The Papaya Fruit Fly - A Failure of Quarantine

Research Paper 29 1995-96

Tom Bellas
Consultant to
Science, Technology, Environment and Resources Group

Papaya Fruit Fly photo


Major Issues

On 17 October 1995, Mr John Crawford, a pawpaw (papaya) grower near Cairns, found fruit fly maggots infesting his crop. The culprit was identified as the papaya fruit fly, a species not previously reported from mainland Australia. This fruit fly has been described as the worst pest of fruit in Asia and the Pacific. Surveys of the surrounding areas found flies in Mossman to the north, Bingil Bay to the south and inland at Mareeba.

A quarantine zone was established that extended 500 kilometres along the coast and up to 200 kilometres inland. Susceptible fruit and vegetable fruits-and that meant almost all varieties produced in the area-leaving the quarantine zone for markets in the south had to be treated with insecticide. Japan and New Zealand banned the import of fruit from the area.

Little is known about the biology of this species and how it will adapt to Australian conditions. The papaya fruit fly is thought to be capable of extending its range southwards as did the Queensland fruit fly last century. The flies are claimed to infest all of the varieties of fruit grown in the area except pineapples and to infest vegetables such as tomatoes and melons. Observations to date indicate that it is out-competing the Queensland fruit fly and attacks fruit that this fly rarely attacks making it a far worse pest than the indigenous species. Control costs will be substantially greater than they have been with the existing fruit fly pests.

The flies almost certainly arrived in Cairns in fruit brought from areas to the north. The nearest infestation was on islands in northern Torres Strait 800 kilometres away. It is clear from the distribution of the flies in October 1995 that they had been present for one to two years.

Male lures provide a sensitive and efficient detection system for the papaya fruit fly. Traps had been in place in the Cairns area as well as in other places in northern Queensland from 1976 to 1988 when they were removed. Despite a recommendation contained in a report of the Horticultural Policy Council in 1991 that was considered by the Standing Committee on Agriculture in February 1992 the traps were not replaced.

The reason why this decision was taken is unknown since the papers of the Standing Committee are confidential. If the reason was related to cost, the savings of less than $200 000 per year on traps is far less than the estimated $40-50m cost of an eradication program. Even if an eradication attempt does not proceed, the cost of measures needed to control this new pest will quickly amount to more than the cost of a system of detection traps.

Fruit flies have invaded many areas around the world and they have been eradicated many times. An efficient procedure for eradication has been developed using male annihilation, female suppression and the release of sterile insects. These techniques have been used to eradicate Mediterranean fruit fly from Carnarvon and the Queensland fruit fly from Perth.

Eradication is easiest to achieve if the infestation is found when the extent of the invasion is small and before the population has increased which it would have been in this case if traps had been in place. The Horticultural Policy Council was concerned that '. . .if an exotic species succeeded in gaining entry at one of these ports and becoming established it is probable that its presence would remain undiscovered for a considerable period-perhaps until eradication had become too difficult or too costly to contemplate'. The arrival of the papaya fruit fly in Cairns has confirmed their worst fears.

Population suppression measures were commenced within a month of the discovery of the fly and these measures have greatly reduced the population. At the time of writing (May 1996), there has been no decision made about whether to proceed with an attempt to eradicate the fly.

In December 1995, funding was announced so that monitoring traps for exotic fruit flies could be set out near airports and seaports in northern Australia. While that action is too late for the papaya fruit fly, there are several other pest species in areas to the north of Australia that would become problems if they were allowed to become established. An adequate system of traps to detect exotic fruit flies backed up by contingency plans for their eradication and a mechanism for rapidly putting those plans into effect will prove of great benefit to Australian agriculture.

Responses to pests and diseases of animals are covered by the Australian Veterinary Emergency Plan (AUSVETPLAN). There is no comparable plan for pests and diseases of plants nor does there appear to be any group with the responsibility to prepare contingency plans for pests like the papaya fruit fly.

It is highly desirable that there be a co-ordinated program to produce contingency plans and preparedness plans for pests and diseases of plants. This program could be organised through the Standing Committee on Agriculture and Resource Management and use the resources and expertise of the Australian Quarantine and Inspection Service, the Bureau of Resource Science, State Departments concerned with agriculture, and the CSIRO.


ABARE                Australian Bureau of Agricultural and Resource Economics
ACIAR                Australian Centre for International Agricultural Research
AQIS                 Australian Quarantine and Inspection Service
AUSVETPLAN           Australian Veterinary Emergency Plan
CSIRO                Commonwealth Scientific and Industrial Research Organisation
DPIE                 Department of Primary Industries and Energy
HPC                  Horticultural Policy Council
Medfly               Mediterranean fruit fly
NAQS                 Northern Australian Quarantine Strategy
OTA                  Office of Technology Assessment, US Congress
PHC                  Plant Health Committee
QFF                  Queensland fruit fly
SCA                  Standing Committee on Agriculture
SCARM                Standing Committee on Agriculture and Resource Management
SIRM                 Sterile Insect Release Method
SIT                  Sterile Insect Technique

Chemical names of specific fruit fly lures
cue-lure             4-(4-acetyloxyphenyl)-2-butanone
methyl eugenol       1,2-dimethoxy-4-(2-propenyl)-benzene



It is of some concern however, to find that such [efficient detection] systems are not being used at many of the most likely entry points for these species around Australia-the major and minor seaports and airports. Consequently, if an exotic species succeeded in gaining entry at one of these ports and becoming established it is probable that its presence would remain undiscovered for a considerable period-perhaps until eradication had become too difficult or too costly to contemplate.

The impact of fruit flies on Australian agriculture, Horticultural Policy Council, April 1991.

The papaya fruit fly, Bactrocera papayae, was found infesting pawpaw at Yarrabah, near Cairns, in October 1995. A quarantine zone was promptly established extending 20 kilometres around the Cairns Post Office. Within ten days the fly was trapped at locations to the north, west and south of Cairns and the quarantine region had been extended to cover a region about 500 kilometres along the coast and up to 200 kilometres inland (see Figure 1). Japan and New Zealand imposed bans on the import of produce and controls on movement of produce from the area were established within Australia.

Thus were the first few days after the confirmation that a new, highly damaging fruit fly pest had arrived on the Australian mainland. The papaya fruit fly is described by Dr R. A. I. Drew of the Queensland Department of Primary Industries, one of the world's leading investigators of fruit flies, as the worst fruit pest in the whole of Asia and the Pacific.

Within days of the discovery a team of people was assembled in Cairns to investigate the extent and the severity of the infestation. On 16 November a program was initiated to control the fruit fly (AQIS Bulletin 1995a). Finance has been provided through the Standing Committee on Agriculture and Resource Management (SCARM) to enable investigations into the species' host preferences and into the possible hosts among indigenous fruits.

There are still many areas where our knowledge of the papaya fruit fly is deficient.

At the time of writing (May 1996), the control program begun in November has already had an impact on the populations of the fly in areas that it has infested and the numbers of flies in these areas has been much reduced. But flies have been found another 50 kilometres further south at Kennedy.

This paper provides background information on the arrival of this new insect pest, examines how the the invasion has become as serious as it is, and looks at the chances of success in repulsing a pest that has the potential to have a major adverse impact on Australian horticulture.

Fruit flies

The name 'fruit fly' has been applied to two groups of flies.

To orchardists and entomologists it means those flies that lay their eggs in immature or ripening fruits so that their larvae can feed and grow within the fruit. The damage arising from the feeding of the larvae makes the fruit unmarketable. These flies are placed in the taxonomic family Tephritidae (1). These are the fruit flies that are the subject of this paper.

To geneticists it means Drosophila melanogaster and related species that have been the subject of their study for many years. The larvae of most of the Drosophilidae feed on fungi and while the larvae are found in fruit it is only in those fruits that are over-ripe and decaying and usually fallen. Entomologists prefer the names ferment flies or vinegar flies for these insects (Carne et al. 1987).

