn fly in Brazil.

Technical Abstract: The horn fly, Haematobia irritans (L.) (Diptera: Muscidae) was introduced to Chile early in the 1990s. Since their introduction, farmers have controlled this pest almost exclusively with insecticides. In order to understand the consequences of different control strategies on the development of insecticide resistance and their persistence, a field survey was conducted at eight farms in the south of Chile to characterize insecticide resistance in field populations and resistance mechanisms. Horn fly samples were assayed to determine levels of resistance to pyrethroids and diazinon, genotyped for kdr and HiaE7 mutations, and tested for general esterase activity. All field populations, including ones that were not treated with insecticides for the past five years, showed high levels of cypermethrin resistance and high frequencies of the kdr mutation. None of the fly populations demonstrated resistance to diazinon and the HiaE7 mutation was not detected in any of the fly samples. Esterase activities in all populations were comparable to those found in the susceptible reference strain. The findings of high frequencies of the kdr mutation in both the treated and the untreated fly populations suggest complex dynamics among field populations of the horn fly in Chile.