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Thermal tolerance threshold, potential distribution, and management of pasture mealybug associated with pasture dieback
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Thermal tolerance threshold, potential distribution, and management of pasture mealybug associated with pasture dieback

Parbati Ghising
Southern Cross University
Masters by Thesis, Southern Cross University
2026
DOI:
https://doi.org/10.25918/thesis.588
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Abstract

Pasture dieback Pasture Mealybug CLIMEX Modelling Thermal tolerance
Pasture dieback is a disorder affecting grasslands in Queensland and New South Wales, Australia, and it has immense economic impacts due to reduced livestock feed availability. The pasture mealybug (Heliococcus summervillei) has been implicated as the primary causal agent of this disorder; however, the mechanism(s) by which it induces pasture dieback remain unclear. H. summervillei is a small, soft-bodied sap-sucking insect that feeds on grasses and leads to pasture dieback. The thermal tolerance threshold, potential climatically suitable areas for H. summervillei, and the effects of different slashing heights on reducing H. summervillei and dieback remain poorly understood. To address this knowledge gap, a series of studies was conducted. (1) A water bath experiment was conducted using static and one-way ramping assays to determine H. summervillei's upper and lower lethal temperature thresholds (2) Give an update on pasture dieback distribution in NSW using data from government agencies (North Coast Local Land Services, Department of Primary Industries and Regional Development) and a modelling exercise using CLIMEX software to predict the potential distribution of H. summervillei in Australia and globally (3) Field trial across three sites, Grafton, Alstonville, and Yorklea, was established to assess the effect of slashing at three different heights (2-3 cm, 7-8 cm, and non-slash) on H. summervillei density and dieback severity. The thermal tolerance study shows that H. summervillei’s cold and heat tolerance is method-dependent, with significant differences between the static and dynamic methods. It was found that the upper lethal temperature thresholds and lower lethal temperature thresholds in the static method were 48.41 ˚ C and -5.67 ˚ C, and were 50.17 ˚ C and -8.04 ˚ C in the dynamic method. Looking at the records of pasture dieback spread in Australia, by July 2025, pasture dieback has spread to the North Coast, Hunter, Tablelands and Northwest local land service regions of NSW. From the CLIMEX results, eastern Australia, most of South America, Central Africa, and South Asia are modelled as highly favourable for H. summervillei establishment. The field study showed that pasture dieback and H. summervillei incidence were low across all trial sites, and thus it was not possible to determine whether different slashing heights reduced H. summervillei populations or dieback severity. The thermal tolerance study identified the survival temperature range and threshold temperature for mortality of H. summervillei in controlled environmental conditions. This temperature gives valuable insight into their potential habitat and can be used in modelling potential distribution using software such as NicheMapR, etc. CLIMEX modelling results identified climatically suitable areas for H. summervillei establishment and subsequent pasture dieback occurrence, including eastern Australia, southern Asia, Most of Africa, South America, and southern North America. These CLIMEX findings provide an early warning for areas modelled as climatically suitable for the growth and development of H. summervillei, enabling early monitoring. The results of the field trial were inconclusive, as there was no incidence of H. summervillei and dieback. Overall, findings from this thesis are crucial for industry policymakers, related stakeholders, and farmers.

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