Parthenium weed can produce 100,000 seeds from a single plant. This beetle has a big job to do

Australia has been infiltrated by six-legged foreign agents, but instead of spying for the enemy, these beetles are working to defeat invasive weeds and protect the Great Barrier Reef.

In the 1980s the federal government started recruiting a sleeper cell of insects and fungi from Central and South America to assassinate one of the world’s hardest-to-kill pest plants, parthenium weed.

Originally from North America, the annual herb takes over where native plants are weakened, is resistant to herbicides, toxic to animals, and causes allergic reactions in humans.

And while it’s unlikely to ever be completely eradicated, what is now one of the country’s longest-running biocontrol programs is keeping the enemy at bay, even as recent weather conditions once again put it to the test.

The enemy

It took two attempts for parthenium weed to gain a foothold in Australia, but once it arrived its effort to spread was relentless.

First found – and contained – in 1940, it arrived again in 1958 on contaminated seed from the United States used in Clermont, central Queensland.

Growing up to 2 metres tall and producing 100,000 seeds from a single plant, it quickly invaded other Queensland regions and into northern New South Wales, more than 800 kilometres away.

“The amount of seeds in the soil is so high it’ll keep coming back over the next 10 to 15 years,” Biosecurity Queensland senior principal scientist Kunjithapatham Dhileepan said.

This was no benign invasion.

In central Queensland parthenium weed hosted tobacco streak virus which spread to oilseed crops effectively ending the local sunflower industry.

Dr Dhileepan says it also produces a toxin that poisons and stunts other plants, clearing a path for it to spread further.

And while it can be killed by herbicides, the dosage required not only makes it too expensive to spray entire paddocks, it also poses a risk to the nearby Great Barrier Reef.

So with no chemical option to combat it, and a looming environmental and agricultural disaster if it continued to spread, authorities turned to biocontrol.

The fixer

Australia’s track record on using introduced pests to combat other pests is patchy at best, but there have been successes.

In 1975, a program was established to investigate what could work against parthenium, and it remains one of the longest-running programs of its kind in the country.

Seeking to avoid the mistakes of the past, like the introduction of cane toads, the program observed potential control species in their habitats in Central and South America before selecting its recruits.

After import permits were issued by the federal government, the species were kept in a high quarantine facility where they could not escape, and rigorously tested before controlled field trials.

“It has to be approved by every state because once you put an agent in one state it will go everywhere,” Dr Dhileepan said.

Between 1980 and 2004, 11 different species of insect and two rust fungi were released into areas impacted by parthenium weed.

In true secret-mission style, each of the agents brings a unique skill set to the fight, by targeting different parts of the plant.

For example, there’s a moth that eats the stems and its larvae create a cancerous growth that stunts the plants’ growth, while a weevil targets the seeds and a beetle eats the leaves, supported by the rust fungi that also attacks the leaves.

So far there have been no double-agents that have turned invasive.

The handler

Now under Biosecurity Queensland’s jurisdiction, the insects self-sustain their population.

And like any good spy, they need handlers to direct their missions.

Among them is Central Highlands Regional Council, which works with landcare groups and neighbouring regions to deploy the bugs.

But first they need to find, and awaken, the sleeper cells.

“The zygogramma [beetle] is a little bit unpredictable where it’s going to turn up,” rural lands officer Susan Walters said.

In the past 18 months, drought followed by big wet seasons provided the perfect conditions for the weed to spread, prompting the council to restart a program to distribute the leaf eating beetles and rust.

“Last year it wasn’t really at the place where we found it [before],” Ms Walters said.

“We’ll try and collect a little bit more of it and move it around … it would be nice if we can establish more areas around the district.”

The council also swaps its insects with its neighbours, trading its parthenium-killing zygogramma beetles for jewel beetles, skilled in the eradication of another invasive weed, the cat’s claw creeper

“I think we can all work together and try and combat it and try and contain it,” she said.

The Queensland government estimates parthenium costs Australia’s beef industry $16.5 million per year and cropping industries several million dollars per year.

The battlefield is pastures like that of grazier David O’Connor, who has been dealing with a major outbreak on the central Queensland property he bought 17 years ago.

Farming in a Great Barrier Reef catchment area, using biocontrol means less chemical runoff in the waterways.

“It just really wasn’t feasible to spray it because we had so much of it,” he said.

Instead Mr O’Connor focused on making native grasses and pasture healthier so it could fight of the invasion, supported by the rust and zygogramma beetles sourced from the local council.

“That was really the best option,” He said

“We still have parthenium … but it’s not a problem.”

While no silver bullet is going to take out parthenium weed completely, Dr Dhileepan said the impact of biocontrol has been significant.

Not just halting the spread, parthenium weed plants have become smaller, and less vigorous, and the number of seeds in the soil has reduced.

Now, he said after the success of the program other countries struggling with the invasive weed are turning to this Australian team for help.

“Not all countries can do this sort of research,” he said.

“We will provide all the data that we have and the insects.”