Screw-worm fly

Screw-worm fly (SWF) is an aggressive insect pest of warm blooded animals (including people, wildlife and livestock). The fly lays eggs on wounds or moist body openings. The eggs then hatch to become aggressive flesh eating maggots. SWF is related to the blowflies that cause fly-strike in Australian sheep, however SWF prefer hot, humid climates and cannot survive in frost-prone areas.

SWF is widespread through tropical regions, including some of Australia’s closest neighbours – Papua New Guinea, Indonesia, Malaysia and the Philippines. The potential spread of this pest from Papua New Guinea is one of the major biosecurity threats to northern Australia. Establishment of SWF in Australia would have major impacts on northern livestock production, livestock export trade and public health. 

In order to eradicate SWF, Australia would need to establish a facility where sterile male flies could be produced in sufficient numbers to interrupt the SWF breeding cycle. The construction of such a facility could take a number of years by which time costs could approach $500 million a year in lost production and control measures. It has been estimated that up to 15 per cent of cattle could be struck at any time in the potential zone of infestation, with the greatest losses due to the deaths of newborn calves as a result of navel strike.

Keeping Australia SWF free relies upon early detection, containment and ideally eradication before it can spread to other areas. The Northern Territory Department of Primary Industry and Resources (DPIR) maintains a number of traps close to East Arm Wharf which specifically target SWF. In addition, biosecurity officers regularly inspect cattle for evidence of fly strike.

Regular sampling and laboratory examination of maggots from infested wounds anywhere in Australia is also critical to early detection. Maggot collection kits will be distributed by your local Livestock biosecurity officer in the next few months during property visits. If you see an animal with maggots in a wound please support surveillance by sending maggots to Berrimah Veterinary Laboratory – early detection and response is the key to control.

Adult Old World screw-worm fly (Chrysomya bezziana). screw worm fly egg masses (white) deposited in a wound (Animal Health Australia, 2017)

Figure 5. Left- the adult Old World screw-worm fly (Chrysomya bezziana). Right- screw worm fly egg masses (white) deposited in a wound (Animal Health Australia, 2017)

For more information go or contact Peter Saville, DPIR SWF Coordinator, on 08 89518181 or 0401 118 181.

Last updated: 23 September 2019


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