When a carcass is seen teeming with blowfly maggots, it seems logical that poisoning them should decrease flystrike risk. However, this is rarely the case.
Lucilia cuprina, the common Australian sheep blowfly, has evolved differently to the other common blowflies. It is a primary strike fly and in Australia it initiates over 90% of the strikes on live sheep. However, there are times in some regions when the Calliphora species can be a significant primary strike fly.
Once a strike is in progress, the odours attract secondary blowflies; the most common are the brown blowflies (various Calliphora species) and the green hairy maggot blowfly (Chrysomya rufifacies).
Carcasses are dominated by the maggots of secondary strike flies and there is considerable competition between maggots for available food. The secondary flies play a valuable role by preying on the maggots of Lucilia cuprina, and because secondary strike flies rarely initiate a strike, their presence is actually helpful. Also, the heat generated in the carcass will kill Lucilia.
Lucilia have evolved to overcome this competition by striking the sheep while it is alive. Once the sheep dies, generally the only Lucilia maggots that survive are those that were already well-developed and crawled off the carcass relatively early.
Killing flies or maggots is rarely effective at reducing flystrike because the size of the fly population is usually not the limiting factor. Generally, it is the number of susceptible sheep that limits the amount of strike.
In warmer areas with sufficient rainfall and no seasonal break, maggots can burrow into soil, pupate and emerge as flies all year round, maintaining a population ready to strike susceptible sheep. Because part of the fly life cycle involves larvae pupating in the ground, burying carcasses may actually help Lucilia.
There are two specific situations where flies are naturally at the low population levels needed for them to be the limiting factor for flystrike (i.e. sheep can be susceptible, but there are too few flies for strike to occur). The first is in areas cold enough for flies to be inactive for about two months during winter, and as the temperature increases during spring, there may still be insufficient numbers of flies to strike susceptible sheep. The second situation is in very hot, dry areas.
Actively trapping adult flies in these two situations can maintain a lower fly population for longer than would naturally occur and can reduce flystrike. However, large numbers of traps are required and trapping must start before the population increase can get a start each year.
When strategically used after winter, trapping is useful in conjunction with the ‘early season flystrike prevention strategy’.
In hot, dry areas where sheep only become susceptible for a few weeks (such as following heavy rainfall), the traps may keep the population suppressed long enough to get through that period.
However, the economics of using traps or bait bins to keep the fly population suppressed must be carefully considered due to the large number of traps and amount of labour required to service them. At least 1 trap/100 sheep is required and the traps should generally be moved with the sheep. In the hot, dry areas, traps could be strategically placed around the homestead and along watercourses, where Lucilia are more likely to be found.
In areas where sheep become susceptible for lengthy and more frequent periods, such as high summer rainfall areas, trapping cannot keep the fly population suppressed to low enough levels to significantly reduce flystrike.
Covert strikes (small strikes that are seen only with very close inspection) on untreated sheep can also contribute enough Lucilia maggots to build the population over spring and summer.
In the past, open home-made bait bins were used to trap and kill blowflies. With the understanding that secondary flies were useful, bins were improved by leaving only a small opening covered by mesh that allowed Lucilia to enter, but excluded the secondary flies. In due course, a commercial fly-trap specific for Lucilia cuprina was produced: the LuciTrap (at late 2016 this is currently not in commercial production, but may be coming back).
Smaller numbers of traps are very useful as a means of monitoring the fly population as an indicator for when preventative treatments should be applied.
It is useful to kill maggots on any struck sheep, particularly if the sheep has been previously treated and these fly larvae could be resistant to the chemical used.
Collect any shorn maggot-infested wool into a plastic bag and leave this in the sun for some hours so that the maggots die.
Sheep that are struck should have a knockdown treatment applied to the strike area to kill any developing maggots before they can drop off the sheep and burrow into the soil.
In summary, poisoning carcasses is not generally helpful in reducing the risk of flystrike. However, collecting and killing the maggots from struck sheep is useful, particularly if a flystrike prevention or treatment product had already been applied to the sheep.
Trapping adult flies can reduce flystrike, but generally only in specific situations and it can be quite costly and labour-intensive.
Trapping (with relatively few traps) is a useful means to monitor the fly population as an indicator for when to apply flystrike prevention measures.