Tiny little pest, great big problem

For most of the past two years, my friends who live a 12-minute drive away from me in the inner suburbs of Adelaide couldn’t bring any fruit over when they visited my house.

The reason? My friends’ house was in a fruit fly outbreak ‘red zone’, while mine was in a green zone. That means: no moving fruit from one suburb to the other, lest you unwittingly spread the pest.

South Australia has been working to control numerous outbreaks of fruit fly since the end of 2019.

“December 2019 is when we first found Mediterranean fruit fly outbreak in metropolitan Adelaide,” says Nick Secombe, general manager of the Fruit Fly Response program at the South Australian Department of Primary Industries and Regions (PIRSA).

More Mediterranean and a Queensland fruit fly outbreak in Adelaide followed, and there were further outbreaks declared at Port Augusta and in the SA Riverland – making a total 18 different outbreak declarations since December 2019.

Sixteen of those, including all in metropolitan Adelaide, have now been successfully eradicated. But the two remaining outbreaks of Queensland fruit fly in the Riverland are still of concern – in no small part because this region is a horticulture hotspot.

According to the federal Department of Agriculture, Water and Environment (DAWE), horticulture is Australia’s third largest and fastest-growing agricultural sector. DAWE says Australia has long enjoyed “a domestic and international reputation as a producer of premium safe food.”

It also says that fruit flies are the world’s most economically significant plant pest for horticultural commodities, impacting production, domestic trade and international market access.

The fly in situ

Approaches to controlling fruit fly can vary depending on where you are in Australia.

“Australia is a hub for fruit fly biodiversity, especially in the north,” explains Katharina Merkel, an entomologist at PIRSA. “We have over 100 fruit fly species in Australia and not all of them are pests.”

Fruit flies do have useful ecological roles, Merkel says. For example, some species act as pollinators. The fly larvae can also be a source of nutrition and protein for other animals in the ecosystem.

“In areas where they are native, you wouldn’t want to try to eradicate them, but I think you can still try to control them in your agricultural landscapes,” Merkel says.

“We have over 100 fruit fly species in Australia and not all of them are pests.”

Katharina Merkel, PIRSA

The Queensland fruit fly (Bactrocera tryoni) is, as the name suggests, native to Queensland and northern New South Wales (though still controlled in designated Pest-Free Areas within those states). Mediterranean fruit fly (Ceratitis capitata), meanwhile, is an introduced species currently found mainly in Western Australia.

In regions like South Australia and Tasmania, where neither fruit fly species is established, there are strong incentives to keep them out entirely and to eradicate them when they appear. Not only can fruit flies ruin your backyard veggie garden or fruit tree crop, but there are significant implications for local agricultural industries.

“The citrus industry in South Australia exports anywhere between $80 million [and] $130 million, depending on the year, out to other countries every year,” says Secombe.

“Most of that comes out of our Riverland, and we’re able to say, ‘we don’t have fruit fly, we don’t need to spray it, we don’t have to put these chemicals on it’.

“If we do get outbreaks, it’s an additional cost of that to put through that market as well.”

The value of the South Australian horticultural industry at risk from recent fruit fly outbreaks was put at $1.3 billion by the state’s Minister for Primary Industries and Regional Development David Basham in December 2021.

He added that the outbreaks had been threatening 37,500 local jobs, 4,000 businesses and thousands of livelihoods.

The science of fighting fruit flies

The first step is monitoring: you need to know how many fruit flies are around, and where they are. This can include checking fruit for larvae and trapping adult flies.

Monitoring traps contain both a ‘lure’ to attract the flies – for example, by mimicking fly pheromones – and a toxicant to kill them once they land.

Once an outbreak has been established, there are several complementary strategies that can be used to try to stamp it out. Restrictions on fruit movements, such as those imposed in Adelaide, are one.

“We do a lot of work with hygiene,” says Merkel. “If we know that an area is infested, we can remove host fruit from that area by removing all the fruits from the ground and picking them up as soon as they arrive to avoid the flies fulfilling their full life cycle.”

More traps can be set up to bait the fruit flies and bring numbers down. Like the monitoring traps, these contain both a lure and a toxicant, but the targets are slightly different.

For example, the monitoring traps target male flies because they can be lured from further away. Population control traps use a protein- and sugar-based bait to attract both males and females (in fact, Merkel says that female flies tend to show a higher response to this bait, since they require more protein for their maturing eggs).

You need to know how many fruit flies are around, and where they are.

The fly bait is paired with a toxicant called spinosad, which is an insecticide derived from compounds produced by an environmental bacterium, Saccharopolyspora spinosa.

