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

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.

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Three dicyclanil products failed to protect against dicyclanil resistant larvae.

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The standard dose dicyclanil product had an 83% reduction in protection period.

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This resistance also reduced efficacy of cyromazine and ivermectin jetting fluids.

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These products protected sheep against dicyclanil susceptible implanted larvae.

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This study confirms the observations of some sheep producers regarding dicyclanil.

Control of the sheep blowfly in Australia and New Zealand--are we there yet?

The last 50 years of research into infections in Australia and New Zealand caused by larvae of the sheep blowfly, Lucilia cuprina, have significantly advanced our understanding of this blowfly and its primary host, the sheep. However, apart from some highly effective drugs it could be argued that no new control methodologies have resulted. This review addresses the major areas of sheep blowfly research over this period describing the significant outcomes and analyses, and what is still required to produce new commercial control technologies. The use of drugs against this fly species has been very successful but resistance has developed to almost all current compounds. Integrated pest management is becoming basic to control, especially in the absence of mulesing, and has clearly benefited from computer-aided technologies. Biological control has more challenges but natural and perhaps transformed biopesticides offer possibilities for the future. Experimental vaccines have been developed but require further analysis of antigens and formulations to boost protection. Genetic technologies may provide potential for long-term control through more rapid indirect selection of sheep less prone to flystrike. Finally in the future, genetic analysis of the fly may allow suppression and perhaps eradication of blowfly populations or identification of new and more viable targets for drug and vaccine intervention. Clearly all these areas of research offer potential new controls but commercial development is perhaps inhibited by the success of current chemical insecticides and certainly requires a significant additional injection of resources.

Effective control of a suspected cyromazine-resistant strain of Lucilia cuprina using commercial spray-on formulations of cyromazine or dicyclanil

OBJECTIVE

To demonstrate the protection of Merino sheep from flystrike by Lucilia cuprina with cyromazine or dicyclanil in an implant study and in the field.

METHODS

In the implant study, sheep were treated with cyromazine or dicyclanil and implanted with 1st-stage larvae from a newly isolated field strain of L. cuprina (CYR-LS) or a reference strain (DZR50), then assessed over 3 days and compared with the implants on untreated control sheep. In the field study, weaner lambs were treated with cyromazine or dicyclanil and monitored weekly for flystrike over 18 weeks of grazing on the same farm from which the L. cuprina were isolated.

RESULTS

Implant study: cyromazine (6%) provided effective protection against CYR-LS and DZR50 L. cuprina for a minimum of 13 and 10 weeks, respectively. Dicyclanil (5%) provided at least 18 weeks' protection against both strains. Field study: only 1 of 386 lambs in the cyromazine-treated group was struck in the first 14 weeks of the trial. No strikes occurred in the 198 sheep treated with dicyclanil (5%). Rainfall, temperature and flytrap data indicated consistent fly pressure during the study.

CONCLUSIONS

Based on the results of these studies, there was no evidence of reduced susceptibility to cyromazine or dicyclanil and the periods of protection of sheep against L. cuprina were unaffected and consistent with the registered label claims.

Selective sweeps at the organophosphorus insecticide resistance locus, Rop-1, have affected variation across and beyond the α-esterase gene cluster in the Australian sheep blowfly, Lucilia cuprina

