Scientists believe they have identified what is killing tens of thousands of hectares of pasture in Queensland.

Key points: Researchers are linking pasture dieback with a destructive mealybug species

Researchers are linking pasture dieback with a destructive mealybug species The lead researcher on the project is warning against the use of chemicals to control the mealybugs

The lead researcher on the project is warning against the use of chemicals to control the mealybugs It is believed a number of factors have contributed to the outbreak of mealybugs

They say outbreaks also occurred in the 1920s and 30s.

When dead patches of grass began creeping over the landscape, the cause was a mystery. But researchers have since found tiny sap-sucking mealybugs at every site of pasture dieback that has been investigated.

In the past two weeks, Australia's leading entomologists have confirmed that the common link is a destructive mealybug species called Heliococcus summervillei.

The species is named a after the late Sir William Alan Thompson Summerville, who first discovered and comprehensively documented the species in the 1920s.

Entomologist Sir William Summerville first identified and documented the destructive mealybug. ( Supplied: Queensland State Archives )

Queensland University of Technology microbiologist Caroline Hauxwell is leading research into the insect, which can be spread hundreds of kilometres by air and survive nearly one metre underground.

"We find the mealybug consistently in [pasture] dieback [and] we've identified it as a single species," Dr Hauxwell said.

"We have reared it in the laboratory [where] we killed buffel grass and, if we kill the insects, the grasses can recover."

With such a vast area affected — from the New South Wales border to north of Townsville and west to Roma — Dr Hauxwell warned against using chemicals to control mealybug infestations.

"We can't use pesticides; they are non-registered and there are no emergency permits for pesticide use," she said.

"We need a long-term perpetual solution for this, not band-aids that will disrupt other pest systems in pasture.

"Pesticides are not the solution for this one; for large-scale control, it's not appropriate."

Newly hatched mealybugs are microscopic and extremely hard to detect. ( Supplied: Andrew Dickson QUT )

Mealybugs have a waxy covering protecting them being wet by sprays. They can also survive cold and dry conditions buried in the soil to emerge in spring and re-infest pastures.

Breeding predator insects

Dr Hauxwell said breeding beneficial insects as biological controls could have potential in the battle against pasture dieback.

Heliococcus summervillei has natural predators in parasitic native wasps, lacewings, and some ladybeetles.

"We're working with the Queensland Museum to identify those species and hopefully, we will be looking at their distribution and their biology to see if they can be cultured at scale," Dr Hauxwell said.

"We know that there are a couple of companies who do that large-scale rearing of natural enemies.

"They're very interested in what we find and will certainly jump in if we find anything substantial."

The tiny white spot on this piece of grass is a destructive mealybug nymph, showing how hard they can be to find. ( Supplied: Ray Morgan, Caroline Hauxwell QUT )

Dr Hauxwell said she had also been sent samples of dying grass by the NSW Department of Primary Industry, but the symptoms were "not right for dieback as we see it in Queensland".

"The things that can also kill pastures — pathogens, aphids, ground pearls, leaf hoppers — haven't gone away. There are still things popping up as they always have and graziers get very anxious, as they would.

"We keep an open mind," she said.

"I'm not going to argue down any line of research that has a sound evidence base and the evidence is still out on that particular NSW outbreak."

The dieback is destroying precious pasture. ( ABC Rural: Jennifer Nichols )

Some of Australia's brightest minds are collaborating with the Queensland University of Technology on pasture dieback.

These include scientists with Meat and Livestock Australia, Biosecurity Queensland, Australian National University, the University of Sydney, and the New South Wales Department of Primary Industries.

Mealybugs from history informing modern research

Heliococcus summervillei was first documented as causing pasture dieback in 1926, in paspalum grass at Cooroy and Kin Kin on the Noosa hinterland.

It was also observed on the Atherton Tablelands in the 1930s.

