Investigating the role of blow fly olfaction in flystrike in sheep

Breech flystrike is a painful, debilitating and potentially lethal disease caused by the larvae of the blowfly, Lucilia cuprina, and, despite many years of research, it remains a serious financial and animal-welfare issue for the Merino sheep industry in Australia. The common methods of prevention, namely insecticides, crutching and ‘mulesing’, are problematical, so alternative approaches are needed. Breeding for resistance to breech strike is a fundamentally attractive proposition, but the trait itself is difficult and expensive to quantify in large numbers of sheep in extensive production systems. Several indirect traits are correlated with susceptibility to flystrike, but a large proportion of the variation in susceptibility remains unexplained. The common thread through those indirect traits is odour, so we turned to the biology of insect olfaction and its role in fly–sheep interactions. L. cuprina uses odours to detect and locate potential hosts over long distances, to guide orientation and landing behaviour, and to select egg-laying sites. Preliminary studies demonstrated the importance of confining our work to gravid female L. cuprina, and also validated the use of flies reared in the laboratory for experimentation. Using laboratory-reared flies and a combination of gas chromatography–mass spectrometry with electroantennographic detection, we identified odouriferous compounds from sheep that the antenna of L. cuprina can detect. To determine whether the identified compounds were attractive or repulsive, we needed to use a behaviour test. In preliminary studies, we compared four behaviour bioassays (Y-tube, landing time, visiting frequency, and trap) and found the trap to be the most effective. We observed that L. cuprina was attracted by several compounds in Merino wool, including octanal, nonanal and dimethyl trisulfide. We also found that the wool levels of octanal and nonanal are heritable in Merino sheep, suggesting that these compounds might be useful as traits in selection for flystrike resistance. Another possibility is that these olfactory-active compounds might guide efforts to modify the genome of sheep, or perhaps even L. cuprina. Success in these endeavours could save as much as A$200m per year for the Australian Merino-based industries, while also improving the image of wool in world markets.