Pasture dieback is a scourge in contemporary grazing, with the income lost running into billions of dollars.
Pasture dieback isn’t new in Australia — it was first observed in 1926 — but despite over 80 years of documented outbreaks, the cause of the recent outbreak has remained elusive.
Researchers at QUT have worked quickly to identify what they believe to be the main cause: a small yet lethal mealybug.
With the support of $1.2m in funding from Meat and Livestock Australia (MLA), the research comes at an important time: as more farmland is lost to pasture dieback, options are needed now to combat the spread.
Associate Professor Caroline Hauxwell and her team are investigating the biology, pathology and natural enemies of mealybugs in the hope that they can find a solution.
From little things…
The particular mealybug causing so much trouble is Heliococcus summervillei, an insect in the Pseudococcidae family. These small, sucking pests grow on and around the affected plants, feeding on sap.
While they’re found in warmer climates, mealybugs can live underground and survive frosts and drought.
The main difficulty for graziers is the insect’s size: at only 0.2 mm and often hidden in soil or plant thatch, they’re very difficult to see.
“This means that the mealybug can set in and start causing significant damage before farmers and graziers realise they’re there, and early symptoms can look like many other causes,” said Hauxwell.
“We’ve found these mealybugs at every site at which dieback is active, and we’ve worked with taxonomy experts at Biosecurity Queensland to confirm their identity.
“It’s the same mealybug that was reported in 1926, was found causing pasture dieback in 1998 in New Caledonia, and has emerged more recently in Barbados and Puerto Rico.
“The other pests and diseases that affect pastures — diseases like kikuyu yellows, Fusarium and other pathogens — are still present, but they don’t kill grass at this scale and with this pattern of spread and symptoms.”
The root of the problem
The exact mechanism by which mealybugs kill grasses is still to be discovered, but there are several clues that Hauxwell is following.
In the lab, Hauxwell’s team has introduced mealybugs to Buffel grass — an important introduced pasture grass — with consistent results: the symptoms produced are congruent with pasture dieback.
“The damage that mealybugs cause to a plant can’t come from feeding on it alone — it takes very few mealybugs to cause damage, and they’re so small the plant wouldn’t even notice the loss of sap from feeding,” said Hauxwell.
“We think that the mealybug is interfering with the plant’s defence systems — either by generating a toxin or from something in their gut microbiome that interferes with signalling pathways in the plant immune responses.
“Pathogens such as fungi or viruses may be dormant within the plant or soil, but once the mealybug is present they can become more active, exacerbating the damage.”
The team is waiting on further funding to support fundamental research into the biology of these interactions, which will provide further insights into potential management.
So, how do we control this pest?
Hauxwell’s team have obtained an emergency permit with MLA for the systemic insecticide Spirotetramat, which reduced mealybug numbers and led to grass recovery in field trials.
“Insecticides aren’t always the answer, though — they’re expensive, and can disrupt beneficial insects like parasitoid wasps, which we need to help kill the pest,” said Hauxwell.
“What’s more, adult mealybugs don’t appear to feed much, and they spend winter in the soil, which limits the window for effective insecticide application.”
Beyond pesticides, Hauxwell’s team are investigating sites that aren’t affected by dieback to identify any protective factors that may exist in the environment.
Endophytic fungi — a type of fungus within the plant that prevents feeding or kills mealybugs in temperate grasses — provide a possible avenue to control the insects. QUT researchers are working with plant breeders to identify endophytes that can stop the mealybug and protect tropical grasses.
Evidence also points towards more diverse pasture use providing a natural buffer against mealybugs.
“Forage brassicas and legumes are not susceptible to this mealybug and can provide valuable fodder for cattle,” said Hauxwell.
“The flowers from diverse vegetation also feed parasitoid wasps, which in turn can control the mealybug population, and they may support healthier soil microbiota to protect the grasses.”
Into the future
As the mealybug and pasture dieback spread into northern New South Wales, Hauxwell’s team continues their investigation.
Under the MLA program, the team are working with seed companies and producers of biocontrols, as well as with experts at the University of Queensland, CSIRO and the Department of Agriculture and Fisheries, to share information and improve programs to detect, prevent and treat pasture dieback.
Hauxwell’s team have moved quickly: in just six months, they have developed a robust sampling method for graziers to quantify mealybugs on their farms, identified natural enemies that may lead to long term control, and developed a wealth of knowledge about mealybug biology.
“We don’t yet know why the mealybug and dieback have become so severe in recent years,” said Hauxwell.
“Ecosystems are like Jenga: you can remove some blocks with no impact, but as soon as you pull in the wrong place the whole thing can collapse.”
“For now, farmers need to be aware of the problem and actively looking for mealybug infestations in their land.
“Our research will provide clear guidance on what to look for so they can act early.”