“Spinosad works through neural mechanisms. It binds to a receptor and attacks the nervous system of insects,” Merkel explains.

“Using it in combination with baits is what makes it so much safer. It’s already organic, and we avoid non-target effects by mixing it with baits that will actively attract fruit flies.”

Putting fruit flies on birth control

Sterile insect technique (SIT) is another important tool. It has been used to successfully control both Mediterranean and Queensland fruit flies in New South Wales, South Australia and Western Australia, according to DAWE.

In SIT, flies are raised in captivity and exposed to x-rays, which cause DNA mutations that render the flies sterile. The sterilised flies are released into the outbreak area in large numbers. When they mate with the wild flies, no viable offspring are produced.

Interestingly, it’s a technique that’s much more efficient if the sterile flies are male.

Male flies can mate very often, Merkel explains. Producing sterile females is inefficient, because a wild male may mate with a sterile female but then produce offspring by mating with another, wild female straight away. All that work you put into raising, sterilising, transporting and releasing the sterile female is moot.

“Therefore, it is a huge economic saving if we can produce single-sex strains, especially at an early stage [in the life cycle].”

The sterilised flies are released into the outbreak area in large numbers.

A facility in Western Australia producing sterile Mediterranean fruit flies is actually raising a strain with a sex-dependent temperature-sensitive lethal gene, which causes female larvae to die when exposed to high temperatures.

However, SIT isn’t a cure-all. For one, it works best when the wild populations are low enough for reproduction to be effectively disrupted by the sterile flies. Even where that’s true, there are challenges in how to use SIT most efficiently.

“One of the limitations currently is the amount of data you can work with in a really fast time,” says Merkel. “I would hope that in the future we can use better data collection and models to help us to make even faster data-driven decisions of when to use SIT in combination with other tools.”

Sterile Queensland fruit flies in a petri dish. Credit: PIRSA.

Another challenge is reliably and easily distinguishing between wild and sterile flies when they’re caught in traps.

Currently, sterile flies are marked with a fluorescent dye before release. But this means that each fly caught in PIRSA’s traps has to be looked at under a microscope to be identified as wild or sterile. In 2021, that meant inspecting more than a million individual flies.

Future ID techniques under consideration include an isotope analysis based on captive-bred flies’ diet, and using the same temperature-sensitive lethal gene that scientists use to control sex ratios as a genetic marker.

However, all these techniques work to specifically identify the sterile flies, which is the opposite of what you really care about for monitoring purposes. Ideally, you’d want to find a marker that indicates how many wild flies are hanging around. Bonus points if it can be used without having to physically look at a million individual flies under a microscope.

Harnessing nature’s weapons

Some researchers are looking to nature for new fruit-fly control methods. Merkel describes such biological control mechanisms as working through a “natural enemy”.

One such enemy of fruit flies is parasitoid wasps, which lay their eggs inside the fruit fly at its egg or larval stage. The wasp larva then slowly consumes the developing fruit fly larva, eventually killing it and emerging as an adult wasp.

The wasps are generally quite specific to their host fruit flies, making their use a highly targeted strategy. Parasitoids that target fruit flies are widespread in Queensland and northern New South Wales, where flies are endemic.

In 2021, Agriculture Victoria launched a research project with help from researchers at Queensland University of Technology (QUT) and DAWE to trial raising and releasing parasitoids in Victoria and southern New South Wales. If it’s successful, parasitoids might one day be introduced as a control technique in South Australia as well.

Another option could be “mycoinsecticides”: fly-killing fungi. These infect the fruit fly directly through its cuticle – the outermost layer of its body. They are often more specific than chemical insecticides, Merkel says.

Another option could be “mycoinsecticides”: fly-killing fungi.

Mycoinsecticides can be applied to the soil, where they can attack fruit fly larvae that pupate underground. Adult flies can be targeted by luring them to a station containing fungal spores, which attach to the cuticle and can then be transmitted from fly to fly, especially during mating.

A research collaboration between DAWE and the Western Australian Department of Primary Industries and Regional Development is currently investigating the use of such fungi to target Mediterranean fruit fly.

We wish them gone

Controlling fruit fly in Australia requires both a lot of science and a lot of resources. DAWE reports that the federal government has committed $30 million over three years to support fruit fly management capability around the country.

“Securing long-term, sustainable funding for the management of fruit fly nationally will require national agreement across all states, territories, government and industry groups,” a DAWE spokesperson said.

Further research into innovations like biological control and more efficient outbreak data collection will also help.

While it might seem an inconvenience to be told where you can and can’t take your fruit, considering what’s at stake, it was a small price to pay.