SHEEP producers have been warned that resistance to a blowfly treatment has been discovered in the Eastern States.
The discovery in NSW has prompted the Department of Agriculture in Western Australia to issue a reminder to growers to manage their use of blowfly treatments carefully to reduce the risk of resistance.
Department development officer Di Evans said the recent report by NSW Agriculture of fly resistance to diflubenzuron, which includes Magnum, Strike and Fleececare, was the first time resistance to an insect growth regulator had been reported in Australia.
"What is interesting about this discovery is that low level diflubenzuron resistance was found in fly populations that had not been previously exposed to diflubenzuron, but had been exposed to diazinon," she said.
"This suggests that there could be cross-resistance between both chemical groups."
Resistance to diflubenzuron was first suspected when farmers reported a reduced fly protection period from the label claim of 12 weeks to only seven to eight weeks.
Ms Evans said although no surveys had been undertaken in WA to determine whether diflubenzuron resistance was present in the State, she said that it is likely given that diazinon resistance is present in WA.
"Where diazinon resistance exists it is too late," she said. "The swing away from the use of diazinon is mainly due to occupational health and safety concerns and an increased awareness of the resistance issue."
Ms Evans said to avoid resistance in general, growers should only use a chemical when necessary and rotate the use of different chemical groups.
"For those properties where diazinon resistance is known to exist or is suspected, it is possible that diflubenzuron resistance is also present," she said.
"Research in the Eastern States indicates that there has been fly resistance to diazinon for more than 30 years, with recent field surveys indicating that no fully susceptible fly populations exist in many areas of Australia.
"Where properties believe that they don't have diazinon resistance, diflubenzuron should still be fully effective, but where diazinon resistance is suspected, then diflubenzuron may not be fully effective."
The recommended flystrike treatment for individual animals is to shear or clip the affected area and to apply an insect growth regulator, such as Vetrazin, which provides longer protection against re-strike or a spinosyn, like Extinosad, to provide short term protection.
The shorn wool and maggots should be placed in an airtight bag and left in the sun for a couple of days to kill off the maggots, which would otherwise pupate in the soil and later emerge as adults.
For more information about effective fly treatments, read Farmnote 47/2001 and Factsheets 1/2001 and 10/2001.