How important is odour in attracting blowflies?
Author correspondence: johan.greeff@dpird.wa.gov.au
This article describes the different ways the role of odour in attracting blowflies is being investigated.
Introduction
The Breech strike project was initiated in 2006 with the establishment of the research flock on the Mt Barker research station in Western Australia and supported by Australian Wool Innovation. Six hundred Merino ewes were sourced from 10 industry and three research station flocks from DPIRD.
It has been demonstrated during the project that breech strike is a repeatable trait and any animals that has been struck should be culled as these animals are more likely to be struck again. This factor together with the large differences that exist between sire progeny groups in breech strike susceptibility, and the fact that we could not visually differentiate between susceptible and resistant sheep, indicated that the susceptible animals are likely to have an additional factor(s) that attracts blowfly strike.
Thus the focus of the experimentation shifted towards identifying these elusive factors, the most obvious being odour. A proof of concept experiment was previously carried out with Hanrob Dog Academy where three dogs were trained to determine whether they would be able to distinguish between wool from resistant and susceptible sheep (see WA Breech Strike Newsletter No. 6). Wool from unstruck sheep taken prior to the breech strike season was collected from the breech of the extreme ewes mentioned above in the repeatability study (published in the September 2018 edition of the Ovine Observer), and regularly forwarded to Hanrob Dog Academy to train the dogs.
Identifying wool from struck sheep using dogs
After completing their training, one dog was 100% accurate in differentiating between the wool from susceptible and resistant sheep that was used to train him. In a double blind test (neither the handler nor the dogs knew what the samples were), the dogs were then tested on wool from the CSIRO breech strike flock in Armidale to which they have never been exposed.
The best dog was 82% accurate in identifying the resistant sheep and 92% accurate in ignoring the susceptible samples and other dummy wool samples used to distract the dogs. This result indicated that an odour component was likely and that odour may contribute in attracting flies to susceptible sheep.
If an odour is confirmed, then it opens up additional opportunities depending on whether the odour components act as repellents or as attractants. If a repellent, then it offers opportunities to find new chemicals to protect sheep from breech strike; if they act as an attractant, then it offers opportunities to develop more effective methods to trap blowflies than are currently available, and to identify sheep that are be less attractive to blowflies.
The University of Western Australia (UWA) has extensive experience in identifying odour that attracts insects, and in the identification of volatile chemical components that contribute to germination of specific seeds. A joint project with UWA was thus initiated to identify the specific odour components that are secreted by breech wool.
More than 1500 different compounds have been identified across both the DPIRD (formerly DAFWA) and the CSRIO Breech strike flocks, using gas chromatography. A large number of these volatile components are unknown and are currently being investigated to determine which will attract flies.
Two different methods are being used: the electro-antennagram detector and the behaviour of blowflies when exposed to these compounds.
Electro-Antennagram Detector (EAG)
The EAG is presented in the following picture. It is connected to a Mass spectrometer (MSD).
The arista of a blowfly is placed between two electrodes and the specific volatile component is determined in the MSD and blown over the arista through a glass tube. The electrodes measure the voltage differential when the neurons fire and the resulting pattern is shown in Figure 6.
When it has been shown that the flies recognise specific volatile components through the firing pattern of its neurons, then it indicates that they have specific neural receptors for those compounds. However, it is still unknown whether these compounds will elicit a specific behavioural response (repelled or attracted), so likely candidates still need to be tested using behavioural methodology.
Blowfly behaviour
The behaviour of blowflies as they find susceptible sheep and lay their eggs is an important area of research and an area of specialist skills. We invited Dr Bekka Brodie from Ohio University (USA) to participate in this work. She has studied the ordinary bottle green blowfly, Lucilia sericata, a cousin of the sheep blowfly Lucilia cuprina, and she identified specific chemical odour signatures that attract L. sericata.
She assisted us in fine tuning our EAG methodology to get better neural electrical responses but, more importantly, she demonstrated very quickly that L. cuprina is not attracted to the same odour compounds that attract L. sericata. This was a major finding which explains why L. cuprina prefers live animals while L. sericata prefers carrion (dead animals). Clearly, there is something specific in live sheep that attracts L. cuprina as they do not lay eggs on carrion.
Up to now, we have assessed fly behaviour and what attracts them in an artificial environment, but we need to determine what attracts, or repels them under natural conditions. Their behaviour and how they identify susceptible sheep under natural conditions will be an important future area of research as we determine how they are attracted or repelled by these factors.