The competitive abilities of seven calliphorid and one sarcophagid species were examined by investigation of various Characteristics of their larval growth curves. The relative success of each species is thought to depend to a great extent on the ability of the larvae to attain quickly the minimum weight for viable pupation. The ovoviviparous species (Calliphora nociva Hardy, C. augur (F.) and Parasarcophaga crassipalpis (Macq.)) were found to have the most rapid growth and Chrysomya megacephala (F.), Calliphora nociva and C. augur the highest assimilatory rate during the period of most rapid growth. Further, C. nociva, C. augur, Ch. megacephala and Ch. rufifacies (Macq.) were the species best adapted to pupation at low larval weight, which suggests that their food requirements for successful pupation were less (relative to maximum larval weight) than those of the other species. In this respect, Lucilia cuprina (Wied.) was also well–adapted and Ch. varipes (Macq.) was the least successful species.

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Resistance to diflubenzuron in the Australian sheep blowfly, Lucilia cuprina , has rendered this insecticide incapable of preventing flystrike in sheep from a few districts in eastern Australia. Wool producers affected by this situation must find suitable alternatives to protect their flocks. Results of laboratory bioassays against one population demonstrated that, despite extremely high diflubenzuron resistance (Resistance Factor > 791), it had only very low (2x) tolerance of cyromazine and dicyclanil. It is unlikely that this level of tolerance would have any practical impact on field control with either insecticide. Consequently, wool producers in districts where diflubenzuron-resistant flies are common can rotate insecticide treatment to either of these compounds to prevent flystrike in their flocks. However, unlike the highly diflubenzuron-resistant field strain, a laboratory strain selected for resistance to diflubenzuron (Resistance Factor = 617) was 10 times more resistant to dicyclanil than a susceptible strain but, like the field strain, was only two times more tolerant of cyromazine. Conversely, a field-derived strain selected in the laboratory for cyromazine resistance was 20 times more resistant to dicyclanil and 362 times more resistant to diflubenzuron than the reference susceptible strain.

Cyromazine resistance was detected in a field population of L. cuprina. Low-level cross-resistance to dicyclanil was also confirmed. Until more is known about the resistance, the prudent recommendation to control flystrike by this blowfly population is topical treatment with ivermectin.

A treated surface technique to measure the response of the sheep body louse, Dumaliniu ovis to contact insecticides is described. The responses of 30 populations of D. ovis to the synthetic pyrethroid cypermethrin showed wide variation at LCs0 and LCPr. Half the populations sampled were considered as pyrethroid susceptible, based on 100% mortality at 5 ppm (or less) to cypermethrin. This suggests that factors other than pyrethroid resistance were responsible for inefficient lice control on the properties from which these populations were obtained. Lice surviving 5 ppm or greater were considered provisionally as resistant.

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.

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