Considered one of the world’s most destructive pests, the Mediterranean fruit fly (Medfly), Ceratitis capitata, is highly adaptive and has a broad host range of more than 200 fruits and vegetables and can devastate market access and trade.

In Australia, Medfly is only established in Western Australia. The presence of Medfly requires host fruits exported interstate or overseas from WA to undergo costly postharvest treatments. Ongoing production losses and measures to control Medfly incursions in commercial orchards can also have a significant economic impact.

Thought to have originated in Africa, Medfly established in Western Australia in the late 1800s, where it almost wiped out the region’s horticulture industry. It soon spread to Sydney by shipping in the 1890s but then disappeared. One theory for its disappearance is that Queensland fruit fly (Qfly) displaced Medfly in the area.

While Medfly is established only in WA, growers should be aware of potential incursions of Medfly in other parts of Australia as indicated by the sporadic outbreaks of Medfly that occur in metropolitan Adelaide. Growers on the eastern seaboard cannot be complacent about the threat to commercial and home orchards.

Border security is the first line of defence in keeping out serious plant pests like fruit fly. The Department of Agriculture, Water and the Environment applies strict biosecurity measures offshore, at the border, and onshore to protect Australia. This includes specific import conditions like phytosanitary treatment and certification of shipments by international governments. There are also domestic quarantine restrictions in place to prevent further spread of Medfly outside WA.

Highly adaptive

Western Australia’s Department of Primary Industries and Regional Development entomologist Dr Darryl Hardie says Queensland fruit fly (Qfly) may be a stronger and more virulent species, but Medfly has proved to be a highly adaptive survivor.

“Medfly learned to co-exist with three other major pest fruit fly species in Hawaii and if it got into one of Australia’s eastern growing regions it would find a synchronous niche for itself somewhere,” Dr Hardie said.

Dr Hardie and his colleague Dr Touhidur Rahman have numerous examples of Medfly adapting its behaviour to survive in different habitats. One of the more startling is its willingness to infest olives in WA.

“When we told Israeli entomologists that olives were a host fruit for Medfly locally, they thought we had misidentified another species as Medfly,” Dr Rahman explained.

“Olives had never been observed as a Medfly host fruit anywhere around the Mediterranean, despite centuries of geographical overlap between plant and fly. When we provided our evidence, they were shocked by the implications for Israel’s olive industry.”

This high level of adaptability makes Medfly a considerable threat to horticulture in all of Australia.

Cool climate survival

The pest’s ability to survive cold winters is another aspect of its threat to Australia.

“Eastern growers cannot rely on a cool climate to protect them from Medfly,” Dr Rahman said.

Medfly is widely established around the Mediterranean, including countries with sub-zero winters like Greece, Italy and Spain.

It has spread steadily southward in WA and now occupies cold apple growing regions like Donnybrook (mid-way between Perth and the south coast), as well as semi-arid areas in the north and inland. In fact, it has spread to all parts of settled Western Australia excluding Kununurra.

Medfly becomes active as temperatures exceed 12 degrees Celsius and under ideal summer conditions will complete its entire life cycle within 28 days.

When temperatures start to fall, Medfly can survive for extended periods as eggs or larvae in fruit, or as pupae in the ground.

“We have conducted tests where we put adults in cages in a tree canopy during winter and they survived for more than 110 days,” Dr Rahman said.

“They seemed to survive the longest when there was a live branch in the cage, so non-deciduous fruit trees are ideal for them.”

Citrus trees can provide a year-round shelter for Medfly. The heavy canopy provides cool shade during summer and a warm micro-climate can insulate them during winter.

“Medfly will survive in the citrus tree and use the fruit to breed, so the population will be ready to explode as soon as temperatures rise.”

For this reason, baiting and setting traps in citrus trees during winter and early spring is a crucial Medfly control strategy.

Myths and misconceptions

Primary Industries and Regions South Australia manager of plant health operations Nick Secomb said one of the most dangerous things about Medfly is the perception that it is a tropical pest.

“It definitely has the potential to extend its range from tropical and sub-tropical areas all the way into the cool pome and stone fruit growing regions of eastern Australia, including parts of Tasmania,” he said.

Like his WA counterparts, Mr Secomb doubts the theory that Medfly cannot establish itself in areas where Qfly is prevalent.

He said eastern growers should assume Medfly is able to establish itself in their area and cause damage, rather than just hoping it will not.

Vigilance essential

Phytosanitary controls on interstate movement of fruit and government fruit fly trapping networks are important measures in preventing the spread of Medfly to eastern states in Australia. 

Mr Secomb said growers must inspect their fruit frequently in order to detect the presence of fruit fly species early. “The most likely way for Medfly to reach an area is with travellers bringing infected fruit and discarding it,” he said.

Under the national fruit fly protocols, finding Medfly larvae in locally grown fruit is considered evidence of a breeding population and appropriate control measures will be taken by regulatory agencies.

The other criteria include catching a gravid female – i.e. a pregnant adult female – or trapping three males within a kilometre of each other within 14 days.

However, a grower’s normal surveillance traps are not reliable in established Qfly areas since the para-pheromone lures used for Qfly are unattractive to Medfly.

Area-wide Medfly surveillance needs to be complementary to Qfly surveillance trapping and control, although many protein bait sprays are attractive to both species in varying degrees.

The most important precaution is not to be complacent or think Medfly incursions are limited to tropical climates. All parts of Australia are vulnerable to Medfly.

Read more on the Prevent Fruit Fly website.