Strategic fly control – the best way into Spring
The seasonal nature of fly activity provides unique opportunities for strategic control that can significantly reduce fly pressure and reduce the risk of flystrike. Treating sheep early in the season with an effective and long lasting chemical prevents fly numbers from building up by removing the host environment required for the flies to reproduce. Treating sheep late in the season reduces the number of flies that can contribute to the fly population in the following season. In some ways, early and late season treatments for fly prevention provide long-term benefits in the same way that summer drenching can provide long term benefits for worm control. Both strategies exploit a weak point in the natural life cycle of the parasite and take advantage of environmental conditions at those times.
The Australian sheep blowfly Lucilia cuprina is the primary fly species responsible for initiating flystrike in sheep in Australia, and this species is largely dependent on sheep as a breeding (1) – with female flies laying eggs in moist wool subsequent to a protein meal and mating. After hatching, fly larvae then moult through three larval stages (also known as maggots) and eventually leave the sheep to pupate in the soil. In the warm conditions of a normal fly season, the blowfly lifecycle takes 2.5–3 weeks2 but can be as fast as 12 days under ideal conditions – with variation depending on exact environmental and soil temperatures. Fly numbers can therefore build up very rapidly under suitable environmental conditions, assuming susceptible sheep are available as a breeding resource.
In cooler conditions at the end of the fly season, development is inhibited or “arrested” at the prepupae stage and development is not resumed until conditions are again suitable. The increase in soil temperature in spring in south-eastern Australia has been shown to trigger synchronous emergence of the first generation of flies for the new season,3 regardless of exactly when
the larvae were deposited – as can be seen in below in Figure 1.
Lucilia cuprina therefore has the ability to adapt to the environment and changing seasonal conditions. Flies will keep developing well into the colder months if temperatures remain relatively mild – and the reemergence of flies at the start of the fly season will vary depending on the actual temperatures experienced.
Early season treatment
If chemical treatment is applied to sheep before flies emerge at the start of the fly season, sheep are essentially removed as a resource for the propagation of the fly population when the first generation appear. As emergence is relatively synchronous and the first generation is typically small, this can have a significant effect in reducing fly numbers and the risk of flystrike
for the rest of the fly season.
This strategy has been demonstrated to work very effectively on commercial sheep farms in different regions and over a number of years with varying seasonal conditions – and clearly reduces the risk of flystrike. Flies are not capable of moving long distances, so the success of early season treatment can be even further enhanced if all sheep on a farm are treated, if neighbours treat as well and/or if flocks are physically isolated.
Late season treatment
If chemical treatment is applied (or still active) on sheep in late summer and autumn, sheep are again removed as a breeding resource for the fly population. At this time, this has the effect of reducing the number of larvae that enter arrested development and directly reduces the size of the spring generation of flies in the following fly season.
Optimum fly control will also depend on farm management practices. Integrating early or late season treatment with shearing and crutching of sheep during spring, early summer and autumn will further reduce susceptibility to flystrike and contribute to reducing fly numbers.
Depending on the length of the fly season and the product used, a single treatment may provide both early and late season strategic fly control. In areas with a longer fly season, one or the other may be preferable depending on time of shearing – remembering to be conscious of wool withholding periods. In years with a late or delayed start to the fly season due to seasonal
conditions, late season treatment is even more crucial.