The Tephritidae

The papaya fruit fly, Bactrocera papayae, is placed in the family Tephritidae within the order Diptera (the true flies). There are at least 4 000 species in this family of flies, the great majority of which have larvae that feed on fruits or on seed-producing heads of plants of the daisy family (Asteraceae). Of these 4 000 species around 10 percent would be classified as pests and around 10 percent of these (i.e., about 40) as major pests. The adults of most species of fruit flies can live for two to six months, are capable of flying many kilometres-even across water-and produce many young. Many of the species of fruit flies are restricted to one species, or a very few closely related species, of host plants. It is from among this group that biological control agents have been brought to Australia for the control of Bathurst burr, Crofton weed, Bitou bush and thistles.

Fruit flies are of major significance in almost all fruit growing areas of the world either because they are already present or because they are capable of establishing in areas presently free of them. The major pests, e.g., the Mediterranean fruit fly, attack a wide range of fruits and have established themselves in regions far from their native range. Quarantine restrictions have been imposed around the world as countries try to prevent the spread of the pests into new regions.

Figure 1: North Queensland (4)

Figure 1: North Queensland (4)

The Dacinae

The great majority of species in the family Tephritidae are placed in one of four sub-families: Trypetinae; Tephritinae (seed flies); Dacinae; and Ceratitinae. The species in the sub-family Dacinae are found predominantly in tropical and sub-tropical regions and are associated with soft fruits from a very wide range of plants. The more than 800 species in the Dacinae are found in Africa (200 species), Asia/South East Asia (300 species) and the South Pacific (300 species). All of the pest fruit flies native to Australia belong to the Dacinae.

Several synthetic chemical lures for fruit flies have been found through accidental discoveries and systematic searching. The Dacinae can be divided into three groups: those species attracted to methyl eugenol; those attracted to cue-lure; and those not attracted to either of these compounds (Drew & Hooper 1981; the systematic names of the two chemicals are given on page iii). At normal population levels only males are found in traps baited with methyl eugenol or cue-lure. The range of attraction of methyl eugenol for papaya fruit fly males is probably only a few tens of metres downwind although because of the mobility of these flies individuals from much further away will be trapped over time as they fly into the area of attraction of the lure (Cunningham 1981).

Several chemicals that attract males of the Mediterranean fruit fly (in the sub-family Ceratitinae) have been discovered but none is as effective as the two chemical lures that are active for the Dacinae.

Methyl eugenol and cue-lure provide a sensitive and efficient tool for detecting low populations of those species of fruit flies that respond to these chemicals.

Papaya fruit fly

The papaya fruit fly is one of a number of pests that were until recently known collectively as the oriental fruit fly, Bactrocera dorsalis. Flies with this common name have been recognised for many years as the worst pest among the Dacinae. They damage fruit in much of east, south-east and south Asia and on islands to the north of Australia. The oriental fruit fly has been transported to and has established itself on islands across the Pacific; it reached Hawaii in 1946. This fly has been eradicated from California more than a dozen times. This name, however, was applied to a group of species all very similar in appearance-forming a species complex. The more than fifty members of the dorsalis-complex occupy many areas of Oceania and south-east Asia and also infest a wide range of host fruits. Not all members of the species complex are pests. That there is a species complex has been known for many years but it was only in 1994 that a definitive publication on this group of flies was published (Drew & Hancock 1994). Thus the name Bactrocera papayae only dates from that year.

The home range of the papaya fruit fly is south-east Asia-southern Thailand, Malaysia and Indonesia-where it infests a wide variety of indigenous and introduced fruits. Many studies have been conducted on the oriental fruit fly but because of the confused taxonomy it is not always clear as to which of the species within the complex was being studied. Thus results of research on the fly present in Hawaii (which is the true oriental fruit fly) need not apply to the papaya fruit fly. What is known is that the papaya fruit fly attacks a wide range of fruits and attacks them earlier in the ripening process than most other species of fruit flies.

The life cycle of the papaya fruit fly

After mating, females lay their eggs just under the skin of the host fruit. These entry points are known as 'stings'. In summer around Cairns the eggs will within 36 hours hatch into larvae (known as maggots) which feed and burrow into the fruit. Up to 30 maggots of the oriental fruit fly have been found in one fruit. The damage caused to the fruit by the feeding larvae can lead to premature fruit drop. After about seven to ten days, depending on the temperature and the quality of the food, the larvae leave the fruit and burrow into the ground to pupate. Adults emerge from the pupae about ten to twelve days later. Adults are sexually mature after about a week or two and can live for three to four months. Provided the females have access to protein and mates they can reproduce throughout their life. In the oriental fruit fly the potential fecundity is over 1000 eggs per female and this is likely to be true for the papaya fruit fly as well. The long reproductive life of a female means that it can still be laying eggs when its great grandaughters are attacking their first fruits.

Australian fruit flies

Eighty-four species in the Dacinae have been described from Australia (Drew, 1989). All of the species occur in tropical or subtropical parts of the country spread across the wetter northern areas and down the east coast. All are associated with moist forests and depend on the soft fruits produced within those forests. Several of the species are pests. The Queensland fruit fly, Bactrocera tryoni, is the most widespread and damaging of the Australian species. It has become established in New Caledonia and in some of the Society Islands including Tahiti. It also became established on Easter Island from where it has been twice eradicated.

Before Europeans arrived with their fruit trees the Queensland fruit fly occurred probably no further south than northern NSW; a report on Sydney in 1819 commented on the quality and quantity of stone fruits with no indication of the presence of pests. But by 1853 maggoty fruit was reported from Kiama south of Sydney and the present range extends into East Gippsland (see Drew, 1989). The Queensland fruit fly now infests inland towns in NSW. In addition the flies have established themselves many times in irrigation areas along the Murray valley and Adelaide but they have been eradicated as many times. It was recently eradicated from Perth (Yeates, 1990) after its first appearance in the metropolitan area.

The Queensland fruit fly in north Queensland breeds continuously provided suitable fruit is available. It is capable of passing through seven generations a year in the northern part of its range but in East Gippsland there is only one generation per year. Around Sydney there can be three or more generations per year and numbers can increase rapidly over a summer (Meats, 1989).

The Queensland fruit fly's presence has meant that access to many markets for Australian fruit has been denied or made difficult because of control and disinfestation measures that have to be applied to comply with the requirements of importing countries. Several other indigenous species are rated as significant pests (see Table 1) and other species are occasional or minor economic pests. There are restrictions and prohibitions on movements of fruit and vegetables within Australia which aim to prevent the penetration of the Queensland fruit fly and other species into areas where they do not normally occur.

        Table 1: Indigenous Fruit Fly species that can be major economic pests.

Name of species 
Bactrocera         Infests                     Location           Synthetic lure
aquilonis          many fruits                 N & NW Aust.       cue-lure
bryoniae           capsicum, mangoes           E Qld              cue-lure
cucumis            melons, tomatoes, pawpaws   E Qld, NT          none known
jarvisi            several fruits              NW Aust to NSW     none known
melas              several fruits              Central & SE Qld   cocue-lure
musae              bananas                     NE coast, PNG      methyl eugenol
neohumeralis       many fruits                 NE coast, PNG      cue-lure
tryoni             many fruits                 E coast            cue-lure

Extracted from Economic fruit flies of the South Pacific (Drew et al. 1982)

Across Australia the sum of the costs of fruit fly-induced crop losses plus the costs of pre-harvest controls and regulatory costs were estimated in 1994 to be in excess of $150m yearly. When the loss of international markets due to fruit flies was included the estimated cost to Australia was of the order of $300m per annum (SCARM Workshop, 1995).

Non-indigenous species

The Mediterranean fruit fly, Ceratitis capitata, found its way to Western Australia before the turn of the century. After Western Australia it then appeared in New South Wales where it became a major pest around Sydney. For reasons not known but possibly associated with an increasing abundance of the Queensland fruit fly, the Mediterranean fruit fly became less common and the last sighting in NSW was in 1941. It is still present as a major pest in Western Australia.

