Studies on pyrethroid ear tags and pyrethroid resistance in horn fly populations in Manitoba

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Mwangala, Felix S.
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This study was undertaken from 1987 to 1990, in Manitoba. A producer survey was conducted in 1987 to determine the extent of insecticidal ear tag usage in Manitoba. More than 80% of the 200 cattle producers who responded to the questionnaire applied tags to control flies. Ear tags were more rapidly adopted by beef producers than by dairy producers. Most of the producers applied one tag per animal and were satisfied with the control afforded by ear tags. Fenvalerate tags were used extensively followed by permethrin tags. Field horn fly populations were evaluated for resistance to fenvalerate and permethrin using the impregnated filter paper petri-dish method in 1987 to 1989. Resistance factors were calculated by comparison to a susceptible laboratory strain from Kerrville, Texas. The number of resistant populations and intensity of resistance increased during the study. Resistance factors ranged from 0.11- to 5.5-fold and 0.6- to 14-fold in 1987, 0.03- to 38-fold and 0.1 to over 100-fold in 1988, and 1.0- to 62-fold and 0.8- to over 100-fold in 1989, for fenvalerate and permethrin, respectively. Resistant flies were sampled on 3 herds in 1987, 6 herds in 1988, and 10 herds in 1989. Fenvalerate and permethrin residues on hair of cattle at Glenlea Research Station were determined, using gas chromatography, over a 3 month period following application of two tags per animal or one tag for every other animal. Cattle with two tags had significantly higher residues on the head and neck region for one month and on the body and rump for 2 wk than cattle with one tag or without a tag in the same herd. Residues on cattle with or without a tag in the same herd were similar. Residues declined by 80 to 86% on the head, 73 to 78% on the body and 36 to 84% on the rump. The isomeric composition varied from 51-61% SR,RS:39-49%SS,RR for fenvalerate and 61-67% trans:33-39% cis for permethrin. Fly numbers on cattle with two tags per animal and one tag for every animal were similar in 1987 and 1988. Fly numbers increased towards the end of the season and were higher in 1988 than in 1987. The LC50's of these flies increased during each season but the LC50's declined between fall and spring. Susceptible flies were eliminated from the population upon application of tags...