Early season flystrike prevention

Jetting sheep with a Dutjet wand. Source: Peter James.
Jetting sheep with a Dutjet wand. Source: Peter James.

Early 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.

When is it warranted and successful?

1. This strategy is only warranted on flocks in high rainfall areas that already need to treat most mobs or the entire flock for flystrike in most years. If this strategy is not suitable for you view other methods for choosing a treatment time.

2. Winter daily maximum air temperatures must drop below 16°C for most days in an 8-week period to provide a complete break in the fly season. In these circumstances, no adult flies are present during winter, but the parasite survives to the following spring by overwintering as ‘arrested’ larvae in the soil.

3. Proximity to neighbouring properties where sheep could be/have been flystruck could compromise effectiveness of the strategy. While Lucilia cuprina can travel up to ten kilometres or more, flies coming from within a few kilometres pose the most threat.

4. All sheep on the property must be treated. This may preclude the strategy from properties where some sheep, such as lambing ewes, cannot be mustered and treated when required. ‘Covert’ strikes (ones that cannot be detected easily on general inspection) in untreated sheep are not uncommon, and they are an important ongoing source of maggots that build up the fly numbers through a season.

5. The method may not be cost-effective if shearing or crutching is carried out in the first 3 months of the fly season because any protective chemical will be removed and a new protective treatment must be applied immediately after shearing, or crutching.

What does it involve?

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 optimize 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


Withholding Period (meat)


Export Slaughter Interval


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

*Actual length of protection on your property may be shorter and could be influenced by how well you apply the chemical, wool length at application, rainfall amount and intensity during the protection period, and the amount of urine stain, dag and fleece rot where the chemical is applied.

Use the FlyBoss Products tool to search for brands of these long wool flystrike preventative treatments.

What issues must be considered with this strategy?

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.