In 1974 flies collected on the Cape York Peninsula were identified as Bactrocera frauenfeldi, a species known from Papua New Guinea as a pest of mangoes. The arrival of this species in Australia called attention to the possibility of the arrival of pest species from areas to the north. In mid-1975 a trapping program specifically for the oriental fruit fly and the melon fly (Bactrocera cucurbitae) commenced. The discovery in November 1975 of a fly identified as the oriental fruit fly from Melville Island brought fears that there had been another invasion. However this fly was not found in commercial fruits and an intensive study found that it was not the oriental fruit fly but an indigenous species restricted to one local fruit, Opilia amentacea (Morschel 1979). This species is now known as Bactrocera opiliae.

A series of trapping stations was established across the northern coastline from Derby to the Torres Strait. This program, known as the Northern Monitoring Program, also incorporated monitoring for the oriental screw worm and was largely conducted by the States and the Northern Territory using funds provided by the Commonwealth Government.

In 1983 on Christmas Island, nine males of oriental fruit fly were collected at methyl eugenol baits as part of a survey conducted by the Western Australian Department of Agriculture. These specimens were later determined to be the papaya fruit fly (Drew & Hancock 1994). This is the earliest known incursion onto Australian territory.

In 1987 a major review of quarantine in Australia was conducted by a committee headed by Professor David Lindsay of the University of Western Australia. The report of the review was published in 1988 (Department of Primary Industries and Energy 1988). The review was conducted after the reorganisation and consolidation of quarantine functions within the Department of Primary Industries and Energy.

In an interim report (Quarantine Review Committee 1987) the committee noted the special problems associated with Australia's northern coastline. The use of motors on boats and the use of aircraft had increased movements of people and produce between the islands of Torres Strait and between the islands and the mainlands of Australia and Papua New Guinea. There was also an increasing movement of pleasure boats and yachts in the area as well as increased international and domestic tourism. The nearness of Papua New Guinea and the changing practices and variety of agriculture there would thus cause an increase in the number and types of pests that are that much closer to Australia.

As a result of this inquiry there was established in 1989 the Northern Australia Quarantine Strategy (NAQS). It was under this program that the Australian Quarantine and Inspection Service (AQIS) began to conduct surveys in Papua New Guinea in 1989 and also in Irian Jaya, the easternmost province of Indonesia adjacent to PNG. Also in 1989 a series of trapping stations was set up across Torres Strait. Initially there were eight: Boigu, Saibai, Yam, Badu, Thursday, Yorke and Murray Islands, and also at Bamaga on Cape York to monitor the populations of fruit flies and to check on the species present.

In August 1995 the NAQS maintained monitoring stations on thirteen islands in Torres Strait and six locations on northern Cape York Peninsula as well as six stations in Western Australia and nine in the Northern Territory. The Torres Strait stations were on the following islands: Badu, Boigu, Darnley, Dauan, Deliverance, Gabba, Horn, Murray, Saibai, Stephen, Thursday, Yam, and Yorke, with all but Deliverance being checked monthly. The southernmost of the six stations on the mainland was Bamaga (Nairn, 1995) (see Figure 2).

In April 1992 the presence of Bactrocera papayae was reported from Merauke, Irian Jaya, to the south-west of PNG. In May 1992 the species was collected in Papua just over the border from Irian Jaya (AQIS Bulletin 1992).

By March 1993 the papaya fruit fly had reached Australian territory in Torres Strait when it was found on Boigu Island just off the Papuan coast and on Darnley Island further to the east. Within a few weeks traps had been placed on another 58 islands and further flies were trapped on Saibai, Dauan and Stephen Islands. Control measures were commenced and an eradication program applied on the two islands furthest from the Papua New Guinea coastline since reinfestation was certain on the islands close to the coast. Darnley and Stephen Islands were declared free of Bactrocera papayae after no flies were trapped for several months. In early 1995 flies were again detected on Darnley and Stephens Island and soon afterwards on Murray and Yorke Islands. It is thought that seasonal strong winds had carried the flies to these islands from Papua New Guinea. A control program was initiated on these four islands (AQIS Bulletin 1995b).

In October 1995 came the announcement that the flies had been found near Cairns. The extent of the infestation, from Mossman in the north to Bingil Bay to the south and inland to Mareeba would indicate that the flies had been present for many months and had dispersed widely. Cairns seems to have been the hub of the infestation.

It is unlikely that the flies that were responsible for the original infestation near Cairns arrived unaided. The nearest natural infestation was on islands in the northern Torres Strait. And while fruit flies have been known to cross more than 100 kilometres of water it is far more likely that a batch of infested fruit was brought into Cairns from an already infested area to the north. The fact that flies have not been found between Mossman and Cape York, nor on the islands just to the north of the Cape shows that they did not island-hop across Torres Strait onto the mainland. The most likely route for the entry of the papaya fruit fly into Cairns is by aircraft from one of the Torres Strait islands but other routes such as pleasure craft cannot be discounted.

Figure 2: Torres Strait

Figure 2: Torres Strait

The impact of the papaya fruit fly on Australian horticulture

In south-east Asia the papaya fruit fly infests a wide range of commercial and native fruits (White & Elson-Harris 1992). Many of those tropical fruits are also grown in north Queensland. The fly is said to be capable of breeding in all fruits except pineapples; and most vegetable fruits (tomatoes, squash etc.). The rainforests of Queensland have many species of plants in the same plant families as those that occur in south-east Asia so it is likely that this fly will be able to maintain itself on rainforest fruits. Traps placed in rainforest have captured male papaya fruit flies but it is not known whether the flies are breeding in the forests.

The papaya fruit fly attacks fruits that the Queensland fruit fly rarely infests, such as mango and pawpaw. In fruits that both species attack the newcomer appears to be out-competing the Queensland fruit fly. This may be because the papaya fruit fly attacks fruit a few days earlier than the Queensland fruit fly and thus gains an advantage.

The original observation at Yarrabah was in pawpaw. The grower had not had to spray because of fruit flies for several years so when he saw an infested fruit he asked for advice. It was his observation that brought the infestation to notice.

The home range of the papaya fruit fly is Southern Thailand, peninsular Malaysia, Borneo and some other Indonesian islands. It has a more tropical distribution than Bactrocera dorsalis, the oriental fruit fly, which has a distribution range to the north of this species (Drew & Hancock 1994).

The Queensland fruit fly proved to be able to adapt to cooler climates in extending its range from northern NSW to east Gippsland. It seems that the genetic makeup of the species allowed it to adapt to the cooler conditions (Meats 1989). If this is also true of the papaya fruit fly then it could range much further south than the range in south-east Asia would suggest and many important fruit growing areas could be infested by this new pest.

The gross value of annual production in Queensland of fruit and vegetables that are potential hosts of the papaya fruit fly is estimated at $486m (ABARE 1995). Around 70% of the produce is sent outside the State, mainly to southern destinations, but there is a growing market for this produce overseas. Sales to southern markets from within the quarantine zone are continuing following post-harvest disinfestation of the fruit and vegetables. The re-establishment of overseas sales will depend on convincing the importing countries that adequate control measures are in place and that disinfestation procedures currently in use to combat indigenous flies (e.g., for mangoes, by heating them with steam) will also work for this new pest. Regions which are currently free of papaya fruit fly will have to establish a monitoring program to prove that the fly is not present to permit current exports to continue.


Fruit flies have been eradicated from areas where they have invaded more times than any other group of insects. The first time this was achieved was in 1929 in Florida when the Mediterranean fruit fly made its first incursion into North America. The earlier methods relied on fruit removal and widespread spraying with arsenical insecticides. Since then efficient methods have been developed to achieve eradication and it has become almost routine (Bateman 1982).

Three methods have been developed which are usually applied in combination to achieve eradication.

