How did Adelaide eradicate its Queensland fruit fly outbreak?

A year-long outbreak of Queensland fruit fly in metropolitan Adelaide was declared over earlier this week, bringing an end to restrictions on transport of fruit across the city.

It’s a relief for South Australian communities, who have battled several outbreaks of both Queensland fruit fly (Bactrocera tryoni) and Mediterranean fruit fly (Ceratitis capitata) across the state in recent years.

At the height of the outbreaks in 2021, the South Australian Department of Primary Industries and Regions (PIRSA) was eradicating 11 Mediterranean fruit fly outbreaks and one Queensland fruit fly outbreak in metropolitan Adelaide, one Mediterranean fruit fly outbreak in Port Augusta, and five Queensland fruit fly outbreaks in the Riverland. The Mediterranean fruit fly outbreaks in metropolitan Adelaide were declared over in December 2021.

This week’s announcement brings the number of remaining outbreaks down to two: both of Queensland fruit fly in the Riverland.

So how did Adelaide manage to outsmart this horticultural pest?

Fruit flies like a banana, and an apple, and a peach…

The Queensland fruit fly is a wasp-like insect about eight millimetres long and native to eastern Australia, particularly eastern Queensland and northern New South Wales.

The fly is described by the Queensland Department of Agriculture and Fisheries as a “major and frequent pest”. Females lay their eggs in fruit on trees or on the ground, and hatching maggots feast on the fruit.

According to Nick Secomb, General Manager of the Fruit Fly Response Program at PIRSA, the wide host range of the fruit fly is part of what makes it dangerous.

“In South Australia there’s about $1.3 billion worth of produce grown every year that the fruit fly can infect,” he explains.

The impact of fruit flies on fruit trees and veggie gardens is another concern.

“There’s that social impact as well, if you can’t go out and grow your summer tomatoes, bring them inside and eat them without eating into a maggot,” says Secomb.

“It’s a significant thing for people to be able to maintain their veggie gardens and enjoy clean, green homegrown produce.”

Getting the numbers down

Effectively combating a fruit fly outbreak requires a few different strategies, targeting different stages in the insect life cycle.

Strategically placed traps across the state emit pheromones to attract flies for monitoring purposes.

Adult flies are targeted with a bait containing an insecticide called Spinosad, derived from a soil bacterium.

“It’s really specific to fruit fly and they come and feed on it and die,” says Secomb. “Because it’s organic, we can use it in residential areas.”

Fallen fruit is collected and composted at high temperatures to kill off any fruit fly eggs or larvae that may be lurking within.

Once the fly populations have been brought down sufficiently, it’s time to send in the sterile flies.

The killing blow

Sterile insect technique was key to PIRSA’s fruit fly control effort.

To start, you need to breed and raise a large number of fruit flies in captivity.

“It starts in a really specialised factory,” says Secombe. “For the Queensland fruit fly, we’ve got our own facility in Port Augusta.”

Sterile Mediterranean fruit flies, meanwhile, are grown in Western Australia.

The captive flies are sterilised using x-rays while in their pupal (cocoon) stage.

“It’s really handy because they kind of package themselves up in a little package that’s easy to transport around the country,” Secombe laughs.

The sterilised flies are sent to an operations team, who release them into the outbreak area, either with a plane or from the back of a specialised ute. When the sterile flies, which are otherwise “fit and healthy”, mate with wild flies, no viable offspring can be created.

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For eradication, it’s important to be able to flood the wild population with enough sterile flies to disrupt reproduction for the whole population.

“So if you can imagine you’re filling a room up with all these sterile flies and there are two wild flies within there, they’ll just never find each other,” explains Secomb.

“It’s like being at a football ground, you can never find your friend because there’s so many people in the way.”

Sounds pretty effective, so why isn’t this being used to eradicate Queensland fruit fly all over the country?

“Sterile flies really struggle to be effective if there’s millions of wild flies out there anyway,” Secomb says.

“They can’t overwhelm the population as they need to. So the places where there’s lots of fruit fly naturally, it’s really hard for sterile flies to do their job.”

Luckily for Adelaideans, in this case the fruit fly numbers were controlled to a degree that allowed the sterile flies to fulfil their destiny.