The mealybug samples Summerville collected all those decades ago have proved a valuable reference for scientists who found them after a search of the insect archives at the Department of Environment's Ecosciences Precinct in Brisbane.

Juvenile mealybugs are microscopic and the insects can be lethal to pasture in low density — the first thing most graziers notice is grass turning yellow with purple streaks.

The first signs of pasture dieback are grass turning yellow with purple streaks. ( Supplied: Ray Morgan )

In 1998 a mealybug outbreak caused a major pasture dieback in New Caledonia, where natural predators eventually got it under control.

Dr Hauxwell suspects dieback has been moving steadily through Queensland pastures over the past 20 years, but she said the problem had accelerated after Cyclone Debbie in 2017.

"That's consistent with how mealybugs spread; they are blown on the wind and can get sucked up to very high elevations in storm fronts," she said.

"They can literally rain down across the landscape and start new populations."

Why has the outbreak occurred?

She said a combination of changes could have led to a boom in mealybug numbers, including the introduction of pasture species like buffel and rhodes grass that now dominate pasture landscapes.

"It's quite possible that the mealybug has made a jump into some of those species," she said.

"It may have acquired gut symbiont as part of its gut microbiome that has allowed it to expand its range [or] it may be that the natural enemies are in decline.

"We're seeing problems with insect decline across the board [or] it may be that heat shock is killing or preventing the reproduction of parasitoid wasps."

MLA's Doug McNicholl says the drought is making it hard to determine how much land the mealybug has affected. ( Supplied: Doug McNicholl )

A reduction in the diversity of pollen and nectar sources supporting the adult wasps while they are hunting mealybugs could be another contributing factor.

"There are a number of different explanations and we'll probably never really know why," she said, stressing that a multi-pronged approach to research with analysis of potential plant pathogens like fungi, bacteria, and phytoplasmas be continued.

Meat and Livestock Australia's (MLA) manager of supply chain sustainability innovation, Doug McNicholl, said improving detection methods was an important part of their research program.

He said drought made it even harder to identify just how widespread pasture dieback was.

"It's very difficult — particularly when, at this stage, we're heavily reliant upon people reporting dieback," Mr McNicholl said.

"When you're in the middle of unprecedented drought, you really wouldn't know what you're dealing with unless you have a trained eye.

"I would love to have a silver-bullet solution, but the reality is at this stage we don't.

"The first steps are contacting Queensland Department of Agriculture and Fisheries, the NSW DPI, or MLA and registering your sighting."

No answers for graziers

MLA is investigating natural enemies, mealybug-resistant grass varieties and endophytes — organisms, often fungi and bacteria, that live inside plants.

New Zealand researchers have been working on endophytes that live within rye-grass and prevent mealybugs from eating the plant.

Recently moulted adult female mealybugs are pink. ( Supplied: Ray Morgan, Caroline Hauxell QUT )

"It lives in a symbiotic relationship so it looks after the plant and the plant looks after it," Mr McNicholl said.

"We know that, generally speaking, mealybugs are deterred by plants that have an endophyte population but that's a long and slow burn, developing endophytes technology for Queensland or northern NSW conditions.

"We don't have conclusive evidence to say this particular pasture variety's more resistant than another.

"Resistance studies are part of our program of work at the moment but, at the same time, we need to ensure that those varieties lend themselves to red meat production as well."

Are some pastures more resistant than others?

Gympie grazier Mick Seeney, president of the Gympie District Beef Liaison group, said this year pasture dieback on his property had not been as bad as last year.

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"I've had paddocks where the grasses have come back but the dieback's still there and affects a proportion of pasture that we could be using for grazing," he said.

"I've heard of some people who have lost 80 per cent of their pasture, and that has a huge effect on their grazing ability."

Anecdotally, Mr Seeney believed that introduced species of softer, more palatable grasses were suffering more than natural pasture in his region, but he said graziers were hamstrung by a lack of knowledge.

"There's nothing worth doing until we get a plan to control it, a definite plan," he said.