Annihilation of males. This method makes use of the synthetic chemical lures that have been discovered. Traps baited with a lure and containing an insecticide (usually malathion) are placed throughout an area (see Figure 3). Any males that are attracted to the lures are killed. Because the females can mate with several males a very large proportion (more than 95%) of the males has to be removed to reduce the size of the next generation. Provided enough traps are installed this is not difficult and reductions by more than 99% have often been achieved. Because the flies can live for months it may be several weeks before there is a decline in the number of females.

This method used alone was successful in eradicating the oriental fruit fly from the 85 square kilometres island of Rota in the Marianas Islands in 1965. The usual technique is to load a mixture of the lure and an insecticide into a medium which can be a block of porous material, a length of cord, or viscous fluid formulations (Cunningham 1989).

Insecticidal bait sprays (female suppression). This method is based on the need in female flies for a meal of protein before they can develop their full load of eggs. By providing a source of protein and adding insecticide to it, females can be selectively removed from the population. The most common lure is a hydrolysed protein solution. The mixture can be placed as individual lures or spot baits or more commonly sprayed over the foliage of host trees (Bateman 1982). This method is capable of achieving eradication but requires an intensive effort (Scribner 1983).

Sterile Insect Technique. Neither of the preceding methods is selective. The use of male lures means that other fruit fly species that respond to the same chemicals will be attracted to the baits and the protein hydrolysate is attractive to other fruit flies as well as other types of insects. The third method is selective in that only flies of the pest species are the target of the technique.

Figure 3: Fruit Fly Trap

Figure 3: Fruit Fly Trap

The sterile insect technique (SIT)-or sterile insect release method (SIRM) as it was earlier called-was conceived by E. F. Knipling in the 1930s and was first applied against the New World screwworm fly in the USA in the 1950s. The theory behind the technique is that if sufficient sterile male flies are released then the chances of a wild fertile fly mating with a fertile male are reduced. Matings with a sterile male would result in sterile eggs and no progeny. The method worked very well against the screwworm because in that species each female mates only once. However it has been shown that even with species where females mate many times, as with fruit flies, provided enough sterile males are provided the technique can still work. The method requires the production of very large numbers of insects that are sterilised by irradiation before their release into the environment (Gilmore 1989).

It is best to apply the SIT when the population is low. This can be during a part of the year when the numbers are normally low or it can be achieved by applying first one or both of male annihilation or bait spraying. The lower the population the easier it is to achieve the high ratio of sterile males to fertile males necessary for the method to work. The people who conducted the successful eradication of oriental fruit fly from the island of Guam took advantage of the fact that there had been two hurricanes which had drastically reduced the amount of fruit on the island and had blown most of the fruit flies off the island.

The ratio of sterile males to fertile males should be as high as possible but there is no firm information on a minimum value. A ratio of 50 or more to 1 appears to be necessary for success: values lower than this have rarely given the desired result (Hooper 1982). It is suggested that for the Queensland fruit fly a ratio of around 1000:1 would be required (Horticultural Policy Council 1991, p. 106). Eradication is not claimed until no flies are trapped at lures for a period of time equal to three generations under the prevailing conditions.

Most of the successful eradication exercises have been conducted on islands or with infestations caught early before the numbers have increased or the flies have dispersed very far. The oriental fruit fly has been eradicated from California at least 13 times: in each case the flies had arrived from Hawaii. This species has been eradicated from Japan where it was present on several islands in the vicinity of Okinawa. The Japanese campaign commenced in 1968 but was not completed until 1986. The melon fly has been eradicated from Rota and a few of the southern Japanese islands.

Fruit flies have been eradicated many times in Australia. The Queensland fruit fly has often infested fruit growing areas in southern NSW and in Victoria and South Australia over the years and in each case eradication has been achieved. Adelaide has been the scene of many successes. A major eradication was achieved in Perth in 1989-1990 after the first ever infestation was discovered in the Perth metropolitan area.

Eradication of the Queensland fruit fly in Perth

An area of about 300 square kilometres was treated. The program involved using all three of the methods outlined above. The population was suppressed using bait blocks (with cue-lure as the attractant) and protein foliage sprays and then, over a period of ten months, 1 504 million sterile flies were released. The operation commenced in February 1989 when the flies were discovered and the program was declared successful when no further flies were discovered after December 1990. The operation had a budget of $5m. It had been estimated that the annual cost of control measures in addition to those already in place to combat the Mediterranean fruit fly would be about $2m per year (Yeates 1990; Ayling 1989). Some of the techniques used in this campaign were developed in the successful project to eradicate the Mediterranean fruit fly from Carnarvon in 1984 (Fisher 1985). Traps baited with cue-lure are now in place to detect any further arrivals of the Queensland fruit fly in and around Perth.

The campaign in Mexico and Central America

The Mediterranean fruit fly was discovered in Brazil in 1904. From that infestation the fly has spread over most of South America and into Central America. It was detected in Costa Rica in 1955. An attempt was made to stop it spreading northwards but in 1975 it appeared in Honduras, El Salvador and Guatemala. In 1977 it was detected in Mexico in the State of Chiapas just north of the border with Guatemala. In order to protect the large Mexican horticultural industry and to prevent the pest's movement northwards an international program was launched with the support of international organisations (Organizacin de los Estados Americanos, Organism Internacional Regional de Sanidad Agropecuaria, Food and Agriculture Organisation, International Atomic Energy Agency) and the co-operation of three countries: Guatemala, Mexico, and the USA. The stated aims of the program were to halt the spread of the fly, to eradicate it from Mexico and Guatemala and ultimately to eradicate it from Central America. By 1979 the flies had penetrated into the next State north (Oaxaca) about 250 kilometres north of the border and efforts to eradicate the flies were intensified.

From 1977 the program employed bait spraying from the ground and from the air, and confiscation of fruit from infested trees to reduce the population. Fumigation of fruit was conducted to prevent spread of the flies. Vehicles were inspected and fumigated at ports of entry into Mexico. A large mass-rearing facility was constructed at Metapa in Mexico and came into operation in 1979. After some initial fine tuning this facility from 1981 to 1985 produced on average 500 million sterile flies or more per week (Schwartz et al. 1989). In addition to servicing the local campaign several thousand million flies were provided to California where two infestations had been discovered in 1980 and where a campaign was conducted until 1982 when the insect was declared eradicated (see Appendix) (Scribner 1983).

At its peak the project was being conducted over 4 600 square kilometres. Northward expansion of the fly was stopped by 1980. In 1982 the fly was declared eradicated from Mexico and the major campaign moved into Guatemala. However small numbers of flies were occasionally found in Mexico and some areas had to be re-treated.

Because of social, political and financial problems the ambitious long term objective of eradicating the fly from Central America has been indefinitely postponed. The program at its peak in 1981 cost US$19m a year but this was reduced to about US$10m per year within two years. The money was provided by Mexico and the USA (Schwas et al. 1989).

Is eradication worth the cost?

An analysis of the economics of the program in the Mexico/Guatemala border regions was reported in 1989 (Vo 1989). The direct control costs were given as being US$62.76m. The estimate of the benefit/cost ratio with the benefits calculated up to 1991 is given as 49:1. A similar analysis had been conducted for the campaign against the Mediterranean fruit fly in Santa Clara county, California, that began in 1980. High and low benefit scenarios were estimated as at 1991 and the benefit/cost ratios are calculated as between 19.62 and 12.75 at these limits (2). The figures quoted in the original publication (Conway 1982) have been reassessed in 1991 (OTA 1993).