A major theoretical consequence of selection at a locus is the genetic hitchhiking of linked sites (selective sweep). The extent of hitchhiking around a gene is related to the strength of selection and the rate of recombination, with its impact diminishing with distance from the selected site. At the Rop-1 locus of the sheep blowfly, Lucilia cuprina, polymorphisms at two different sites within the LcαE7 gene encode forms of the protein that confer organophosphorus insecticide resistance. To assess the impact of selection at these two sites on variation around LcαE7, we sequenced regions within six other genes along chromosome IV across isogenic (IV) strains of L. cuprina. High levels of linkage disequilibrium, characterized by low haplotype number (K) and diversity (H), and significant R(2) values were observed for two genes, LcαE1 and LcαE10, both members of the same α-esterase gene cluster as LcαE7. A significant R(2) value was also observed for a gene predicted to be the next closest to LcαE7, AL03, but not for any of the other genes, LcRpL13a, Lcdsx, or LcAce. Skews in the site frequency spectra toward high-frequency variants were significant for LcαE1 (Fay and Wu's H = -2.91), LcαE10 (H = -1.85), and Lcdsx (H = -2.00). Since the selective sweeps, two forms of likely returning variation were observed, including variation in microsatellites in an intron of LcαE10 and a recombination event between LcαE7 and LcαE10. These data suggest that two incomplete soft sweeps have occurred at LcαE7 that have significantly affected variation across, and beyond, the α-esterase gene cluster of L. cuprina. The speed and impact of these selective sweeps on surrounding genomic variation and the ability of L. cuprina to respond to future environmental challenges are discussed.

Isolation of a Drosophila melanogaster desiccation resistant mutant

Mutagenesis provides a powerful way of isolating genetic and physiological processes underlying complex traits, but this approach has rarely been applied to investigating water balance in insects. Here, we describe the isolation of a desiccation-resistant mutant of Drosophila melanogaster. Mutagenesis of a desiccation sensitive line resulted in the isolation of a mutant with two-fold higher resistance. The mutant was partially dominant and mapped to the second chromosome. Mutant flies showed lower rates of water loss, and had a higher water content, but showed no change in body mass, glycogen content, hemolymph volume or water content tolerated at death from desiccation. These physiological differences are contrasted to changes in lines of D. melanogaster mass selected for altered stress resistance. Isolation of this mutant provides an opportunity to identify a gene involved in water balance in insects.

Genetics and fitness costs of cyromazine resistance in the house fly (Diptera: Muscidae)

The genetic basis of cyromazine resistance was investigated in the house fly, Musca domestica L. The ED-R strain, which was collected in Mississippi and selected further in the laboratory, was 116.5-fold resistant compared with the laboratory susceptible strain, OR-S. The SEL strain, which was created by crossing ED-R with OR-S followed by three cycles of reselection and backcrossing to OR-S, was 84.7-fold resistant relative to the susceptible strain. Mortality data from reciprocal crosses of resistant and susceptible flies indicated that resistance was autosomal and not influenced by maternal effects. The relative position of probit lines from the parental strains and reciprocal crosses showed that resistance was expressed as an incompletely dominant trait with D = 0.30 and 0.32 for ED-R and SEL, respectively. To determine the number of genes involved, models of one, two, three, four, and five loci were used to compare observed and expected mortality of F1ED-R x susceptible backcross. Resistance was best described by a polygenic model of three loci when equal and additive effects of loci were assumed. Another approach, which was based on phenotypic variances, showed that nE, or the minimum number of freely segregating genetic factors for ED-R, equaled 3.07. ED-R showed greater reductions in fitness compared with SEL independent of the presence or absence of sublethal concentrations of cyromazine. These data suggested that reduced fitness was not due to deleterious pleiotropic effects of the resistance genes themselves but arose from other loci in the ED-R genotype.

Toxicity and behavioural effects of diet-borne alkaloids on larvae of the black blowfly, Phormia regina

Larvae of the black blowfly, Phormia regina (Meigen) (Diptera: Calliphoridae) were exposed for 24 h to artificial diets that contained one of the following alkaloids: arecoline, caffeine, nicotine, quinine, sparteine or strychnine at either 1000 or 100 p.p.m. Each of the alkaloids caused reduced weight gain, relative to a control population in a no-choice bioassay and, with the exception of quinine, all alkaloids caused reduced larval weights in a choice bioassay. Larvae were unable to move away from diets containing arecoline (1000 and 100 p.p.m) and congregated away from diets containing 1000 p.p.m. quinine. Arecoline (1000 p.p.m) and both concentrations of nicotine caused significant mortality of larvae. Over a longer period (120 h), 10 and 1 p.p.m. nicotine resulted in significant numbers of larvae congregating away from a treated diet. Ten p.p.m. nicotine caused reduced weight gain over 120 h, although larvae provided with a choice were less affected. Exposure of larvae to dried residues of nicotine for 2 h did not affect subsequent development.

