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Skeleton weed is a serious problem which can significantly reduce grain crop yields. Together with the Department of Primary Industries and Regional Development (DPIRD) we've released a tiny midge fly at field sites around the Western Australia grain belt.

It's a safe, specific and targeted biological control (biocontrol) agent for skeleton weed.

A sneaky weed

Skeleton weed, Chondrilla juncea, is a tough, deep-rooted perennial weed that originated in Europe, the Mediterranean region and south-west Asia. The plant looks a bit like a dandelion. It starts out as a ground-hugging leaf rosette and then sends up tall flowering stems with yellow, daisy-like flowers.

The stems are leafless so they look like the skeleton of a plant. There are three different genetic forms present in Australia (narrow, intermediate and broad leaf forms).

It was first recorded near Wagga Wagga in NSW in 1917. By the 1950s, it was considered Australia's worst weed for agricultural production.

Natural enemies are best

We worked on this weed for more than 30 years, looking at the natural enemies of this weed in both its introduced and native environments.

We identified three targeted biocontrol agents, including the gall midge Cystiphora schmidti, and a rust fungus, Puccinia chondrillina. These were tested and released in Australia's eastern states in the 1970s. They significantly decreased the density of the narrow leaf form of skeleton weed.

One of the benefits of the midges is that they can damage all the different types of skeleton weed present in Australia.

The midges look like little mosquitoes, and the adults only live for 24 hours.

Female midges lay up to 200 eggs on the underside of the leaves or on the stem. This irritates the weed which forms a growth or gall around the eggs. The larvae then feed on the leaf material.

The adult midges emerge after 20–50 days. The empty galls then become necrotic, killing the surrounding leaf tissue. When affected by multiple galls, whole leaves can die. This reduces the growth of the weed.

The midge, the whole midge and nothing but the midge

Fast forward to now and we're working with DPIRD to introduce the skeleton weed midge to the West Australian grain belt.

Step one was to find some midges in Eastern Australia! Mick Davy is our midge mission controller. He traveled from Western Australia to Canberra to join his Canberra colleagues collecting midges from skeleton weed populations around Cowra in NSW.

"It's amazing to think these are the descendants of the midges released in the original biocontrol program decades ago," Davy says.

It's not all smooth sailing though. In NSW there's a tiny wasp which parasitizes the midge larvae. Fortunately, this wasp hasn't been found in Western Australia. So, the team had to make sure the midge colony had strong biosecurity practices and there weren't any hitchhiker parasites.

"We had to make sure we were working with clean colonies for field release," says Mariana Hopper, our Environmental Biosecurity Lead.

"It was an added step that good biocontrol tries to implement so that you don't accidentally bring in the enemy of the enemy of the weed."

Davy and colleagues checked every single adult under a microscope.

"We did find one or two parasites. They're very obvious little members of the wasp family with beady red eyes," Davy says.

Flying in the tiny flies

The midges didn't get their own seat on the plane, but they were given first-class treatment.

"I had them on the plane, and I held the little box the whole way," Davy says.

"I was too scared to put them up in the overhead locker in case I left it up there and someone ended up with an unexpected box of midges," he said.

The insects were brought into WA in accordance with biosecurity regulations and were checked by Quarantine WA when the plane landed. The packaging was also clearly labeled so there was never any concern it would end up in lost luggage!

Midge mission success

Hopper says the field release for this new biocontrol agent in Western Australia has been a highlight.

"It's been a while since there was a field release like this in Western Australia, so it's been really exciting," Hopper says.

This project was also the perfect sequel to the original releases of the midge in Eastern Australia.

"The team did so much detective work, tracking back through archival material. We found the original letters between department heads on the initial release programs. It was just amazing to tap into the original CSIRO release program," she says.

An unexpected benefit of the project has been the increased East-West collaboration, bringing teams closer together on this shared project.

"The collaboration on this project helped us all learn from each other's skills and capabilities," says Gavin Hunter, our Plant Health Diagnostics Lead.

The team talked to CSIRO Honorary Fellow, Jim Cullen, who worked on the original release program.

"We were so fortunate that we could consult with Jim and find out where would be a good area to go and collect these insects in the field," Hunter says.

There's no place like bone

The midge was released in spring 2023 at four sites around Narembeen and one in Dandaragan. The site near Dandaragan was in a location where skeleton weed has been virtually impossible to eradicate for decades due to the difficult terrain.

"This first year of monitoring is just to see if it can survive, basically," Davy says.

The good news is that the midges have not just survived, they're actually thriving! They've spread at three out of five field sites, moving up to a kilometer!

"It's been a really interesting project, and we're so happy to have success at this first field release attempt," Davy says.

We and DPIRD will continue to monitor the establishment, impact and spread of the gall midge from the release sites.