Meeting the insecticide resistance challenge
The development of resistance to insecticides in blowfly populations has created major challenges for wool producers and researchers alike
The development of resistance to three classes of insecticide by the sheep blowfly in Australia has changed wool producers' options when it comes to protecting their sheep from flystrike.
Dr Garry Levot (pictured), of the NSW Department of Primary Industries, works on monitoring and detecting chemical resistance in blowfly to help prevent the failure of products in the field. While he says resistance is a "big headache" for producers and the chemical industry, he adds that there are a number of reasons, in addition to resistance, why products can fail, namely:
- poor application;
- inadequate dosing; and
- wrong chemical dilutions.
"Insects evolve resistance through a process of 'artificial selection' rather than natural selection," Dr Levot says. "Chemicals select among a genetically diverse blowfly population for those rare insects with natural tolerance. The selective pressure increases given an over-reliance on any one chemical group."
He adds that the problem can be exacerbated by multiple treatments with the same chemical class directed separately at lice control and flystrike prevention. This was the case in the most recent development of resistance, to diflubenzuron in a population in Tara, Queensland.
In that situation, the producer relied exclusively on diflubenzuron for flystrike prevention in long wool and lice control in short wool.
Split shearings of different mobs meant sheep with declining amounts of diflubenzuron in their wool, providing local blowflies with year-round exposure to the chemical. The result was complete failure of diflubenzuron to protect the sheep from flystrike within three years.
"One fear is that all chemicals, if subject to sufficient selection pressure, will succumb," Dr Levot says. "We were funded by AWI to look at that potential in sheep blowfly. Despite more than 26 years of use across all sheep-grazing areas of Australia, resistance has not developed to cyromazine, which is still an extremely effective chemical."
Cyromazine is water-soluble while most other chemicals are lipophilic (soluble in fat) and can persist in blowfly tissue, including the reproductive organs. Dr Levot suggests the water-solubility may have contributed the most to the apparent inability of blowfly larvae to develop resistance to cyromazine, as most other chemicals of its era have resistance problems. There is also no reported blowfly resistance to several newer products based on spinosad, dicyclanil and ivermectin.
"Not all chemicals will succumb to resistance, but all chemicals need to be managed effectively so that you minimise the opportunity," Dr Levot says, adding that there are a few simple rules producers can follow:
- if you need to treat sheep for the control of both lice and blowfly, choose products from two different insecticide groups to minimise the selection pressure on both insects; and
- most importantly, producers should decide if a chemical is really needed, given that blowfly strike is a bad problem one year in five, on average.
"The ability to pick the problem years is helped by understanding the blowfly lifecycle and population dynamics," Dr Levot says.
Blowflies become active in spring when the over-wintering pupae present in the soil emerge in synchrony over a couple of weeks. If they find susceptible sheep - generally, animals that are wet for whatever reason - those animals are liable to be struck, providing the first generation of flies for the season. If favourable conditions persist for another generation, then the stage is set for a blowfly wave.
"There is a misconception that blowflies move progressively across the state, striking sheep," Dr Levot says. "But blowflies pretty much remain within a region - it is the formation of favourable conditions across a region that forms the blowfly wave."
He urges producers to use blowfly traps to detect the first appearance of flies and to look at their rainfall gauges as an indication of whether sheep are likely to be susceptible to flystrike.
"They then have to make their decision," Dr Levot says. "If sheep are susceptible, producers are going to need a strategy and the judicious use of an effective chemical might be an important part of that strategy."
Once the eggs hatch and larvae have fed on the sheep's skin for up to four days, they drop off at night in the sheep camps and crawl into the soil. If the soil temperature is warm - high teens and above - they will continue their lifecycle; under favourable conditions, the egg-to-adult interval is only 12 days.
In autumn, if they fall into cool soil (less than 15°C) their development is halted and they will remain in a suspended state until the soil temperature increases to more than 17°C. These maggots will subsequently develop together in spring, resulting in the emergence of a potentially massive over-wintering population.
As mulesing is phased out, Dr Levot is confident that producers have strong options among the "chemical armoury", but he wants producers to avoid thinking that chemicals will be their sole alternative. He has surveyed producers who manage without chemicals and found they all had one thing in common: they had selected very hard on their breeding stock for plain-bodied, easy-care sheep with tight fleeces.
"I have enormous faith in the woolgrowers of Australia to manage the transition from conventional mulesing. From an industry point of view they will come out the other end just fine."
More information: Dr Garry Levot, garry.levot@dpi.nsw.gov.au
Return to Battling the Blowfly supplement index.