CASE NOTES

Flystrike, insecticide resistance and dressings

Narelle Sales, Elizabeth Macarthur Agricultural Institute

Posted Flock and Herd November 2024

Introduction

Resistance to cyromazine was detected in Lucilia cuprina, the Australian Sheep Blowfly, in 2010 (Levot et al. 2010). Of more importance to NSW producers has been the development and widespread occurrence of dicyclanil resistance. Of the recent submissions of maggots collected off struck sheep by NSW producers, 25% were classified as displaying moderate resistance to dicyclanil and low-level resistance to cyromazine; 64% were classified as having 'higher'-level dicyclanil resistance and low-level cyromazine resistance; and 11% displayed higher-level resistance to dicyclanil and a previously undetected higher level of resistance to cyromazine (Sales 2024 unpublished data). The practical implications of higher-level dicyclanil resistance were demonstrated by a 69% to 78% reduction in protection periods depending on the product used (Sales et al. 2020). Of added importance to NSW producers are the links identified between dicyclanil resistance and resistance to cyromazine and decreased efficacy of imidacloprid and diflubenzuron (Sales 2020).

With dicyclanil resistance a common reality for sheep and wool producers in NSW, chemical options to prevent or treat flystrike are restricted. There are currently six chemical groups available. However, the organophosphates are currently under review, the synthetic pyrethroid (alpha-cypermethrin), the neonicotinoid (imidacloprid) and the insect growth regulator (IGR) dicyclanil are not registered to treat existing strikes. These registration limitations further reduce options for chemical selection and rotation. For example, producers have few alternatives to the use of dicyclanil-based products following mulesing or marking of lambs.

Increasingly, there are reports of producers mixing products contrary to manufacturer's instructions and applying these mixtures prophylactically and/or to treat existing strikes. Often producers are attempting to increase efficacy or lengthen the protection periods of the alternative, shorter-acting chemical groups. This concept of combinations and mixtures is a familiar one for producers as it is a common approach for drenches and drench products. Disturbingly, there are also reports of producers increasing the application concentrations from the recommended rate based on their prior experiences of reduced protection periods. The following information is provided to District Veterinarians to counter these 'off-label' practices which will impact, and be impacted by, resistance.

METHOD

To provide preliminary information on mixtures and doubling of the registered concentration, we determined the efficacy of a representative product from each of the chemical groups that are registered as dressings. These were the macrocyclic lactone Ivermectin (Akula -Ivermectin Concentrate for Fly and Lice by Zagro), the IGR Cyromazine (iO Venus Liquid Sheep Blowfly Treatment- Independents Own), the spinosyn Spinosad (Extinosad Lice, Fly & Maggot Eliminator - Elanco) and the organophosphate Diazinon (Coopers Diazinon Sheep Blowfly Dressing And Cattle, Goat & Pig Spray - MSD Animal Health). The efficacy of each product was determined singularly and as a mixture with another product against three strains of blowflies. These were a susceptible laboratory strain (LS), a composite strain of low-level cyromazine-resistant field strains (CYRL) and a composite strain of field strains displaying higher-level dicyclanil resistance and low-level cyromazine resistance (DRH). The maggots from each strain were treated as full crop, third instars according to the technique of Levot et al. (1999). Once treated the maggots were allowed to pupate and mortality was assessed based on the number of adult flies that emerged compared to the number of maggots that were treated.

In addition, we measured the efficacy of these products and mixtures when each chemical was at twice the registered rate, the registered rate and half the registered rate. We examined the half rate option as producers mixing two chemicals, each at their own registered rate, would effectively halve the concentration of both chemicals once combined.

Results

As expected, mortalities increased with the doubling of a products recommended concentration and decreased with the halving of the recommended rate. At half the recommended rate 100% mortality was only achieved against the LS strain, with the exception of spinosad (54.9% mortality) and the ivermectin+spinosad mixture (95.7% mortality). The combination of half the recommended rate of ivermectin against the CYR strain also achieved 100% mortality. At the recommended rate only the ivermectin-based dressing delivered 100% mortality against all three strains. Conversely, as little as 7% mortality was achieved against the DRH strain when subjected to a half rate of spinosad or diazinon. In general, as the resistance status of the strain increased the efficacy of both the individual products and their mixtures decreased with few exceptions and despite small variations in the size of the larvae that may occur between strains.

