Parasiticide resistance in flies, lice and ticks in New Zealand and Australia: mechanisms, prevalence and prevention

This review outlines the history of parasiticide resistance in the principal ectoparasites of sheep and cattle in New Zealand and Australia, blowflies, buffalo fly (Haematobia irritans exigua), sheep biting louse (Bovicola ovis) and cattle ticks, and discusses recent changes in their response to insecticides and acaricides. Mechanisms of resistance and evaluation methods are described, with ways in which insecticide resistance can be avoided or ameliorated also discussed. Resistance in sheep blowflies (Lucilia cuprina; L. sericata) to organophosphates and benzoylphenyl urea compounds is widespread in Australia and New Zealand, but there are lesser concerns about a pyrimidine carbonitrile product as well as cyromazine and macrocyclic lactone actives which still offer the promise of long-term protection. In Australia the effectiveness of synthetic pyrethroid and benzoylphenyl urea products against the biting louse of sheep have been reduced by widespread resistance but effective temephos, macrocyclic lactone, imidacloprid and spinosyn-based products are now available. Pyrethroid-resistant sheep lice are also present in New Zealand. Buffalo fly remains a significant problem in Australia and control relies heavily on insecticide use. Resistance to synthetic pyrethroids is widespread and organophosphate resistance common, but less evenly distributed. There is no evidence of resistance to acaricides used against the New Zealand cattle tick, Haemaphysalis longicornis and experience in Japan with the most commonly available active, flumethrin (a synthetic pyrethroid), suggests that this three-host tick is, in the short term, likely to remain susceptible. The same cannot be said for Rhipicephalus australis (formerly Boophilus microplus) which, in some strains, is highly resistant to many of the active ingredients in the acaricides used against it. A formamidine, a benzoylphenyl urea and macrocyclic lactones still show efficacy although some resistance is being detected to macrocyclic lactones in a few strains and the occurrence of resistance to the benzoylphenyl urea active in Brazil is ominous for Australian producers. Insecticide resistance is a price paid for improved livestock health, welfare and cost-effective animal production, but it can be managed through judicious and targeted use of insecticides, consideration of refugia, and observance of the principles of integrated pest management. The use of traps, biological control agents and judicious husbandry all have a role to play, but strict adherence to the correct application of insecticides and recognition of their capabilities and limitations is also required. Alternative methods of ectoparasite control and recommendations for their integration into preventative programmes are discussed briefly in this review.