In December 1995 the Australian Bureau of Agricultural and Resource Economics published a report on a cost-benefit analysis of a proposed eradication program (ABARE 1995). The analysis was conducted using a set of fourteen fruits and vegetables (3) which account for 84% of Australia's total gross value production of potential papaya fruit fly hosts. The lack of knowledge of the biology of the papaya fruit fly in the Australian environment means that firm predictions cannot be made of the possibility of eradication so there are considerable uncertainties in the analysis. If the papaya fruit fly is capable of spreading into southern fruit-producing areas then high benefits are obtained. If however the fly will not spread much further southwards then lower benefits arise. The analysis used a variety of scenarios. These comprised two estimates of the cost of the eradication campaign, differing estimates of the chances of success of the eradication and allowing only half the number of sprays recommended by the Queensland Department of Primary Industries. The results showed that the publicly funded eradication campaign yielded positive net benefits for most of the scenarios considered.

Usually a decision would wait until firmer information was available but under the circumstances a decision may have to be made assuming the worst case as being most likely.

Possibility of eradication of the papaya fruit fly in Australia

The area infested by the papaya fruit fly in north Queensland by early November (from Mossman, 70 kilometres north of Cairns, to Bingil Bay, 110 kilometres south, and to Mareeba, 30-40 kilometres inland) is already comparable to the area treated in Mexico. Use of both male lure baited traps and protein bait treatments can drastically reduce the population of the papaya fruit fly. But it will do little to reduce the area of infestation or stop the fly from invading new territory particularly if the flies can breed in rainforest fruits.

The conditions in Queensland offer some advantages for eradication over southern Mexico. The rainforests are bounded to the west by dryer country and few fruit trees thus limiting the spread of the flies in that direction. The social and political conditions that caused problems around the Mexico/Guatemala border do not exist (Schwarz et al. 1989). The population is well educated and the fruit growers are eager to participate.

The first steps towards eradication were initiated on 16 November 1995 by the Queensland Department of Primary Industries which is overseeing a program using methyl eugenol baited traps and protein bait spot treatments in areas where breeding populations have been identified or where they could become established (AQIS Bulletin 1995a). 'The next stage involves a study by relevant Commonwealth and State agencies to examine the feasibility of broad-scale eradication measures' (from the same bulletin). These measures would involve the use of the sterile insect technique and the building of a mass-rearing facility for the production of the very large number of flies that will be required for the area that has been infested.

As in Mexico the first aim of a SIT program would be to prevent the extension of the area infested: in this case towards the large fruit growing areas to the south. The first step has been the establishment of a quarantine zone from which only fruit that has been treated is allowed to leave the quarantined area. Thus the first areas to be covered using the SIT method would be just to the south of the area known to contain flies. As fruit fly-free zones are established then the treatment area would be moved northwards until fly-free areas are reached. The southern Torres Strait islands would be treated if by that time the flies have invaded them. Any population suppression measures using insecticide have to be applied in advance of the SIT area so that sterile flies are not removed before they have a chance to do their job.

The number of sterile flies required and hence the size of the breeding program (and the size of the factory) will depend on the population of the papaya fruit fly. In orchard and urban areas control measures can lower the population as has already been achieved but there are greater problems, including effects on non-target organisms, in reducing the population in rainforest areas. If it becomes necessary to attempt to attack the flies in the rainforest then the attack will have to be conducted so as to minimise damage to non-target insects and other animals. The decision on the feasibility of eradication will depend on accurate estimates of the numbers of papaya fruit flies in the rainforest.

An analysis of the cost of conducting an SIT program was conducted for a Horticultural Policy Council report (Horticulture Policy Council 1991 p. 118). For the production of 20 million flies per week for at least 40 weeks a year the annual production costs were estimated to be $230 500. The construction and set-up costs of the factory were calculated as $475 000. The costs for conducting the eradication program were estimated as $382 000. These estimates were made in 1989 for eradication of the Queensland fruit fly in southern Australia. These costs were compared with the then costs of eradication activities in NSW, South Australia and Victoria which were calculated as $590 000 a year. In addition there were suppression campaigns which cost an additional $400 000 a year. An advantage of the proposal was that when sterile flies were not required for eradication they could be redirected towards suppression thus giving overall savings. These figures were estimates made for small outbreaks. The successful eradication of the Queensland fruit fly from 300 square kilometres of Western Australia operated with a budget of $5m over two years.

An eradication campaign against the papaya fruit fly would be substantially larger than any conducted previously in Australia. Figures of $35m and $52.5m were used in the ABARE benefit-cost analysis as estimates of the cost over five years.

Control of fruit flies

If the papaya fruit fly cannot be eradicated then in those areas it occupies it will have to be controlled to enable the production of fruit and vegetables to continue. The aim of control is to maintain the population at a level below which the damage can be tolerated ('the economic threshold').

A well established program for the control of indigenous fruit flies already exists (Beavis et al. 1989; Thwaite et al. 1995). Where the flies are endemic, or where an infestation has become established, cover sprays with systemic insecticides are the usual recommendation for most fruits. Systemic insecticides are absorbed by the plants and will translocate into the fruits where the fruit fly maggots are hidden from other types of insecticides. Male annihilation and protein bait sprays and spot treatments can all have their place in the control of fruit fly infestations, particularly in those areas where fruit flies are not always present (Bateman 1982). The SIT method could have a place in the control of small outbreaks and the maintenance of zones free of flies (Horticulture Policy Council 1991, pp. 74-76).

The recently established Tri-State Fruit Fly Strategy will employ the SIT method to control and eradicate the Queensland fruit fly in the Riverina, Sunraysia and Riverland areas of south eastern Australia so as to maintain fruit fly-free status that will allow the export of produce without the need for disinfestation.

Mr John Crawford, the grower who found the first infestation by the papaya fruit fly just south of Cairns is reported to have said that he had not had to use sprays for several years (Roberts 1995). He will now have to undertake measures in order to control this new fruit fly and it has been estimated that even small growers will face additional costs of $15 000 a year (Granger 1995). A substantially greater effort will be needed than before if the papaya fruit fly becomes established.

In some fruits cultural practices also play a role in reducing the impact of fruit flies. In bananas the technique of bagging the bunches at an early stage affords significant protection from fly attack. Removing infested fruit from the trees and the collection and treatment of fallen fruit are important factors in reducing the numbers of flies that emerge.

Exports of fruit from fruit fly infested areas are still possible provided that the importing country can be convinced that the produce has been treated to ensure its freedom from living flies. Various disinfestation methods currently in use include dipping or flood-spraying with insecticides; fumigation; low temperature storage; and various heat treatments by exposure to heated vapours or hot air; or by dipping in hot water, all of which add considerably to costs incurred by the producer. Irradiation has been proposed as a method for disinfestation but it is not currently used.

Biological control

In 1904 the Western Australian Government sent Mr George Compere to search for biological control agents for the Mediterranean fruit fly which by then had established itself as a severe pest in the state. He was the first of many to conduct such searches. He introduced several potential agents but few became established and little impact on fly populations was observed. While no fruit fly pest has ever been under complete classical biological control the use of these agents has contributed to significant reductions in populations of some pest species. The most successful was the introduction of parasitic wasps to Hawaii where populations of oriental fruit fly and the Mediterranean fruit fly were reduced and the incidence of attack on the less favoured hosts of the flies declined. The major agent was the wasp Fopius arisanus but when this species was introduced into Australia its effect was negligible on the Queensland fruit fly even though it was successfully established (Waterhouse 1993). It is not known whether this species of wasp has any effect on the papaya fruit fly.

Other possible techniques

Sex pheromones. In many species of insects interactions between the sexes are governed by volatile chemicals known as pheromones. In moths the females produce blends of chemicals which result in males of the right species being able to find the females from many metres distance. These substances have been used to specifically detect males of pest species to provide early warning of their presence and in some cases have provided a means of controlling the pest. In southern districts of NSW and in Victoria, the oriental fruit moth, a major pest of peaches and nectarines, is controlled without the application of insecticides by using this approach (Thwaite et al. 1995).

Tephritid fruit flies are known to possess sex pheromones. From a gland attached to the hind gut the males produce a blend of chemicals that is released during courtship. The identity of the chemical compounds involved has been established for several species. While the chemicals undoubtedly have a role it seems that sight is the most important factor in establishing contact between the sexes. To date pheromones have not been used in controlling the pests although a place may still be found for them in an integrated pest management system.