Resistance to antiparasitic drugs: the role of molecular diagnosis

Chemotherapy is central to the control of many parasite infections of both medical and veterinary importance. However, control has been compromised by the emergence of drug resistance in several important parasite species. Such parasites cover a broad phylogenetic range and include protozoa, helminths and arthropods. In order to achieve effective parasite control in the future, the recognition and diagnosis of resistance will be crucial. This demand for early, accurate diagnosis of resistance to specific drugs in different parasite species can potentially be met by modern molecular techniques. This paper summarises the resistance status of a range of important parasites and reviews the available molecular techniques for resistance diagnosis. Opportunities for applying successes in some species to other species where resistance is less well understood are explored. The practical application of molecular techniques and the impact of the technology on improving parasite control are discussed.

A genetic analysis of cyromazine resistance in Drosophila melanogaster (Diptera: Drosophilidae)

Flies resistant to the insect growth regulator cyromazine were selected in the F1 generation from a cyromazine-susceptible strain of Drosophila melanogaster (Meigen) treated with ethyl methanesulfonate. Four resistant strains were isolated by screening with cyromazine at a concentration > LC100 of susceptibles. In each strain, resistance is conferred by a single gene mutation. Cyromazine resistance in two of the mutants (rst(1a)cyr1 and rst(1a)cyr2) localizes to map position 17 of the X chromosome. Evidence is presented that these mutations are alleles of the gene rst(1a)cyr. Cyromazine resistance in another of the mutants (Rst(1b)Cyr) is also X-linked, and localizes to map position 49 of the X chromosome. The location of the gene conferring cyromazine resistance in the other mutant (Rst(2b)Cyr) is map position 66 of chromosome II. This is possibly an allele of a previously characterized cyromazine resistance gene, Rst(2)Cyr. Dosage-mortality analyses demonstrate a low level of cyromazine resistance is conferred in all strains.

The character or the variation: the genetic analysis of the insecticide-resistance phenotype

In this critique it is argued that the genetic basis of the evolution of resistance is dependent on how the phenotypic, and underlying genotypic, variation is channelled during a selective response. A polygenic response is preferentially favoured if selection acts within the phenotypic distribution of susceptibles; a monogenic response is predicted if selection screens rare mutations with phenotypes outside that susceptible distribution. The relevance of this model to the method of genetic analysis, the prediction of resistance mechanisms to novel insecticides, the generation of resistant beneficial insects and the development of the most effective resistance and integrated pest management programmes is discussed.

Predicting insecticide resistance: mutagenesis, selection and response

Strategies to manage resistance to a particular insecticide have usually been devised after resistance has evolved. If it were possible to predict likely resistance mechanisms to novel insecticides before they evolved in the field, it might be feasible to have programmes that manage susceptibility. With this approach in mind, single-gene variants of the Australian sheep blowfly, Lucilia cuprina, resistant to dieldrin, diazinon and malathion, were selected in the laboratory after mutagenesis of susceptible strains. The genetic and molecular bases of resistance in these variants were identical to those that had previously evolved in natural populations. Given this predictive capacity for known resistances, the approach was extended to anticipate possible mechanisms of resistance to cyromazine, an insecticide to which L. cuprina populations remain susceptible after almost 20 years of exposure. Analysis of the laboratory-generated resistant variants provides an explanation for this observation. The variants show low levels of resistance and a selective advantage over susceptibles for only a limited concentration range. These results are discussed in the context of the choice of insecticides for control purposes and of delivery strategies to minimize the evolution of resistance.