Recent rains have brought with them an increased risk of flystrike for sheep producers. The Australian Sheep Blowfly (Lucilia cuprina) is responsible for initiating at least 90% of all cases. This metallic green fly lays eggs that hatch into the flesh-eating maggots that begin the strike. The flystrike wound then attracts other species of fly, including the Hairy Maggot Fly (Chrysomya rufifacies).
The mature maggots (larvae) drop from the infected sheep and burrow into the soil to pupate. This development is usually halted over the winter months, as soil temperatures below 15°C are not favourable for the development into the pupae and onto the immature fly. Once soil temperatures warm up, usually mid spring, the immature fly emerges and develops into an adult to continue the life cycle.
The risk of flystrike depends on a range of factors, including the susceptibility of the sheep and environmental conditions. Ideal conditions are temperatures between 15-38°C, recent rain and wind speeds below nine kilometres per hour.
Proactive monitoring and management is the best approach to tackle flystrike. The Flystrike Risk Simulator and Optimise Treatment tools on the Flyboss website are great for helping predict fly waves and making decision on the timing of treatments and other management strategies.
Non-chemical options are important to flystrike management. A long-term solution is to breed for reduced wrinkle and dags. More short-term measures include shearing, crutching and worm control to decrease the areas of damp/urine stained wool and dag build up. Shearing and crutching can provide up to six weeks protection from breech strike, although this is reduced to three weeks if the sheep are scouring. These are best timed for just prior to the start of an expected fly wave.
There are a range of preventative chemical compounds available that vary in application methods, protection period and withholding periods. The use of such chemical compounds prior to, or at the first sight of blowflies, also has the potential to decrease fly pressure later in the year by breaking their lifecycle. When considering which item to use, consider how long you want coverage for, the timing of the application and the withholding period (WHP) or export slaughter interval (ESI) on products.
Preventative treatments can be applied routinely or opportunistically. Routine treatment involves timing the application so that the protection period best matches the highest risk time for flies. This should be considered once non-chemical management, such as shearing or crutching, has been explored. If choosing to monitor the flock, the threshold for applying a preventative treatment is usually once 0.5% are struck within one week.
Recent collaborative research by AWI and the NSW Department of Primary Industries has found widespread resistance to cyromazine and dicyclanil, although the extent varied greatly between states. From the 12 samples tested in SA, 58% were resistant to cyromazine and 25% resistant to both cyromazine and dicyclanil. These samples were not taken at random, so it is unclear how well they represent the prevalence of resistance for the state. Adult flies do not normally travel more than three kilometres from where they hatch, meaning that resistance to chemical can vary significantly from property to property.
Rather than just complete loss of use of a product, resistance can result in shorter periods of efficacy for preventative products.
To reduce the progression of resistance, it is important that chemicals are applied according to the label instructions, including the correct dose rate and complete application. Be sure to kill all maggots when treating a struck sheep.
Reducing the reliance on chemicals through some of the other strategies mentioned above is also important. Know your chemical groups, not just active ingredient names, and rotate where possible. Keep in mind that some products that kill other parasites such as lice may also work on flies, so these need the be considered when planning rotations.
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