Resistance to dicyclanil and imidacloprid in the sheep blowfly, Lucilia cuprina, in Australia

Abstract

Background The sheep blowfly, Lucila cuprina, is a myiasis‐causing parasite responsible for significant production losses and welfare issues for the Australian sheep industry. Control relies largely on the use of insecticides. The pyrimidine compound, dicyclanil, is the predominant control chemical, although other insecticides also are used, including imidacloprid, ivermectin, cyromazine and spinosad. We investigated in vitro resistance patterns and mechanisms in field‐collected blowfly strains. Results The Walgett 2019 strain showed significant levels of resistance to both dicyclanil and imidacloprid, with resistance factors at the IC50 of 26‐ and 17‐fold, respectively, in in vitro bioassays. Co‐treatment with the cytochrome P450 inhibitor, aminobenzotriazole, resulted in significant levels of synergism for dicyclanil and imidacloprid (synergism ratios of 7.2‐ and 6.1‐fold, respectively), implicating cytochrome P450 in resistance to both insecticides. Cyp12d1 transcription levels were increased up to 40‐fold throughout the larval life stages in the resistant strain compared to a reference susceptible strain, whereas transcription levels of some other cyp genes (6g1, 4d1, 28d1) did not differ between the strains. Similar resistance levels also were observed in flies collected from the same property in two subsequent years. Conclusion This study indicates that in vitro resistance to both dicyclanil and imidacloprid in this field‐collected blowfly strain is likely mediated by cytochrome P450, with Cyp12d1 implicated as the enzyme responsible; however, it remains possible that another P450 also may be involved. A common resistance mechanism for the two drugs has important implications for drug rotation strategies designed to prolong the useful life of flystrike control chemicals. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The current status and outlook for insecticide, acaricide and anthelmintic resistances across the Australian ruminant livestock industries: assessing the threat these resistances pose to the livestock sector

“…On the other hand, the responses of field strain larvae to ivermectin and spinosad were normally distributed, with the response of the susceptible reference strain falling within the distributions for the field strains for both chemicals. The existence of in vitro resistance to imidacloprid was reported by Kotze et al 40 who found that blowflies collected in the Walgett district of NSW showed resistance to both dicyclanil and imidacloprid in in vitro assays. This pattern of in vitro dicyclanil and imidacloprid resistance occurring together has subsequently been found in a further five strains collected from various regions across NSW (Kotze, unpubl.…”

“…There has been considerable research on the molecular basis of chemical resistances in some of the parasites examined in the present study. This information would potentially allow for the development of molecular-based resistance tests (for example, nematodes, 55,56 cattle tick, 57,58 sheep blowfly 40 ). However, the use of this information on the molecular basis of resistance has been largely limited to basic research into mechanisms of resistance or small-scale surveys of resistance mechanisms in field-derived parasite samples, with no commercial tests available for Australian livestock producers.…”