Cosmos
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By Cosmos
Skeleton weed is a long lived perennial with a reputation as a serious problem which can significantly reduce grain crop yields.
Together with the Department of Primary Industries and Regional Development (DPIRD), Australia’s national science agency, CSIRO, has released a midge at field sites around the Western Australia grain belt as a biological control agent.
CSIRO’s extensive research confirmed it’s a safe, specific and targeted biocontrol.
A sneaky weed
Skeleton weed, Chondrilla juncea, originated in Europe, the Mediterranean region and south-west Asia. The plant looks a bit like a dandelion. It emerges in autumn and winter as a ground hugging leaf rosette and then in spring and summer sends up tall flowering stems with yellow, daisy-like flowers.
The stems are mostly 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
CSIRO has worked on this weed for more than 30 years, looking at the natural enemies of this weed in both its introduced and native environments.
It 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 midge 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 24-48 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 and developing larvae. The larvae then feed on the leaf material from inside the gall.
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..
The midge, the whole midge and nothing but the midge
CSIRO is working with DPIRD to introduce the skeleton weed midge to the West Australian grain belt, particularly in the Narembeen and Yilgarn shires. These areas are in control rather than eradication zones and where the broad leaf type is dominant.
Step one was to find some midges in Eastern Australia! Michael Davy, CSIRO midge mission controller, travelled from WA to collect 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.[MD1] . So, the team had to make sure the midge colony had strong biosecurity practices and there weren’t any hitchhiker parasites.
Davy and colleagues checked every 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,” he 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.”
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 labelled so there was never any concern it would end up in lost luggage!
A skeleton weed gall midge female © CSIRO
There’s no place like bone
The midge was released in spring 2023 at four sites around Narembeen and one in Dandaragan, in WA. 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 established in the field. [GH2] [ID3] They’ve spread at three out of five field sites, moving up to a kilometre!
CSIRO and DPIRD will continue to monitor the establishment, impact and spread of the gall midge from the release sites.
A version of this article first appeared at CSIRO news
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