Fly Management in Sheep
Sheep flystrike or myiasis is predominantly caused by the Australian Sheep Blowfly, Lucilia cuprina. It is more common in the warmer months from late Spring to early Autumn. Strike occurs when blowflies lay their eggs on moist or faeces stained wool (dags) or wrinkles on sheep. Larvae emerge from eggs and damage skin tissue. Eventually they can burrow into deeper tissues. This results in painful, bleeding wounds, secondary infection and sometimes death. The most common types of strike are body and breech strike but other parts of the body can be affected. The life cycle can be as short as 2-3 weeks under ideal conditions. Adult females lay eggs on susceptible sheep and maggots emerge to feed and burrow under the skin for several days. The first stage maggots cause irritation but the serious damage is caused by the second and third stage maggots. They then drop off onto the soil and form pupae which are protected in a hard case. When temperatures are warm enough new flies emerge. Larvae can overwinter in the soil when temperatures are too low for development and emerge the following season. Flies do not travel more than 1-2 kms so there is no such thing as a true “fly wave” however the environmental climatic conditions suitable for fly emergence gradually move allowing flies to develop in different regions progressively giving the impression of a fly wave. Fly strike is more likely to occur when climatic conditions are optimal.
Suitable conditions include temperatures between 15 and 38°C, recent rain (which predisposes to wet sheep, scouring, foot problems), winds less than 9kph and the presence of susceptible sheep.
Merino sheep have more wrinkle and more susceptible to strike. Preventing the devastating impact of flystrike in sheep requires attention to multiple factors. Firstly, certain lines of sheep are more prone to flystrike because of their conformation, degree of wrinkliness and/or tendency to have daggy breeches. Breeding for plainer bodied sheep reduces susceptibility as does selection for sheep with lower dag scores and better conformation. Sheep in short wool and/or are crutched are less prone to strike and correct tail docking is important in minimising attractiveness of the breech are to flies. Internal parasite control also reduces faecal staining of the breech area.
Mulesing, where necessary, is a valuable procedure because it surgically removes wrinkled skin around the breech and tightens the skin in this area, reducing moisture and soiling. Whenever lambs are mulesed, pain relief should be employed to minimize the animal welfare impacts of the procedure.
Finally, there are a range of chemical treatment options and application methods which form part of an integrated flystrike control strategy.
Available chemicals for flystrike prevention fall into different chemical groups. Insect Growth Regulators (IGRs), Macrocyclic Lactones (MLs), Synthetic Pyrethroids (SPs), Spinosyns, and most recently Neonicotinoids such as Avenge®. Organophosphates (OPs) have been widely used for flystrike treatment and prevention in the past however OP resistant blowfly have become common. Newer chemicals are highly effective and have fewer human and animal health safety risks. There are also different options in delivery methods including backliner, spray on or jetting fluid. Treatments can be applied at different times depending on the presence of flystrike potential. Some chemicals can be applied in short wool (even off shears) or in long wool.
Avenge® (imidacloprid) is approved for flystrike control and provides up to 10 weeks protection against body and breech strike when applied in short wool (up to seven days off shears) or up to 14 weeks protection when applied in long wool (6 weeks to 8 months).
When to treat for flystrike prevention
It is important to be vigilant when climatic conditions are suitable for flystrike. Regular monitoring of the flock for struck sheep and the use of fly traps to monitor fly activity and numbers are all valuable. By the time obviously struck sheep are detected fly numbers have already increased. Ideally, with a knowledge of climate and other factors predisposing to flystrike, preventive treatments should be applied in anticipation of this occurring.
Apart from organophosphate compounds (OPs), there has not been resistance detected in sheep blowflies to the other chemical groups. Nonetheless, to avoid resistance developing it is important to use all chemicals according to recommendation, do not under dose and avoid over reliance on chemicals from a single chemical class. Use of chemicals with a shorter period of protection at times of the year when fly pressure is declining may give the other chemical groups a rest and slow down selection for resistance.