Managing flystrike

By District Vet, Kelly Wood

Sheep producers will already be aware of the increased risk of flystrike during wetter conditions. Rainfall received so far this season has led to a rise in reports of flystrike across the Central West region. Of concern is the increase in blowfly resistance to insecticides.

What to do this Summer

Given the small number of available compounds for preventing and treating flystrike, it is important that producers strive to slow the development of resistance. This should include an integrated fly management plan that identifies, and treats strikes as soon as possible, manages the risk of strike though non-chemical strategies, and uses insecticides sparingly and judiciously.

1. Increase surveillance of high-risk mobs

Integrated flystrike management hinges on accurately identifying early strikes. This means targeted surveillance of high-risk mobs. Sheep should be checked for strike at least every two to three days but increased surveillance of at-risk animals will deliver greater returns for effort.

Flystrike risk is not uniform across sheep and risk can vary with a number of factors:

  • Sheep in 4-6 months of wool have twice the risk of fleece rot and therefore body strike as animals with either more, or less wool;
  • Shedding breeds are susceptible to body strike and persistence of insecticides on these breeds may be reduced due to the decreased lanolin in their wool;
  • Animals less than 1 year old are at an increased risk of fleece rot and therefore body strike, especially if shorn as lambs;
  • Sheep that have been exposed to climate events that result in skin wetting for more than two days are at an increased risk of body strike;
  • Animals with open wounds are at an increased risk of strike. This can be poll strike in fighting rams, strike around the feet for animals with foot abscess and strawberry foot rot and recent mulesing or marking wounds;
  • Animals with short tails (less than 3 palpable joints) are at an increased risk of dag and breech strike; and,
  • Sheep that are scouring are at in increased risk of breech strike. An integrated worm management plan, as well as identification and treatment of other causes of diarrhoea, is vital to the success of fly risk mitigation.

2. Effectively treat struck animals

The first strikes following application of a chemical will be hard to identify but will likely contain a high percentage of resistant maggots. Any pulled or discoloured wool should be examined for strikes. Once identified, the struck area should be:

  • Clipped with wide margins (5cm minimum) and tracks investigated;
  • Treated with an appropriate dressing product; and
  • The maggots and wool placed in a black garbage bag in the sun to ensure they are killed.

The efficacy of dressing products to kill full grown dicyclanil-resistant and dicyclanil-susceptible maggots was investigated as part of the Sheep Blowfly Resistance Update. Sales and her colleagues found that in a laboratory setting, the ranking of products from most effective to least effective were:

a) Dicyclanil susceptible maggots: Diazinon Powder > Spinosad aerosol > ivermectin jetting fluid > cyromazine jetting fluid > Diazinon spray > Propetamphos > Spinosad jetting fluid.

b) Dicyclanil resistant maggots: Spinosad aerosol > Ivermectin jetting fluid > Cyromazine jetting fluid > Diazinon powder > Spinosad jetting fluid > Propetamphos > Diazinon spray.

3.  Cull struck animals from the mob

Strike is highly repeatable. This means that a small number of sheep disproportionately contribute to the number of strikes each year. It is recommended that farmers consider culling for strike as part of a wider genetic selection strategy.

4. Prioritise paddocks

Low risk sheep (i.e. mature animals that have been shorn or crutched in the last 6 weeks, or have been recently treated with an effective preventative) should be prioritised into high risk paddocks (e.g. paddocks with long, wet grass) and vice versa.

5. Make the most of shearing/crutching

Shearing or crutching provides up to 6 weeks protection for strike. If practical, delay the application of an insecticide until this protection starts to wane to get the most “bang for your buck”. Depending on your enterprise and season, an additional crutching may be economically beneficial.

6. Time insecticide preventatives

Careful planning of the timing of chemical application is key to slowing the development of resistance. As the concentration of chemical tails off over time, resistant maggots will start to survive on sheep and, if left untreated, will contribute to the resistant fly population on the property. Increased surveillance to identify strikes during this “tail” period is therefore critical. The timing of the “tail” for chemicals on your property will depend not only on any existing fly resistance but will also vary with:

  • Weather conditions as excessive wetting can reduce the longevity of the product; and,
  • Success of chemical application. Producers should always apply according to label directions as poor application can expose maggots to sub-optimal chemical concentrations.

Applying an insecticide so that the sub-effective “tail” falls in a period when maggots are unlikely to survive anyway (e.g. when temperatures have dropped to a point that inhibits fly activity) will slow the development of resistant. Alternatively, producers can apply a different chemical class as a “tail-cutter” to kill maggots that are exposed to sub-effective concentrations of chemical.

7. Rotate chemical classes

Producers shouldnot reapply the same chemical class within a single wool growing cycle. This includes fly prevention products, strike dressings and lice treatments. It should be noted that as dicyclanil and cyromazine belong to the same chemical class, there are high levels of cross resistance to these two insecticides in NSW fly strains.

8. Develop a long-term strategic fly management plan.

FlyBoss has recently released the Flystrike Risk Simulator that allows producers to model the risk of strike on their property, specific to their classes of stock, management calendar and geographic location. The simulator is free to download through the FlyBoss website at http://www.flyboss.com.au/sheep-goats/tools/flystrike-risk-simulator-downloadable.php.

There are also some good resources for developing a genetic strategy to reduce susceptibility to flystrike without going backwards in production traits. This is discussed in the Beyond the Bale article “Low wrinkle – high fleece weight” from June 2019 (https://www.wool.com/globalassets/wool/sheep/research-publications/welfare/flystrike-research-update/btb-june2019-low-wrinkle-high-fleece-weight-p36-37.pdf ) and a Sheep Connect webinar (https://www.sheepconnectnsw.com.au/events/741/

The future of flies

While the idea of widespread blowfly resistance is a little bleak, there are a number of potential new avenues for fly control currently being investigated as part of the AWI 2020 Flystrike Prevention RD&E Program:

  • A new class of chemicals (HDACs) have successfully undergone small-scale clinical trials that found they were effective at concentrations similar to those in currently registered products (AWI PROJECT NO: ON-00454).
  • Controlled-release nanotechnology may enable chemical classes previously not suitable for fly or lice control to withstand the environmental challenges of farm environments (AWI PROJECT NO: ON-00549).
  • A blowfly vaccine is currently undergoing clinical trials in sheep (AWI PROJECT NO: ON-00619)
  • Fly population control through biopesticides and genetic manipulation of the fly genome are being investigated (AWI PROJECT NO: ON-00620, AWI PROJECT NOs: ON-00373 & ON-00516)

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