Host odours. Female flies have to be able to locate fruit so that they can lay their eggs. Further the fruit has to be at the right stage of development to provide the best environment for the growth of the larvae. Fruit flies use both sight and smell. Smell is of more importance with flies with narrow host ranges. The apple maggot fly, Rhagoletis pomonella, of eastern USA, is attracted by odours produced by apples and in the olive fly, Bactrocera oleae, a major pest of olives around the Mediterranean, smell plays a major role. In both of these species the performance of traps is improved by the addition of host odours. Nothing is known of the behaviour of the papaya fruit fly in this regard.

Improved lures. While protein hydrolysate has been very useful it loses its effect after a two or three days exposure on leaves. The search for lures with improved performance is underway in several places around the world. There are similar projects in Australia and, while the immediate target is the Queensland fruit fly, a successful result will be of benefit in the program against the papaya fruit fly.

Potential pest fruit flies

Three pest fruit fly species have found their way to Australia: the Mediterranean fruit fly late last century probably from Africa or India; and two species from north of Australia, the mango fly by 1974 and now the papaya fruit fly. There are several other species of fruit flies in Asia, Papua New Guinea and Oceania that could become serious pests if they established themselves in Australia (Table 2).

The melon fly, Bactrocera cucurbitae, is rated as one of the world's most serious pests and the most important pest of vegetables especially melons (Waterhouse 1993). It is present in Irian Jaya, mainland Papua New Guinea and several islands to the east of there as well as on Hawaii. Soon after emergence the adults are capable of very long flights-30 to 60 kilometres but even longer flights over water have been recorded. The 150 kilometres gap between Papua New Guinea and Cape York is not a substantial barrier to this species.

The oriental fruit fly, Bactrocera dorsalis, has very similar capabilities to the papaya fruit fly. It is a major pest in Hawaii and from there makes regular incursions into California. This fly could infest many fruit growing areas in southern Australia. It would be wise to maintain both cue-lure and methyl eugenol baited traps at all cities which receive direct flights from Honolulu.

While it is easy to detect those species that respond to the synthetic chemicals cue-lure and methyl eugenol there are two species (Bactrocera atrisetosa and B. decipiens) that are listed in Table 2 that do not respond to either of these. Like the Australian fruit flies, B. jarvisi and B. cucumis, their presence will only be detected after they are found in fruits at the new location and have already been in the area for some time.

     Table 2: Pest species of fruit flies known from areas to the north of Australia

Species name BactroceraRegion                   Hosts            Chemical lure*
albistrigata           SE Asia                  many fruits      cue-lure
atrisetosa             PNG                      melons           none known
cucurbitae             PNG/Asia                 melons           cue-lure
caudata                SE Asia                  melons           cue-lure
decipiens              PNG                      melons           none known
dorsalis               Asia/Hawaii              many fruits      cue-lure
facialis               Tonga                    many fruits      cue-lure
kirki                  Tonga, W Samoa           many fruits      cue-lure
passiflorae            Fiji, Tonga              many fruits      cue-lure
tau                    SE Asia                  melons           cue-lure
trivialis              PNG                      many fruits      cue-lure
umbrosa                SE Asia/PNG/ Melanesia   many fruits      methyl eugenol
xanthodes              Fiji, Tonga              many fruits      methyl eugenol

Adapted from
The impact of fruit flies on Australian horticulture (Horticultural Policy Council 1991).

* Drew 1989; Drew (pers. comm.)

Could the arrival of the papaya fruit fly have been prevented?

In the Lindsay Report of 1988 it is pointed out that no quarantine system can guarantee total exclusion of pests and diseases while there is trade, movement of people, natural movement of pests and diseases, and intentional or unintentional breaking of the law. The Report suggested a system of risk management to allow the quarantine system to be most effective within these constraints (Lindsay Report, chapter 3).

The NAQS, whose origin arose from the considerations of this committee, has been active in education about quarantine among the people who live in Torres Strait and other parts of northern Australia and also among travellers through the region (Baldock 1989). Information about quarantine has been provided via various media: radio and videos, and also in print to air travellers, to schools, and to the 18 communities that live on the islands of Torres Strait (AQIS Bulletin 1992). The public awareness created by this 'Topwatch' program has been rated a definite achievement (Nairn & Muirhead 1995).

The papaya fruit fly could have arrived in Australia as adults using their own flying abilities perhaps assisted by favourable winds as has been suggested for their arrival on islands in the eastern Torres Strait in 1995 (AQIS Bulletin 1995b). This can be called a natural process and although such arrivals cannot be prevented contingency plans can be in place to repulse the invasion. The alternative is an assisted passage for the eggs or maggots in fruit brought into the country by a traveller. There is some possibility of preventing such occurrences.

The disposition of the thirteen monitoring traps in Torres Strait and the six on the mainland (see Figure 2) indicate that the expectation was that if there was an invasion by the papaya fruit fly or other fruit fly species that they would progress from island to island across the strait in the form of a natural invasion. If the assisted passage was from island to island these traps would have provided warning but they could not if the entry by-passed these traps altogether.

This possibility was considered in the Horticultural Policy Council Industry Report No 3, The impact of fruit flies on Australian horticulture, published in April 1991, which contained a recommendation (No 14) that

Detection systems for threatening exotic species should be established around the most likely entry points (major and minor seaports and airports) throughout Australia.

Some port cities already had extensive monitoring traps in place (Adelaide, Darwin and Perth) and the report names Sydney, Newcastle, Brisbane and Cairns as 'obvious high risk areas where detection systems are seriously needed'. The report suggested that each trapping station should have three traps: one cue-lure, one methyl eugenol and one capilure (a synthetic chemical lure for the Mediterranean fruit fly).

The report was submitted to the then Minister for Primary Industry and Energy who passed the report onto the Standing Committee on Agriculture (SCA). The SCA asked for a critical assessment by the Plant Health Committee (PHC) which analysed the report and found Recommendation 14 (and most of the others) acceptable. The PHC response was considered by the SCA at the meeting held in February 1992. Despite this recommendation no traps were put in place between that date and October 1995.

It has not been possible to ascertain what discussion occurred at this meeting or the reasons why no action was taken as the papers of the meeting are regarded as confidential and will only become available after thirty years have elapsed.

The Queensland Department of Primary Industries had placed monitoring traps in Cairns and some other coastal cities between 1976 and 1988. This system was funded by the Commonwealth Government. The trapping at these locations was discontinued when AQIS took over the direct management of the northern Australia surveys.

The HPC Report expressed concern that detection trapping was not being conducted at many of the most likely entry points (Horticulture Policy Council 1991, p. 48).

Consequently, if an exotic species succeeded in gaining entry at one of these ports and becoming established it is probable that its presence would remain undiscovered for a considerable period-perhaps until eradication had become too difficult or too costly to contemplate.

That is just what appears to have happened with the papaya fruit fly at Cairns where it is likely that the fly has been present for at least one year and perhaps as long as two years.

The Report stated that the detection system should be an integral part of Australia's quarantine system and that it should be funded by AQIS, planned in consultation with fruit fly biologists and carried out by staff of the local state departments of agriculture (Horticulture Policy Council 1991, p. 49).

On 14 December 1995 a media release announced the provision of $14.7m over three years to strengthen quarantine barrier surveillance in northern Australia. Among the measures mentioned was '. . .the development of lures and monitoring for exotic fruit flies around airports and seaports. . .' (Collins 1995).

If monitoring for exotic fruit flies had been in place at Cairns in 1994 the probability of detecting the papaya fruit fly soon after its arrival would have been high, as the monitoring systems installed in California for the oriental fruit fly have proven. The infestation would most likely have been confined within Cairns and a relatively minor campaign for its eradication would have been required. The delay in its discovery has fulfilled the fears of the authors of the Horticultural Policy Committee's report.

