Chemical Resistance in maggots.
A blowfly resistance project conducted by Narelle Sales and her team at NSW DPI throughout 2018-2020 found alarming chemical resistance profiles in most of the maggots tested. However, most of the maggots submitted to this project were from central west NSW.
You can still get your maggots tested through DPI (it will cost $300 - 400). Testing will be carried out on the submitted strain to determine if resistance is present to Dicyclanil (e.g. Clik), Cyromazine (e.g. Vetrazin), Ivermectin (e.g. Coopers Blowfly and Lice), Spinosad (e.g. Extinosad) and Imidacloprid (e.g. Avenge + Fly). In addition, this will ascertain if cross-resistance between chemical groups is present. Knowing the chemical resistance status on your property can aid in the decision-making process for flystrike prevention and control.
The results from the 2020 published study, summarised below, will be helpful to guide your flystrike chemical choices this year.
FINDINGS:
To enable comparison between submitted blowfly strains for each insecticide, a resistance ratio (RR) was calculated for each strain relative to a susceptible reference strain.
1. Regardless of the state of origin, and despite decreased use of OPs for many years, all submissions were resistant to the OP diazinon as expected. A strain from Tasmania had the minimum RR of 8-fold while the maximum RR of 51-fold was a strain from NSW. Comparison of these RR ranges with those determined in 1985 (5 to 60-fold) provide further evidence of the stabilisation of OP resistance.
2. A significant increase in the range of response of blowfly strains to ivermectin has occurred since 2002 when RR’s ranged from 0.6- to 2.8-fold (n=74) while this study determined RR’s ranging from 1.3-fold to 9.2-fold.
3. The response of current NSW submissions to spinosad (n=42) with those reported in 2002 show the range of RR’s has also increased from 0.2- to 2.6-fold in 2002 (n=31) to 0.4- to 4.6-fold in 2020.
4. For many years imidacloprid has been used for lice control but only since 2019 for flystrike. Base line data is usually collected prior to extensive use of an insecticide. In the absence of base line data on imidacloprid, this study reports the current range of response which cannot be assumed to be a susceptible range. The range of RRs is large, being from 3.2-fold (Tas) to 42.5-fold (Vic). Further categorization is required but this data will act as a benchmark for the future.
5. Low level resistance to cyromazine (without dicyclanil resistance) was found in 48% of strains from WA (n=21), 33% from South Australia (n=12) and 9% from Victoria (n=11). The percentage of cyromazine only resistant maggots ranged from “present but below 1%” to 73%.
6. To date 100% of submissions from NSW have concurrent dicyclanil and cyromazine resistance (n=42). This is not the case for other states with only 29% from WA (n=21), 25% from South Australia (n=12) and 82% from Victoria (n=11). In individual strains the percentage of dicyclanil resistant maggots ranged from 2% to 93%. The high prevalence of resistance to dicyclanil and cyromazine in NSW submissions (100%) prompted a trial which determined the % reduction in protection provided by currently marketed products against strike by highly dicyclanil resistant maggots. A 50mg/L dicyclanil spray-on product had protection reduced by 78% while the 12.5g/l and 65g/l dicyclanil based spray-on products had 73% and 69% reductions respectively. Jetting fluids with cyromazine and ivermectin as actives had protection periods reduced by 50% and 33% respectively. Against maggots which were susceptible to dicyclanil all of the products protected for the periods listed on the product labels.
The ability of dressing products to kill full grown, dicyclanil resistant and dicyclanil susceptible maggots was determined in vitro. The ranking of products from most effective to least effective are as follows:
a) Dicyclanil susceptible: Flystrike Powder > Spinosad aerosol > ivermectin jetting fluid > cyromazine jetting fluid > Diazinon > Propetamphos > Spinosad jetting fluid.
b) Dicyclanil resistant: Spinosad aerosol > Ivermectin jetting fluid > Cyromazine jetting fluid > Flystrike powder > Spinosad jetting fluid > Propetamphos > Diazinon.
If you’re worried about parasites, internal and external, in sheep, goats and cattle this spring and summer, I’m giving a free webinar at 1pm on the 20th of September, 2022 for Paraboss to launch their updated website and revamped online tools. I hope you can tune in!
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