Subscribe to the Boss Bulletin
Subscribe the the Boss Bulletin for monthly updates and articles about all things parasite management
Subscribe hereSubscribe now to receive email updates from one or more of our ParaBoss suite of websites, ParaBoss, FlyBoss, LiceBoss and WormBoss.
SubscribeEarly season flystrike prevention is an underutilised strategy that can decrease both the incidence and the overall cost of flystrike, as well as improve sheep welfare. It may also use less chemical throughout a fly season.
The strategy involves applying a long-acting flystrike preventative to the whole flock before any fly activity in spring, followed where possible, with shearing or crutching when the chemical protection period is ending.
This prevents a fly population from building up after a winter dormancy, so that strike is ultimately prevented later in the season by a lack of flies, rather than by making sheep less susceptible or applying another chemical treatment.
Early season flystrike prevention involves application of a flystrike preventative chemical to all sheep before any flies have emerged after winter, to prevent any successful breeding of flies from spring to mid-summer. Treatment must be applied to both the body and the breech.
The treatment, which should contain either dicyclanil, cyromazine or ivermectin, is generally applied in August to October, before flies emerge on that property. Fly-trapping or previous experience can be used to plan when treatment should be applied. If the FlyBoss Optimise Treatment tool is used, then treatment must be applied several weeks before the tool suggests that any flystrike is expected.
If shearing is typically done during summer, if possible, time it to occur just before the residual chemical runs out, to extend the period when strike cannot occur; shorn sheep are rarely struck in the first 6–8 weeks after shearing. Alternatively, crutching at this time (particularly if breech strike or dags are the major problem) provides longer protection.
Choose the chemical that will best match the time of protection required from the treatment time until shearing/crutching, or if shearing/crutching does not occur at the end of the chemical protection period, ideally choose the longest residual treatment, but adhere to the withholding periods. See Table 1 for chemical protection periods.
If shearing is typically done before the start of the fly season, then preventative treatment should still be used before any flies are likely to be active. To optimise the length of protection achieved from the chemical (as they have a shorter protection period when applied to shorter wool), apply the chemical as late as possible after shearing, but it must be before there is any possible fly activity.
By preventing successful breeding of any flies for about 3 months after the end of overwintering, the population does not build up rapidly. Later in the season, when the sheep are more susceptible and no chemical protection is in place, there may not be enough flies to cause flystrike.
Table 1. Maximum protection periods for recommended chemical actives
Chemical active | Application method | Maximum protection period against flystrike* | Wool Harvest Interval WHI | Withholding Period (meat) WHP | Export Slaughter Interval ESI |
Cyromazine 60g/l | Spray on | 77 days (11 weeks) | 60 days | 7 days | 28 days |
Cyromazine 500g/l | Jet or dip | 98 days (14 weeks) | 60 days | 7 days | 21 days |
Dicyclanil 50 g/l | Spray on | 336 days (24 weeks) | 90 days | 28 days | 120 days |
Dicyclanil 12.5 g/l | Spray on | 77 days (11 weeks) | 30 days | 7 days | 21 days |
Ivermectin 16 g/l | Jet or dip | 84 days (12 weeks) | 42 days | 7 days | 7 days |
Use the FlyBoss Products tool to search for brands of these long wool flystrike preventative treatments.
Treatments must be thorough and no sheep should be missed, as an occasional unprotected sheep being struck may provide enough new flies for this strategy to fail.
Treatment times must take into account the relevant withholding periods, in particular, the Wool Harvest Interval, if shearing is to occur near the end of the protection period.
From about 6 weeks post-shearing, sheep will start to become more susceptible to flystrike, but with a very low population of flies the risk of strike is reduced. Nevertheless, monitoring for fly activity later in the season is still essential. In particular, mobs nearer to neighbouring sheep properties may be exposed to a higher fly population, as flies can travel a few kilometres.
Monitoring for fly activity later in the season is still essential
The risk of late-season strike will be lowest where the protection period achieved from chemical (and shearing) has been longest and in locations with the shortest fly seasons because this will limit the time when the fly population can build up after protection runs out.
As with any application of flystrike preventative chemical, there is the potential for flies to develop resistance to the chemical used. The risk may be higher than with typical treatment strategies because the blowfly population will be heavily suppressed and larvae that do survive the treatment will not be ‘diluted’ among a larger population of flies.
Conversely, the intentional shearing and removal of the chemical product just as it is starting to lose effect means that flies with some resistance genes are not exposed to a low-level tail of product to which they might have survived. It is very important to kill any fly eggs and larvae if struck sheep are found, as these may be resistant to the chemical. Shear the affected area and collect all maggots and struck wool into a plastic bag and leave it in the sun all day.
Subscribe the the Boss Bulletin for monthly updates and articles about all things parasite management
Subscribe herewww.wecqa.com.au is a secondary ParaBoss website hosted by the University of New England (UNE). Whilst this is still an official ParaBoss website, UNE is solely responsible for the website’s branding, content, offerings, and level of security. Please refer to the website’s posted Privacy Policy and Terms of Use.