The latest of many

As far as we know the papaya fruit fly is the latest pest insect to arrive in Australia. Since 1971 forty-eight species of insects from overseas are known to have become established in Australia and the great majority of these are pests (Clarke 1996, pers. comm.). It is precisely because they are pests that they have been noticed.

There may well be many other insect species that have arrived but they, being more cryptic, have not announced themselves. Early in 1995 the cause of damage to greens and fairways at the Steelworks Golf Course in Newcastle was identified as a South American mole cricket known to cause many millions of dollars damage to turf annually in the USA. This pest is now known to have been in Australia for at least thirteen years since a specimen, unidentified at the time, was collected in 1983.

In the USA over the period 1980 to 1993 a total of 159 insect and mite species of foreign origin were introduced or first detected. For the 62 species where a pathway had been identified the great majority arrived unnoticed on plants. Of these 159 species 41 were rated as harmful introductions. This list does not include those insects introduced in the same period as biological control agents (OTA 1993).

No similar list is as yet available for Australia. Dr G. M. Clarke of the Division of Entomology, CSIRO, has been seconded to the Animal and Plant Health Branch of the Bureau of Resource Sciences to produce a report entitled "Exotic insects in Australia: introductions, risks, and implications for quarantine". His report is expected to be completed in June 1996.

There are many insects which would prove important pests if ever they were able to establish a foothold in Australia. A list of pests of tropical crops, prepared in 1991, is included in the report of the review of NAQS (Nairne and Muirhead 1995). For some of these potential pest insects there are specific lures such as sex pheromones but for the majority the only weapon is vigilance.

Cue-lure and methyl eugenol make the detection of many of the pestiferous fruit flies an easy task. Unfortunately the detection of almost all the other potential new pests is usually only made after the invasion when the pest has established itself in its ecological niche and calls attention to itself, as indeed was the case with the papaya fruit fly at Yarrabah, and as was also true for another two insects that will prove very damaging over many parts of Australia.

The silverleaf whitefly, Bemisia tabaci biotype B, was first detected in Queensland in October 1994. It is now well established in that State as well as the Northern Territory and New South Wales and has been detected in Tasmania. It is a damaging pest to many vegetable crops as well as a wide range of ornamentals. It is on ornamental plants that it has travelled the world. International trade in poinsettia has been a major route.

It has had its major impact on crops grown under intensive agriculture and in greenhouses. In the Imperial Valley in California between October 1991 and February 1992 crop losses to this insect amounted to US$111m and in addition there were about 6 000 fewer jobs in January 1992 than there were in January 1991 (De Barro 1995). By 1994 another 3 000 local jobs had disappeared and total damage was then estimated as US$300m (OTA 1995, p. 117).

The silverleaf whitefly has a wide host range (more than 600 species), can increase in numbers very rapidly, is resistant to several insecticides, is highly damaging to its hosts, and it is a vector of several plant viruses that, up to date, have lacked an efficient vector in Australia (De Barro 1995).

The western flower thrips, Frankliniella occidentalis was detected in Perth in May 1993 on mixed flower crops (AQIS Bulletin 1993). Restrictions on the shipment of flowers to eastern States were imposed but the insect quickly appeared in the east as well. The thrips on the east and west coasts are not genetically identical so it seems that there have been two separate invasions, both likely to have been on imported cut flowers (Stapleton 1995). This thrips infests a wide range of crops and its feeding causes growth distortions in the host plants. It can also act as a vector for viruses. It is already resistant to a range of insecticides.

Both the silverleaf whitefly and the western flower thrips are expected to be at least as costly as the papaya fruit fly. And as with most insect pests that have found their way to Australia they are here to stay and Australia's horticulturists will have to learn to survive in their presence.

Planning to repel boarders

The Quarantine Review Committee (Lindsay Report 1988) recommended that immediate action should be taken to develop a strengthened, planned and co-ordinated approach to responding to outbreaks of exotic pests and diseases of plants (Recommendation 34). The Committee further recommended that the Bureau of Rural Science (a forerunner of the Bureau of Resource Sciences) should be responsible for developing plans for outbreaks of exotic pests and diseases of animals and plants and for co-ordinating responses (Recommendation 43).

Responses to pests and diseases of animals are covered by the Australian Veterinary Emergency Plan (AUSVETPLAN) which is managed within the Livestock and Pastoral Division of the Agriculture and Forest Group in the DPIE. There is no comparable plan for pests and diseases of plants nor does there appear to be any group with the responsibility to prepare contingency plans for pests like the papaya fruit fly.

Currently each outbreak of a plant pest is considered after it occurs by the SCARM and the Plant Health Committee which reports to the Standing Committee. The PHC usually forms a Consultative Committee (as it did for the papaya fruit fly) to obtain expert advice on the particular pest or disease. Although the Queensland Department of Primary Industries responded promptly after the identification of the papaya fruit fly, control and suppression with funding through SCARM commenced four weeks afterwards (almost a generation on) and a decision on the implementation of eradication procedures is still under consideration (more than three generation times after the discovery). The question of funding is always contentious.

The papaya fruit fly has a generation time of four to five weeks. The suppression measures put in place have greatly reduced the population of the flies but these measures are unlikely to prevent natural spread of the flies at the periphery of their range where suppression measures are unlikely to have been applied so that the area of the infestation will be increasing.

Contingency planning

The major inhibition to the development of contingency plans for pests of plants is the very large number of organisms that could become pests if they were able to establish in Australia. This was recognised by the Quarantine Review Committee (Lindsay Report 1988, p. 114) but nevertheless it still recommended that the effort should be made. The Committee recommended that the then Bureau of Rural Science be responsible for this task but this was never taken up by them or their successors.

The NAQS maintains a target list of exotic insects that is currently being revised and brought up to date. The insects are grouped under high, medium and low priority headings. There are no contingency plans even for those insects in the high priority group. The recent review into NAQS (Nairn and Muirhead 1995) contained as recommendation No 17

that NAQS scientists develop preparedness plans for all the targeted pests and diseases of plants and work with other stakeholders to develop contingency plans for a limited number of more important targets, over a period of five years.

The NAQS reviewers noted that it is often difficult to find an effective forum for development of such plans until a crisis developed but suggested that the NAQS scientific staff are in an excellent position to develop these plans (Nairn and Muirhead, p. 28). Whether the current scientists have the resources or the time to apply to the development of the plans is questionable. The NAQS list covers those insects that occur in areas to the north but insects from other areas could be covered by AQIS or by people in State departments.

It is highly desirable that there be a co-ordinated program to produce contingency plans and preparedness plans for pests and diseases of plants. This program should be organised through SCARM and use the resources and expertise of AQIS, BRS, State Departments concerned with Agriculture, and the CSIRO. The costs of preparing these plans will be minuscule compared with the damage that results from the arrival of a single pest like the silverleaf whitefly on an unprepared horticultural industry.

Another approach is to anticipate the arrival of a pest. It is to Australia's advantage if a pest that occurs in a neighbouring country that is capable of establishing on Australian territory is brought under control in that country because then there is less likelihood that it will find its way to Australia and, if it does, then a control method is already available.

This approach has been undertaken with some insect pests which will at some time in the future arrive in Australia. The banana skipper caused much damage in Papua New Guinea but an ACIAR funded program conducted by the Division of Entomology, CSIRO, has introduced an effective biological control agent that has greatly reduced its incidence (CSIRO 1992, p. 50). The Russian wheat aphid when it arrives in Australia will have a very damaging effect on wheat production just as it did in the USA and in South Africa. A CSIRO program based in the Republic of South Africa has identified biological control agents that will be introduced as soon as possible after the discovery of the aphid here (CSIRO 1992, p. 47). The sex pheromone of a sugar cane borer causing damage in Papua New Guinea has been identified and this can be used to give early warning of the presence of the pest and it might even be able to be used as the basis of a control program (Whittle et al. 1995).


