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Is it better to treat for flystrike on the same date every year, or to wait and see?

Figure 1. Output from the FlyBoss Optimise Treatment Tool
Figure 1. Output from the FlyBoss Optimise Treatment Tool

By Brian Horton, Senior Animal Scientist, University of Tasmania

August 2015


Is it better to treat for flystrike on the same date every year, or to wait and see? There is no best answer, but there are a few rules you can use.

The FlyBoss Tools are intended for long term planning of when to apply preventative treatments and other flystrike management options, not daily decisions. In general, planning ahead is a reliable option, but adjusting the date with the season can reduce costs by 2% if you get it right.

Selecting the right date is critical and depends on how long you expect the fly season to last in your area, when you plan to shear/crutch, and the length of protection provided by the product you plan to use (commonly 3 or 5 months).

If the protective period is about as long as the fly season then treatment
on a fixed date before any strike is expected is generally better.


Fixed date treatment also usually applies if the sheep are going to be shorn during the fly season. This requires careful planning because shearing and crutching reduce the risk of flystrike for a while, but they also remove any chemical treatment applied previously. Thus you must be ready to re-apply protective treatment a suitable time after shearing or crutching. Treatment immediately after shearing or crutching may be easier to organise because the sheep are available, but the protection may not last as long because there is less wool to take up the chemical. The FlyBoss Tool can help plan the best treatment dates, depending on your choice of shearing and crutching dates.

If the fly season is normally longer than the period of protection for the product used then adjusting the date of treatment according to the current season may make it easier to delay treatment long enough to provide protection over the full fly period. In the years when you do need to treat early, you will need to be prepared for flystrike at the end of the season, when the product is losing its effect. It may require planning for more flock checking and individual strike management, or treating them all a second time in advance of the expected strike.

If flies are not a major problem and some or all of your sheep do not require treatment in most years, then, on average, you will save money by waiting to see whether or not you are having a bad fly season. If it is not a bad year then you may get through without preventive treatment, (saving time and money,) provided the number of occasional struck sheep requiring treatment is not excessive.

If flies are usually a problem and you need to treat in most years
then it is generally better to treat at a planned time each year.


Figure 2 (below) shows a typical example of the difference in cost between treating on a fixed date or treating when the level of strike gets to a predetermined level. If treatment is applied when only a very small number have been struck then it will often be done too early. In that case costs may be unnecessarily high because the product loses effect before the end of the fly season, resulting in more sheep struck or the need for an extra treatment. If treatment is delayed until a large number of sheep have been struck then costs are high because many sheep are struck before preventive treatment is applied. If the ‘wait and see’ decision point is just right, then costs are slightly lower than treating on a fixed date. However, the optimum point to treat and the benefits (if any) of using the optimum time will vary depending on the flystrike risk on each property and the date when the sheep are shorn.

Figure 2. Mulesed ewes at Gunning, NSW, in an average flock with a spring shearing on 1 October showing the average result of each management strategy over 20 years. The dashed (blue) line shows the total costs associated with flystrike and its treatment if treatment is always applied on a fixed date, optimised using FlyBoss. The solid (red) line shows the flystrike costs if treatment is applied each year when the indicated number of sheep have been struck in a week. If no treatment was applied the costs due to struck sheep would average $6800/1000 sheep (off the chart).

In general, if about 5–10 sheep in a mob of 1000 are struck in a week, then preventive treatment is probably needed, but this depends on your ability to monitor the sheep, find and treat sheep that have got struck and the time required to treat the entire mob.

A disadvantage of using a variable date is that it cannot be planned for and must take priority over other farm work when strike begins. In general, those who use this method successfully are very experienced at picking when the weather is ideal for strike to start and getting in just ahead of the flies. Those without this experience are probably better to stick with a planned date. Also, the possible benefit (e.g. a 2% gain is $50 off $2500) is associated with a higher risk of getting the timing wrong.

If you do use a fixed date, there is still the possibility that
an unusually bad fly season will start early. 


If this happens you need to treat immediately or risk having a lot of individual strikes to treat, which may take up even more of your time.

Some wool producers treat before any strike occurs to prevent the build-up of fly numbers during spring. This method may be useful provided that all sheep on the property are treated, since it relies on keeping the fly population very low by not having any sheep that can be struck for several months after the flies emerge in spring. On smaller properties, flies coming in from the neighbours can ruin this strategy.

If there are sheep at risk of strike that have not had preventive treatment applied then you need to monitor them closely. If you don’t pick up the few struck sheep very quickly, then a severe fly wave could develop with serious losses as well as animal welfare issues. So don’t continue to wait before treating if close monitoring is not possible.

In summary, if you apply a preventative fly treatment in more than half of the years, then use the FlyBoss Tools to optimise your treatment date, but monitor before that date in case of an earlier than normal fly season. If, on the other hand, you generally treat in less than half of the years, treat when about 5–10 sheep in a mob of 1000 are struck in a week—or sooner if labour to monitor and treat is limited. Checking should be frequent enough so that strikes do not become larger than 5–10 cm in diameter. In flystrike-risk periods this requires mobs to be closely checked each 3 days.