'False crops' trick invasive weed into killing itself

A highly invasive weed discovered in north Queensland has been tricked into triggering its own death.

Red witchweed was found on six sugarcane farms in Habana, north of Mackay, in 2013, the only known detection in Australia.

Scientists say a clever biosecurity strategy of planting "false host plants" has played a significant role in the eradication strategy.

Red witchweed is a parasitic plant. To survive, it must attach itself to the roots of another plant to steal water and nutrients, eventually starving its host.

Michelle Smith, manager of invasive plants at Biosecurity Queensland, said soya beans release a natural chemical into the soil which stimulates witchweed seeds to germinate.

Soya beans have a different root structure, which makes them an incompatible host for red witchweed.

"Without a host connection, the seedlings quickly die," Ms Smith said.

Soil fumigation and herbicide treatments have also been used with success.

Ms Smith said the weed could wipe out grain and cereal crops and had caused multi-million-dollar losses in Africa and the United States.

"It can cause stunting and wilting, and can even cause the death of plants," she said.

"In the space of about four weeks, it emerges from the ground, flowers and releases millions of seeds, and then dies."

Eradication 'within three years'

Biosecurity Queensland said eradication could be achieved within three years.

Eradication efforts started in 2015, with Queensland scientists building on research into "trap plants" that started in South Africa in the 1930s.

The affected area has reduced from about 110 hectares to 4.3 hectares after 11 years of treatment and surveillance.

Ms Smith said it was unknown how the weed arrived in Australia.

"It's a highly invasive plant and its main vector of spread is through infected or infested dirt and mud, so we did a lot of tracing, and we did a lot of surveillance," she said.

Farmers playing important role

Farleigh cane farmers Stephen and Neiola Vella were among the first landholders affected in 2013.

"The first couple of years, it caused us a lot of drama, just trying to get answers on what was happening," Ms Vella said.

While the weed did not visibly damage their sugar cane, quarantine restrictions did.

"We did lose about a quarter of our farming operation that was under quarantine," Ms Vella said.

"We finally got all of that back two years ago, so we're now back under full production for our sugar cane."

Ms Vella said she was grateful to the biosecurity staff who helped the affected land back into production.

"We couldn't have done it as farmers; we wouldn't have had the time. It just would have been impossible," she said.

"We helped where we could with machinery and doing some of the work, but not with the surveillance … we have surveillance people walking around our properties every 10 days sometimes."

Proof-of-freedom plan

The federal Department of Agriculture has approved a proof-of-freedom plan for the final stage of the national eradication effort.

Of the original six properties, four are in the proof-of-freedom surveillance phase, and two are still undergoing treatment.

"When no more plants germinate for several years, we then do three years of surveillance to make sure that nothing actually emerges, that we have actually destroyed all those seeds in the soil."

If no plants are detected in the three-year surveillance period, Australia will be the first country to eradicate red witchweed.

For Ms Vella, that would bring some welcome certainty after more than a decade of involvement in the program.

"It'll be good for all of us affected farmers to know that we're clean."