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Screw Worm Fly
D.R. Thompson, District Veterinary Officer, ORANGE 2800
The screw worm fly (Chrysomia bezziana) is the closest exotic animal pest to Australia. It is present in New Guinea and the South East Asian countries. Its probable range of spread and potentially disastrous effects on Australian livestock if it gained entry to this country should be an area of concern to the whole agricultural community.
THE FLY
This insect is an obligate parasite of all warm blooded animals, including man. It requires a wound on a living animal to lay its eggs. The eggs hatch within twenty-four hours and the maggots crawl into the wound where they feed and grow, burrowing deeply into the flesh. After six days of feeding they crawl out of the wound, burrow into the soil and pupate. After about 7 days (in ideal weather) the adult fly emerges, mates and after six days is mature enough to lay eggs. The adult fly can lay several times during her 20 day life span, depositing up to 250 eggs per sitting. The life cycle can be completed within three weeks under ideal conditions.
THE HOST
All wounds on all warm blooded animals could be regarded as potential egg laying sites. Frequently struck are the still moist navels of new born animals. The minute wound caused by a blood sucking tick commonly provides the site for a strike. The entire range of wound causing management practices in all types of domestic livestock provide additional sites for egg laying.
THE CONSEQUENCES
Any untreated strike can be considered life threatening. The presence of maggots in a wound attracts other females to lay their eggs. Subsequent hatching and feeding enlarges the existing wound, causing greater tissue destruction. Depending on the site, death can result. Mexican cattle owners claim that without treatment 90% of new born calves die. One could assume the same consequences in new born lambs. Apart from death the tissue destruction causes serious production losses. Strike in the genital area of both sexes (a common site) can cause scarring and disfigurement resulting in sterility.
THE COST
It is estimated that if the fly entered Australia and spread over its entire climatic range the annual losses to this country would exceed $101,000,000 in the sheep and cattle industries. This cost includes producer losses, treatment costs and increased costs to the consumers through higher prices.
| Cattle | N.S.W. | Australia |
|---|---|---|
| Production Losses | 10,456,000 | 33,598,000 |
| Treatment Costs | 1,615,000 | 6,663,000 |
| Consumer Costs | 1,573,000 | 5,865,000 |
| $13,644,000 | $46,126,000 | |
| Sheep | N.S.W. | Australia |
| Production Losses | 20,072,000 | 37,040,000 |
| Treatment Costs | 9,031,000 | 14,485,000 |
| Consumer Costs | 2,038,000 | 3,613,000 |
| $31,141,000 | $55,138,000 | |
| Total (per annum) | $45,539,000 | $101,264,000 |
THE CONTROL
The United States Department of Agriculture have developed techniques which have eradicated screw worm fly from large portions of Northern America. These include:
1. Sterile Male Release Technique.
This system utilises the fact that female flies only mate once in their life. If this mating is with a sterile male any eggs produced by the female will not hatch. Large factories have been built to rear flies which are then sterilised with radiation. The flies are then released from aircraft to mate with fertile wild flies. If sufficient numbers of sterile flies are released for sufficient time the wild population is eventually decimated.
2. Adult Suppression System.
This involves the manufacture of small pellets containing an attractant, a food source and an insecticide. These pellets are released from aircraft. Wild flies are attracted to them, feed on them and are killed by the insecticide.
3. Wound Treatments.
An extensive field inspection service is active in encouraging owners to treat with insecticide all new born animals, all animals subjected to wounding (e.g. shearing, dehorning, branding, castrating etc.) practices and all struck animals.
THE THREAT
Many of the islands between New Guinea and Australia are inhabited. Most of them contain animals. The shortest distance across Torres Strait to Cape York is 150 kms. The maximum distance between islands in the Strait is 60 kms. Human and animal movement between these islands is not uncommon. Fortunately screw worm flies tend not to make over water flights and stay perched in windy conditions. Therefore the free flight or wind borne flight onto mainland Australia is considered unlikely.
Animal movements, either by inter-island trade or illegal importation constitutes the most likely method of introduction to Australia.
If introduction occurred in the remote areas of Cape York it might be many months before it was detected. This would give ample time for spread, which American experts calculate at about 50 miles per week.
THE SPREAD
Freezing temperatures and extreme heat (without moisture) are about the only environmental barriers to screw worm fly spread. In America the rough guide is used that any area that supports citrus growth is warm enough to allow fly survival during winter. This implies that much of N.S.W. would support fly activity during winter and all of it would suffer during the other months. The summer temperatures in Western N.S.W. might slow down fly activity a little but the spring and autumn months combined with some rain would constitute almost ideal conditions for the fly.
THE PLAN
CSIRO have a research facility in Port Moresby. They are now able to rear flies and sterilise them. They conduct regular population studies using fly traps in New Guinea to monitor population changes likely to influence the threat to Australia.
The Commonwealth Departments of Health and Primary Industry have established a series of trapping sites in the Torres Strait, Cape York, North Queensland and the Northern Territory. These traps are robbed at regular intervals to check on the presence of screw worm flies.
A model control plan has been drawn up which lists all the actions considered essential to eradicate an outbreak it it ever occurs. The speed with which this plan is activated is vital to prevent fly spread as much as possible. It is proposed that sterile flies from the Port Moresby research facility would be used initially. However, our plans include the possible construction of a factory on mainland Australia should it be required