Dicyclanil resistance in the Australian sheep blowfly, Lucilia cuprina, substantially reduces flystrike protection by dicyclanil and cyromazine based products

Abstract

Late in 2017, field samples of the Australian sheep blowfly, Lucilia cuprina , were submitted by sheep producers from three states of Australia (South Australia, Victoria and New South Wales). Some were collected by submitters concerned about shortened periods of flystrike protection from dicyclanil based products. Neonate larval offspring from the NSW field samples survived and successfully completed their life cycles following exposure to dicyclanil and cyromazine at susceptible discriminating concentrations in vitro . The in vivo study reported here used dicyclanil resistant neonate larvae to assess the flystrike protection provided by a cyromazine jetting fluid and a number of dicyclanil based spray-on products, when applied to sheep six weeks after shearing. The two dicyclanil resistant blowfly strains used in this study showed in vitro resistance ratios, at the LC50, of approximately 13- and 25-fold relative to a dicyclanil and cyromazine susceptible strain. Compared to the levels of resistance that L. cuprina has developed to other insecticides these are relatively low, however, three dicyclanil based spray–on products (active ingredient 12.5 g/L, 50 g/L and 65 g/L) had protection periods reduced by 73%, 78% and 69% respectively when compared to the maximum protection periods claimed by the manufacturer. A 50% and a 33% reduction in protection period was also observed to a cyromazine and an ivermectin based jetting fluid respectively. In contrast, protection periods were attained or exceeded regardless of the treatment used against field derived dicyclanil susceptible neonate larvae. For the first time we confirm that dicyclanil resistance enables the completion of the L. cuprina life cycle following flystrike initiation on dicyclanil or cyromazine treated sheep when insecticide levels are considered high and protective. This study also provides in vivo information on the effect of dicyclanil resistance on the protection provided by a product with an active ingredient belonging to an unrelated insecticide group. Dicyclanil resistance is of major concern to the Australian sheep industry.

Control of sheep flystrike: what's been tried in the past and where to from here

“…While these flocks were not selected randomly and, hence, the results do not represent the real industry prevalence, the results suggest that resistance may be concerningly widespread. Sales 11 and Sales et al 12 also reported on an in vivo trial with strains composed of a mixture of resistant flies from various geographical locations. Significantly reduced protection periods for cyromazine and dicyclanil against resistant blowfly strains were demonstrated in this larval implant trial: strikes were observed (i.e., ‘protection failures’ occurred) at weeks 3, 4 and 9 for dicyclanil products with protection period claims of up to 11, 24 and 29 weeks, respectively.…”

Fleece rot and dermatophilosis (lumpy wool) in sheep: opportunities and challenges for new vaccines

“…However, there is a long history of development of blowfly resistance to various classes of insecticides (Levot 1995;Sandeman et al 2014;Heath and Levot 2015). The risk of widespread resistance developing to currently used insecticides (Sales et al 2020) emphasises the need for constant vigilance and for use of integrated pest management practices to prolong the utility of insecticides.…”

“…Appropriate application minimises the presence of a sublethal dose on sheep, which can increase the risk of resistance development and spread. The ability to maintain control and rotate insecticide use to minimise resistance development may become more constrained with the recent development of resistance to cyromazine and dicyclanil (Levot 2012(Levot , 2013Levot et al 2014;Sales et al 2020). Vaccines that indirectly or directly reduce the likelihood of flystrike may become an important adjunct in the integrated pest management strategy.…”

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

“…The sheep blowfly has shown an ability to develop resistance to some insecticides. 4,5 Resistance to the organophosphate class resulted in inadequate protection against flystrike when applied prophylactically, with their use continuing only as dressing treatments for existing strikes. Low-level resistance to the benzoyl phenyl urea compound, diflubenzuron, emerged in the field soon after its introduction for flystrike control in the early 1990s.…”

“…By 2014, this low level of in vitro resistance to cyromazine was described as being 'quite common'. 8 Sales et al 5 recently described a number of strains of blowfly collected from regions within the state of New South Wales (NSW) that showed survivors when cultured on food sources containing a 'susceptible discriminating concentration' of dicyclanil. This study also reported on an in vivo larval implant trial using a blowfly strain showing high levels of in vitro resistance that showed significantly reduced flystrike protection periods following administration of dicyclaniland cyromazine-based products.…”