The sheep's revenge: using a parasite to catch a pest
Biocontrol agents are being developed against sheep lice but their ability to also kill blowfly maggots has implications for sheep strike prevention
It seems only fair that the flesh-eating blowfly maggots that torment sheep are themselves prey to a creature with a nasty feeding habit. In fact, a maggot-mummifying fungus is just one of a number of organisms showing potential in a biocontrol project focusing on sheep pests.
Under investigation by researchers at the Queensland Department of Primary Industries and Fisheries, the biopesticide agents are targeted primarily at sheep lice. However, lead researcher Dr Peter James says these agents are also being tested against sheep blowfly with the view that applications for lice may also provide some protection against strike.
Overall, the Queensland team is involved in three projects testing biological agents against sheep parasites. There are the fungi, specialist strains of Metarhizium and Beauveria. These are the particular interest of doctoral candidate Diana Leemon; the spores are formulated into water-soluble powders that can be applied to sheep in the same way as chemicals.
Then there are tiny invasive nematodes, which come armed with mutualistic and deadly bacteria that kill both lice and blowflies. Finally, as part of the IPM-sheep project with researchers from the University of New England, they are testing a range of natural plant extracts against flies and lice.
Dr James particularly fancies the fungi as biopesticides and points out that Metarhizium strains, sold as GreenGuard®, are already in use as a low-residue alternative for locust control.
"The attraction of biocontrol and organic strategies is that they fit with the image of wool as 'pure and natural' and address growing sensitivity in some wool markets to even low chemical-residue levels," Dr James says. "In addition, there has historically been a problem with resistance to chemicals in blowflies and lice. But with the fungi's mode of action, resistance is considered to be much less likely to develop."
Ms Leemon has isolated native Metarhizium and Beauveria fungi strains from sheep lice, blowfly and other insects, then selected for further development those strains capable of killing maggots and lice at temperatures found on a sheep's skin.
Although the work is painstaking, Ms Leemon confesses to finding the project fascinating and rather satisfying. "It's the revenge of the sheep, in a way," she says of the discovery that the infected blowflies eventually end up quite furry and come to resemble small, mummified sheep.
She adds that it is the carbohydrate-metabolising enzymes that provide fungi with specificity for different kinds of insects. The strains she has selected attack all stages in the blowfly lifecycle, including soil-based pupae. The spores first lodge on the outside of insects and produce hyphae, which grow through the insect, producing toxins that eventually kill the insect.
"The thinking is that if the spores are put on the sheep for lice control you might also get protection against blowfly strike," Dr James says. "Because these spores stick to the insect's waxy outer skeleton, they have the ready-made ability to attach and persist in greasy wool."
That observation is supported by studies showing that spores persist on sheep for at least 14 weeks at levels that can kill lice. Activity against blowflies is still being examined, but Dr James thinks the formulation should provide protection against new strike.
However, the fungal biopesticide would not be suitable for treating struck sheep, given that the spores require up to 11 hours to germinate and further time to kill the infected maggots.
"The work has progressed past lab-testing and we are in the early stage of sheep trials with a view to commercialisation," Dr James says.
Indeed, the Queensland scientists are already tackling issues of large-scale spore production and are in discussion with interested companies.
More information: Dr Peter James, Queensland DPI&F, 07 3362 9409, peterjames@dpi.qld.gov.au
Photos by Diana Leemon
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