Spinosad alone failed to achieve 100% mortality against any of the strains at any concentration. Even against the LS strain the spinosad-based product only achieved 75.2%, 64.5% and 54.9% mortality following exposure to double, registered and half concentrations, respectively. Doubling the registered concentration of cyromazine did achieve 100% efficacy against all three strains and against LS and CYR as a mixture with spinosad.

The best performing treatments were ivermectin, ivermectin+cyromazine and cyromazine, respectively. However, cyromazine and the ivermectin+cyromazine mixture decreased in efficacy with increasing resistance level in both the recommended and half recommended rates as expected.

In general, the addition of either the spinosad- or the diazinon-based product to either of the ivermectin or the cyromazine based products also reduced efficacy regardless of concentration.

Discussion

A statistically significant correlation was found between the presence of dicyclanil resistance and the response of strains to cyromazine, imidacloprid and the IGR diflubenzuron (n=100) (Sales 2020). A correlation was also present to a lesser extent between cyromazine resistance and responses to imidacloprid, which is understandable as cyromazine resistance appears to be a precursor of dicyclanil resistance. This relationship is supported by our findings of cyromazine resistance in the absence of dicyclanil resistance but never dicyclanil resistance without cyromazine resistance (n=296) (Sales unpublished data). This relationship between dicyclanil resistance and imidacloprid 'resistance' was also observed by Kotze et al. (2022) and reported as cross resistance. This study builds on this information by showing that the presence of cyromazine resistance, and to a greater extent dicyclanil resistance, reduces the efficacy of representative dressing products containing actives from the organophosphate, spinosyn and ML chemical groups. This reduced efficacy may not be wholly attributable to the presence of dicyclanil and/or cyromazine resistance as formulation incompatibilities that may arise when two products are mixed should also be considered.

While only a single representative product from each chemical group was used, this study would indicate that producers will gain little from doubling the recommended dressing rates if resistance is present, particularly to dicyclanil. In general, a decrease in efficacy resulted from the 'off-label' mixing of either the spinosad- or diazinon-based products with the ivermectin- or cyromazine-based products, regardless of resistance status of the strain or doubling of the concentration. Anecdotally, the most common 'mixture' producers use is ivermectin and cyromazine. Of the combinations studied this combination appeared the most effective. However, the efficacy of this mixture was less than ivermectin on its own, particularly against the cyromazine-resistant strain. Overall, these 'off-label' practices increase the cost of treatment per head with little gain. They would also select for resistance at a greater rate than the use of either of the products individually or use of the registered rate of application regardless of the active ingredient in the product.

Acknowledgements

This work was supported by NSW Department of Primary Industries and Australian Wool Innovation (AWI) under contract 4500016328. AWI is grateful for its funding, which is primarily provided by Australian woolgrowers through a wool levy and by the Australia Government which provides a matching contribution for eligible R & D activities.

Acknowledged Individuals

Leanne Bringolf, Samantha Currie, and Elise Carsburg for insect culturing and assisting with insecticide testing.

References

Levot G (2012) Cyromazine resistance detected in Australian sheep blowfly Australian Veterinary Journal 2012 90(11):433-437
Levot G, Sales N and Barchia I (1999) In vitro larvicidal efficacy of flystrike dressings against the Australian sheep blowfly Australian Journal of Experimental Agriculture 1999; 39(5):541-547
Sales N, Suann M, and Koeford K (2020) Dicyclanil resistance in the Australian sheep blowfly, Lucilia cuprina, substantially reduces flystrike protection by dicyclanil and cyromazine based products International Journal for Parasitology: Drugs and Drug Resistance 2020 14:118-125
Sales, N (2020) AWI Final Report August 2020 Sheep Ectoparasite Resistance Update 2018-2020. Project No: ON-00491
Kotze A, Bagnall N, Ruffell A, George S and Rolls N (2022) Resistance to dicyclanil and imidacloprid in the sheep blowfly, Lucilia cuprina, in Australia Pest Management Science 2022 10