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Flystrike and control practices: 2003 and 2011

by Lewis Kahn, Executive Officer, ParaBoss

Introduction

Following on from last month’s article about worm control practices, the blowfly control practices used by Australian sheep producers in 2011 are summarised in this article. These control practices were collected from the national survey conducted by Professor Steve Walkden-Brown and Dr Ian Reeve from the University of New England, which was supported by Australian Wool Innovation Limited and Meat and Livestock Australia.

Results

Blowfly strike remains a common problem for Australian sheep producers with the majority of strikes caused by the sheep blowfly (Lucilia cuprina). Body strike was the most common form of blowfly strike in 2011 affecting 4–5% of ewes and weaners compared with 1–1.5% in 2003, however, as body strike is very seasonal and 2011 was a wet year in many areas, the 2011 prevalence of body strike likely reflects the season rather than an overall increase. Breech strike affected about 3% of ewes and weaners in 2011 with little change since 2003.

Blowfly strike is controlled by an integrated approach that includes breeding for less susceptible sheep, reducing dags, considering breech modification, docking tails to the correct length, timing of crutching and shearing and using chemical treatments. Forty five percent of producers in 2011 were using visual selection to reduce susceptibility of rams to blowfly strike with the most common methods being to select for plain bodied sheep, low breech wrinkle, low fleece rot score and culling sheep that experienced body or breech strike. Only 10% of producers used Australian Sheep Breeding Values (ASBV) to reduce susceptibility to blowfly strike with the most commonly used traits reported as being low coefficient of variation of fibre diameter (also used for staple strength improvement), low breech wrinkle and low dag score.

Research has shown that the most important factor affecting the risk of body strike is the presence of fleece rot caused by moisture and bacterial growth at skin level. The two most important factors affecting breech strike risk are breech wrinkle and dags, although dags are only important in winter and uniform rainfall regions. Selecting sheep for low wrinkle, low fleece rot scores and low dag scores (in relevant regions), and preferably using ASBVs, will reduce the incidence of blowfly strike over time.

The number of sheep producers not undertaking breech modification may have increased since 2003 (although changes in meat versus Merino sheep numbers combined with a difference in survey questions make this unclear). In the first benchmark survey conducted in 2003, 9% of producers who ran Merino sheep indicated that they did not mules. In 2011, breech modification of ewe lambs by mulesing was undertaken by 48% of producers with a range of 28% in the New England to 61% in Western Australia. Where mulesing was conducted, 60% of producers used pain relief for the lambs. Breech modification using anti-flystrike clips was undertaken by 1.5% of producers in 2011. Contractors conducted just over half of the mulesing operations (56%) and 84% of these carried industry accreditation.

Docking the tail to the correct length at lamb marking time is crucial in minimising stain around the breech and reducing flystrike risk throughout the sheep’s life. The recommendation is to dock the tail immediately below the third palpable joint or to the tip of the vulva in ewes. In 2011, 60% of producers docked the tails of lambs at the recommended length, 27% docked tails just shorter, 19% longer and 6% butted tails (much shorter than tip of the vulva). These practices were very similar to that reported by producers in 2003.

Chemical treatments for blowfly strike control are typically conducted as a preventative strategy with routine annual treatments in 2011 used by 46% of producers and 36% applying treatments only when the risk of flystrike is high. Treating the whole mob once flystrike was detected was reported by 19% of producers and 35% only treat individually struck sheep. These values add to over 100% because producers often use more than one strategy.

Where treatments were given as a preventative strategy, dicyclanil (48% of producers), cyromazine (36%) and ivermectin (14%) were the most commonly used chemical groups. When treatments were given to individually struck sheep, the most common chemical groups that were used were spinosad (38% of producers), diazinon (23%), cyromazine (17%) and ivermectin (14%). Producers should be aware that development of resistance in sheep blowfly larvae has significantly reduced the effectiveness of organophosphate products, which include diazinon.

The FlyBoss website can help advise on the factors that make sheep susceptible to strike and the use of breeding and selection to reduce the risk. Other management options to reduce the risk of strike are described, including correct treatment and planning ahead to ensure treatment is applied at the most suitable time.

The FlyBoss Products Tool provides a detailed list of flystrike and lice treatment products currently available, including

  • commercial product name, active chemical and chemical group;
  • suitability as lice or fly treatments;
  • wool harvesting interval, export slaughter interval and meat withholding period;
  • size of pack;
  • estimated cost per pack and per dose.

The FlyBoss tools can also be used to assess the effectiveness of a particular chemical application in reducing risk within the flock, to compare management strategies and to choose the most effective timing of chemical treatments and crutching and shearing to minimise flystrike in different regions.

On a final note, 2% of sheep producers had used FlyBoss to make a change to flystrike control and another 11% had visited the site. But we have to do better because 35% had heard of FlyBoss but not visited the site and a further 51% of producers had not heard of FlyBoss. Even though we have nearly 1,000 people visit FlyBoss each month, it is clear that we need to keep spreading the word.