Uncovering the role of mealybugs in pasture dieback

Having spread rapidly in recent years, pasture dieback is an increasing concern for producers in Queensland and northern NSW.

Although the cause of pasture dieback is still unknown, research has indicated that mealybugs are commonly associated with the condition.

In response to this, MLA is using a grant from the Department of Agriculture, Water and the Environment to fund three research projects being conducted by the Queensland University of Technology which are investigating the possible role of mealybugs in causing pasture dieback.

Dr Caroline [Carrie] Hauxwell is an Associate Professor at QUT and leads these projects. Here, she outlines the projects, what has already been determined about mealybugs and their relationship to pasture dieback, and how this research is informing management strategies.

Project one: Determining the role of the mealybug in pasture dieback

There’s strong evidence to suggest mealybugs could be playing a role in the cause of pasture dieback seen in Queensland and northern NSW; however, the issue is likely to be more complex than one single causal agent.

“There are a lot of things out there that kill grasses, but our research into the mealybug Heliococcus summervillei, as well as historical records of its presence in Queensland causing dieback as early as 1926, strongly indicate they might be a working agent in dieback,” Carrie said.

“We’re looking at how mealybugs kill grasses and running tests on possible controls. The treatment will also provide insights into whether the mealybug causes dieback by observing if the grass recovers or not.

“We found that pasture grass in small plot trials recovers and mealybug numbers decline when sprayed with Movento® (spirotetramat), for example.”

Understanding the mealybug biology in project two will be critical to developing management strategies to control the insect and avoiding the financial and environmental impacts of chemical overuse.

Dr Carrie Hauxwell inspecting Urochloa oligotricha affected by mealybug-induced dieback in an insecticide spray trial. The grass in the foreground is untreated, while the grass next to Carrie has been treated with Movento. Image: Queensland University of Technology.

Project two: Understanding the biology of the mealybug and its natural enemies

“This project aims to get a grip on generation times, grass preferences, seasonality and the natural enemies of the mealybug,” Carrie said.

Establishing the life history of the mealybug and under different seasons will support future research on mitigating their impacts.

“We already know that the mealybugs behave differently as adults and as early instars, and may require different insecticides to target different life stages.

“The damaging early instars feed on roots and deep in foliage and require a systemic insecticide such as Movento to manage them, while reproductive adult females don’t appear to feed but do migrate on the surface and may respond to conventional contact insecticides.“

There are currently no contact insecticides registered for the control of the pasture mealybug, but research such as this will be fundamental to developing future management strategies.

Buffel grass affected by pasture dieback. Image: Queensland University of Technology.

Project three: Identifying resistant grass varieties to mealybugs

“We’re looking for grasses that have survived repeat attacks from the mealybug, and also those that are susceptible,” Carrie said.

“So far, no grasses have been found to be resistant, but farmers have reported small clumps of plants and paddocks have been shown to survive attacks, so we are very interested to identify why this is.

“This project also looks into the role of endophytes and doing a proof of concept to show they can impact the mealybug.”

Endophytes are organisms such as bacteria or fungi that can live within plants without causing disease.

“We know when looking at New Zealand pasture dieback, caused by a different mealybug species, that seed-borne endophytes protect temperate ryegrass against a different mealybug species, and want to find out more about their existence in both temperate and tropical grasses here in Australia,” Carrie said.

How can producers manage mealybugs?

While the projects continue to look into mealybugs as an agent of pasture dieback and the control of them, producers can be proactive in identifying their presence and attempting to manage early incursions of them.

Carrie said it’s important to confirm the presence of mealybugs before deciding on any form of treatment, as treating an area without this knowledge can be costly with no benefit.  

“You need to dig in the ground around affected areas and look for mealybugs. Dig deeper if the ground is dry, because they will be down where the moisture profile is,” Carrie said.

“You won’t identify mealybugs just by walking across a paddock. You need to be looking through the soil, and around the thatch, roots and leaves with a hand lens/magnifying glass.

“If you notice a small patch of pasture showing dieback symptoms and mealybugs present, use Movento. It’s expensive but if you only have a small incursion area, research has shown that it can be successfully used to manage them.” It is important to seek advice before spraying.

The projects will continue to research long-term management strategies for mealybugs including the establishment of naturally resistant grasses and natural enemies.

Identifying the potential causes of pasture dieback is a priority research area for MLA so that control and management strategies can be developed.