Gus's Musings

March 15, 2021

Flies-They get on my nerves.

“Drown the buggars in Rapid Shave” (Paul Hogan)

NB: The above treatment is not recommended by anyone!!



Gee the flies have been persistent and as a merino sheep producer, the poor sheep have had to fend off the black mongrels fairly constantly, especially here through Oct-Dec, for some though the battle continues.  I have put together the below article after discussion with an Elanco Rep, Andrew Brewer, based the Central West of NSW and I will include his expert commentary in Red.

I would like to buy into this discussion because of some of the feedback I receive from producers that “the flies are resistant to most of the chemicals”, or “due to the shearer shortage, I’m unable to keep the flies out of my sheep”.  While there is truth in both of these statements, there are always options and I would just like to highlight some.  With this sort of discussion we are possibly jeopardizing the main chemical that we have for protection of blowflies, Clik Extra (65g/L Dicyclanil).  I am unaware of the development of other chemicals for the protection of sheep, so the correct use and application of all our current chemical groups is very important.

There are many factors that all contribute to the relative susceptibility of sheep to flies, so in order to reduce your exposure to the risk here are a few areas for you to think about:

  • Breed for resistance to flystrike – cull any sheep fly blown, cull any ewes/rams with suint in their wool, or yellow wool. Check with your stud to make sure they cull for flystrike as well.  Aim to reduce dependence on insecticides by making sure that you have the best physical barriers in place (White wool, large bare area around the breech, low wrinkles score on breech).  This season there are distinct lines of yellow, coloured wool that look like rings on a tree stump. You can accurately determine when significant rain events occurred by counting the crimp from the follicle and it is easy to see. Wool growers need to consider classing for better (more resistant) shoulder conformation. I regularly see ‘M’ shaped shoulders in sheep where as the “American barn” or inverted ‘V’ is the more desired shape. Ensure that you take shoulder conformation into consideration when classing sheep. This means you need to get your hands on every animal.
  • Shear or crutch at times that maximise protection against flystrike. – This may differ for each region, look at crutching options, build relationship with contractor/s, purchase crutching/handling facility.
  1. This is a really contentious issue – if choosing to cease mulesing then additional crutching may be required, almost certainly in a year such as we have seen in 2020/2021 summer. 
  2. Time shearing to minimise fly strike risk. Also, don’t assume that 6 month shearing eliminates risk. For example, July/Feb shearing means that 5 months of wool is in place at the peak of the season in a wet summer.
  3. Understand the wool harvesting intervals (WHI) and other withholding times and select appropriate chemical treatments to suit your calendar.


  • Dock tails to the correct length. – Really important, too shorter tail really makes sheep susceptible to flies as they are unable to lift wool out of the way.
  • Manage scouring. – egg counts, pasture management. Changes to diet, seasonal changes that cause digestive upsets can be vectors for strike as can urine stain, lambing stain and dags that build up over fly strike prevention chemicals. These can negate the protection effect as the chemical is essentially covered over.
  • Use breech modification if required, until sheep are genetically resistant to flystrike. Not all mulesing contractors are the same. Speak to your marking and mulesing contractors and be firm on what you want – tail length, size of mules etc.  A large mules is not necessarily the best option.
  • Check livestock regularly, “close shepherd”, that way you can get on top of issues quickly. A sheep can be in real trouble within 48 hours in the worst conditions.
  • When applying insecticides, (notethat dicyclanil [e.g., CLiK] and cyromazine [e.g., Vetrazin] are insect growth regulators [IGR] and do not have any activity on adult flies.  Whereas the other with known efficacy, imidacloprid [Avenge+ Fly], ivermectin [Blowfly and Lice] and spinosyn [Extinosad Eliminator and Extinosad Aerosol] do affect adults and larvae) make sure that you follow instructions and use well maintained equipment that is operating at the correct pressure and/or dose rate.  A quick phone call to the company rep can help you make sure that you are applying the chemical correctly.  Remember the most expensive chemical is the one that doesn’t work.  Make sure the chemical is applied carefully and if any resistance is detected, please contact your supplier.


  1. Resistance is difficult to detect and to identify. Wool growers must give chemicals the best opportunity to work. No chemical should be considered a single pronged approach to fly strike prevention. Our over reliance of the chemical treatments has seen almost every chemical enter the system and tumble out the back door due to either over use, underdosing, incorrect application and/or over reliance which cause flies to become resistant. Fly strike management requires an integrated approach. Any dag, stain or wool in the breach area must be addressed. You are not necessarily treating for the now, you are also treating for the future and if you expect the longest protection, you must prepare the sheep accordingly.
  2. One very important thing to be aware of is that there are no new chemicals in the pipeline for some time into the future. If the current chemicals can no longer be used, there is nothing else coming over the hill to save us. What we have is all we have.


  • Watch the weather and be aware of the impact of multiple rain events in the warmer months, this will reduce the effectiveness of chemicals.  ​Know the rain fast claim on the chemical you have used or intend to use. Rain fastness is a difficult thing to accurately measure but best practice indicates the following:
  1. You should never apply fly strike prevention chemicals (or any pour-on or spray-on chemicals) to wet sheep.
  2. You avoid applying if rain is imminent.
  3. You should consider treating when rain has passed and when the sheep have dried.
  4. You should consider re-treatment with an alternative chemical active when you suspect there are ‘washout’ concerns. 
  5. Avoid treating for fly strike prevention twice with the same chemical in the same wool growing season.
  6. Avoid using the same chemical off-shears for lice then again as a fly prevention strategy. This exposes two pests to the same chemical with different application patterns and volumes which may be a risk to resistance building up.
  • There are fly traps that can assist in reducing the green fly (Lucilia cuprina) on your property. Even if they don’t reduce numbers, when you start seeing green flies in them, it is time to act!  Fly traps can significantly reduce fly populations. One sheep blowfly can lay up to 900 eggs and one fly can kill a sheep.
  1. In general, you own your blowflies on your property. Blowlies only travel up to about 3 km (on average but can be blow on winds a bit further) from where they pupated and they can overwinter in the pupation state to re-emerge when conditions suit. Those paddocks that were bad last year may also be the ones that are bad next year especially with flies around in autumn that pupate in early winter.
  2. This season, dermatitis has been a significant issue for producers and a contributing factor for fly strike. This is particularly the case as young sheep are more easily wet to the skin than adult sheep especially if they are unshorn and/or unclassed. Dermatitis can become a real issue that builds quickly when sheep that have it are yarded wet and hence the bacterial infection is spread rapidly. Consider shearing lambs as early as possible if dermatitis is suspected. Dermatitis trumps chemicals and can render them less effective in the infected area which is commonly the shoulders.



When you run into difficult years like this, with lots of moisture and extended fly periods, not “normal/average” years, you will need different management.  In order to make good decisions you will need good information and the best way I reckon to do this is to access your networks, there are many people/businesses out there ready to assist you to make good decisions, here are a few:

  • AWI – click on the link and search for articles in Beyond the Bale. They also have many valuable resources about classing sheep, fly control and worm resistance
  • Go to ParaBoss and access Lice, Fly and Worm Control resources
  • Fly Boss – click on the link, put in you circumstances and look at options.
  • Local Merchandise store- Speak to your supplier about options.
  • Company Reps – Talk to Coopers, Elanco reps, find out what they recommend.
  • Social Media – Ask questions to find out what other producers are using, I find Twitter is very good for this.
  • Other Landholders – Contact mates and find out their thoughts, most are very comfortable sharing.

Flies cause huge problems in the wool and sheep industry every year and especially when most of us are recovering from a few drought years in a row, this year every dollar is critical.


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