The present invasion is a major rather than a minor problem because of the delay in detecting the papaya fruit fly on mainland Australia caused by the failure to have monitoring traps in place in the vicinity of airports and seaports.

The Lindsay Report noted that a 'no risk' policy was untenable and undesirable and that a policy of 'acceptable risk' was preferred. It also noted that the assessment of risk should be scientific and objective.

If the withdrawal of traps from Cairns in 1988 and the refusal to reinstate them in 1992 despite the recommendation of the Horticultural Policy Council was based on risk analysis, then surely the arrival of the flies in Torres Strait should have rung the alarm bells and led to a reversal of those decisions. The failure to do so has produced the current situation.


In early 1996 a trap baited with cue-lure at Cottesloe in suburban Perth caught one male melon fly. The trap was part of an array across the metropolitan area that was established to detect new invasions of Queensland fruit fly after the successful eradication of the species from Western Australia in 1990. More intensive monitoring was established but no further melon flies were detected. It is unknown how the fly came to be in Western Australia but the lack of further captures after two months indicates that there is not a breeding population. But for the necessity to monitor for the Queensland fruit fly the arrival of this species in Western Australia would have passed unnoticed.

On the other side of the continent a shipment of mangoes and chillies from Fiji was seized and destroyed this year following the detection of fruit fly larvae in the chillies. This was despite the shipment having been supposedly treated with ethylene dibromide as a disinfestation treatment for fruit flies. Imports of ethylene dibromide-treated produce from Fiji were suspended (DPIE, Primary Resource, Issue 96/2).

By the end of March male papaya fruit flies had been found over a wide area but still well within the quarantine zone. Surveys have been conducted for flies breeding in wild guava which is widespread throughout the hinterland of Cairns but only in the vicinity of Cairns, Mossman and Mareeba have breeding populations of papaya fruit fly been found. There have been no flies found breeding in rainforest fruits which indicates that there will not be significant populations of flies within the rainforests. Population suppression measures have greatly reduced the population relative to its level at the time of its discovery. No decision has been made about proceeding with eradication. A meeting of the Consultative Committee is scheduled for June (by which time there will have been about seven generations of flies since the discovery at Yarrabah) (R. A. I. Drew, pers. comm.). Meanwhile, the Federal Government and Queensland are funding the $6 million papaya fruit fly detection and suppression program up to 30 June 1996.


  1. The tephritid fruit flies discussed in this report are given their current scientific names. Prior to 1989 all of the species now in the genus Bactrocera were placed in the genus Dacus and literature before then will refer to these species under this generic name (Drew, 1989).
  2. The estimates were published in 1982 before the program has been brought to finality. The cost is given as US$64m whereas the final cost was close to US$100m (Scribner 1983). The benefit/cost ratios are thus overestimated. If the calculated ratios are in direct proportion to the assumed costs then the benefit/cost ratios calculated with the actual cost are 12.56 and 8.16.
  3. Bananas, mangoes, pawpaw, citrus, apples, pears, wine grapes, table grapes, grapes for dried vine fruit, apricots, peaches, tomatoes, pumpkins and cucumbers.
  4. On 20 October 1995 a quarantine zone was declared over the area 20 km around the Post Office at Cairns. On 26 October a regulation was published (No 301 of 1995) to establish a larger zone above 18 20' south and on 5 November, with regulation No 305, the zone was extended to 19 south. Regulation No 301 contained an error since it referred to a point on longitude 144 15' west and the regulation was probably defective because of that. The zone shown in the map assumes that the intended point was on longitude 144 15' east.


Mediterranean fruit fly eradication from California, 1980-1982: an example of the difference between early and late detection.

On 5 June 1980 males of the Mediterranean fruit fly were trapped at two locations in California.

In Los Angeles the identity of the one fly caught was established the next day. More traps were installed and more flies captured nearby thus confirming the presence of a breeding population. The infestation was discovered in its first or second generation and at the time of the discovery was confined to one loquat tree. Because of the success of a SIT program in 1975 it was decided to use this method again. In July the release of sterile flies commenced and continued until October. The flies were released over an area of 258 square kilometres corresponding to a circle of radius about 8 kilometres around the site of the infestation. Just over 205 million flies were released and eradication was declared in December 1980. This area treated was entirely urban.

In Santa Clara county at the southern end of San Fransisco Bay the two flies were not identified until twelve days later. Subsequently flies were found throughout a 13 square kilometres area. The infestation was probably in its third or fourth generation. Until 1980 it was believed that the Mediterranean fruit fly could not survive during the cooler months in the Bay area and traps were removed in autumn and replaced in late spring. It is suggested that the flies had arrived about the time that the traps were removed and consequently they were not discovered until at least two generations had passed (Gilmore 1989).

The area of infestation was initially underestimated and the number of sterile insects released was too small to be effective. One major problem was the detection of wild flies among the very large number of sterile flies captured in monitoring traps. Even though the sterile flies were marked with fluorescent dusts an inefficient procedure caused an underestimation of the number of fertile flies. This in turn led to low population estimates and the release of too few sterile flies.

Bait spraying was commenced early in the campaign but there were problems of access to backyard fruit trees. The area of infestation increased until it covered several counties around the southern end of the Bay. The total area that required treatment was almost 3 400 square kilometres. After it was realised that the SIT was failing it was decided that aerial spraying should be the method employed.

As well as technical problems with the eradication method and the way in which it was conducted there were numerous others. Because the area was largely urban there was much opposition to the proposed spraying. There were many public meetings opposing spraying and eventually court proceedings. The State Governor initially refused permission to conduct aerial spraying but the possibility of a declaration of a total quarantine on California caused the Governor to grant permission and spraying was allowed. The spray program commenced in July 1981 and continued for just on 12 months. The flies were declared eradicated in September 1982 (Scribner 1983).

This successful eradication program became an immense undertaking and cost about US$100m. In contrast the Los Angeles infestation which was found early the cost was only a small fraction of this amount.

There were many organisational and technical problems that hindered success (Scribner 1983). A detailed account of the campaign and its problems is given by Jackson and Lee (1985). An account of problems, particularly those of the different approaches of the scientists and managers, that gave rise to misunderstandings and conflicts within this program, is given by Lorraine and Chambers (1989).

The northern and the southern incursions offer a salutary lesson in the advantages of catching an infestation early. Although both infestations were eradicated there was probably a hundred-fold difference in the costs due to finding the Santa Clara infestation just a few months too late.

Successful as the 1980-1982 campaigns were there was only a short respite. In 1986 a single fly was captured in Los Angeles which under the established protocols was not enough to initiate action. But every year since then, there have been further catches and despite extensive efforts at eradication using mainly the Sterile Insect Technique there are at present some 4 100 square kilometres of the Los Angeles basin under quarantine.

All of the recent eradications of the Mediterranean fruit fly have depended on the widespread application of pesticides before applying the SIT (Carey 1994). In this largely urban area this option has not been applied. Unlike the chemical lures for the oriental fruit fly and the papaya fruit fly there is no really efficient lure for males and this has proven a major weakness both in early detection of new areas of infestation and the attempts to eradicate the flies. It has been suggested that the Mediterranean fruit fly is on its way to becoming established in the Los Angeles basin (Carey 1991).

In 1983 State quarantine officials discovered that large quantities of illegal produce was being brought into California through Canada. One shipment of longans had travelled from Seattle via Vancouver, Toronto, New York and Miami, where it was repacked into Florida boxes, to California where it was detected at a border inspection station. The identification of this previously unidentified route of infestation and a change in tactics will, it is hoped, lead to eradication of this pest (Dowell 1994).

References & Bibliography

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I thank Carol Kenchington of the Parliamentary Library and Jon Prance of the Black Mountain Library for their assistance in locating and retrieving information from a wide variety of sources. Dr R. A. I. Drew provided the photograph of the female papaya fruit fly taken by Mr Steve Wilson of Mt Nebo, Queensland.


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