Cheap $9 yabby nets might be the unlikely weapon WA horticulturalists need in their fight against Mediterranean fruit fly (Medfly).

A $3.9 million eradication program at Carnarvon's horticultural area, 900km north of Perth, is experimenting with the use of the inexpensive blue yabby nets to release pink, inbred and irradiated Medflies.

It is regarded as one of the most destructive agricultural pests worldwide, and in Australia it is only found in Western Australia.

Medfly has been a pest in WA since its incursion in 1895, and it is a big issue for the Carnarvon region, which produces $100 million worth of fruit and vegetables each year.

But WA Department of Primary Industry and Regional Development (DPIRD) researchers are using the 'Sterile Insect Technique', releasing millions of infertile male flies into the wild to compete with wild males and unsuccessfully breed with females, thus decreasing the population over time.

A bright pink food dye is applied to the flies, so staff can identify them with ultraviolet light in monitoring traps, and easily calculate their survival rate and impact on the wild population.

A simple approach

DPIRD technical officer Brett Renton is among the staff who maintain the program and release the flies.

He said winter was the key period to get on top of the fly population ahead of the coming growing season for widely grown crops such as mangoes and bananas, when the Medfly population explodes.

"Each time a wild fly breeds it can have 10 or more offspring, we need to reduce the population by about tenfold each generation just to keep up with natural replenishment, so we need to get 10 times as many flies out," Mr Renton said.

"If there's a million flies in an area we need to get 10 million flies out in an area to try and out compete them."

The traditional approach is to house the pink pupae in sea-containers at a nearby research facility.

Adult flies emerge from their pupae shells after a week of incubation.

They are then released from the back of a ute when they're at full strength.

Traditionally, the sterile Medflies are kept in a research facility for a week, and are released as adults. ( ABC Rural: Jon Daly )

Mr Renton said this method could release five to six million flies a week but was time consuming, labour intensive, and unable to release the 10 million flies required.

"In order to get twice as many flies out we need to be a bit more inventive," he said.

Mr Renton said about 50,000 fluoro-pink Medfly pupae can be poured into one yabby net, and hung from a host fruit tree, leaving the insects to take flight in their own time.

These blue yabby nets retail for about $9 each.

"This method gives them a bit of protection, gives them good airflow, [and] they're inside a net, so the birds and ants don't get to them because they're hanging in a tree," Mr Renton said.

"They will have to fend for themselves a little bit earlier.

"That being said, there's not much handling of the flies, [and] each time we handle the flies there is a chance of damaging or killing them, this way we can be a little bit more hands-off."

However, it is early days, and Mr Renton needs to gather data on the efficacy of this approach before it can be used more broadly.

Pink, sterile Medfly pupae are shipped up to Carnarvon from DPIRD's South Perth facility. ( ABC Rural: Jon Daly )

Irradiated, inbred flies

Even though they are pink, irradiated, and inbred, these Medflies are not the product of genetic modification.

DPIRD acting manager of plant biosecurity Darryl Hardie said a breeding line of all male flies had been originally created in the 1970s.

As a biological control method, the Vienna 7 strain was bred to combat Medfly.

The Vienna 7 colony is still maintained in DPIRD's South Perth facility.

Dr Hardie said the strain was created using inbreeding to create recurring genetic mutations.

"[We] got this other gene called 'temperature sensitive lethal gene'," Dr Hardie said.

That means all the females die when the pupae are exposed to temperatures above 34 degrees.

The surviving males are made infertile by an x-ray machine and sent to Carnarvon for release.