Across large areas of the Monaro region of NSW, the trees are dying. Over the last decade, the Manna Gums or Eucalyptus viminalis have been gradually declining in health and now stand like skeletons in huge tree graveyards. But what is causing this mass dieback? As an honours student I was given the task of investigating this intriguing mystery, and over the next three weeks I will take you through the tricky detective work that led me to a (possible!) solution.

This post is a summary of my honours research, which I completed at the Australian National University between February 2012 and May 2013. The project was supervised by Cris Brack, and sponsored by Greening Australia who I now work for. This research has been submitted for publication, so when it is available I will link to the paper for anyone interested in further reading. (Edit 8/10/15: the paper is now published and available here)

Part 1 – Crime Scene Investigation

The scene of the crime – The Monaro 

The Monaro region is a high plain extending between Bredbo, Numeralla, Nimittabel and Jindabyne. My first step in solving the mystery was to conduct a road survey of the area, in order to map the extent and severity of the dieback – shown on the map below. The affected area is almost 2000 square km, which is almost the size of the Australian Capital Territory not far to the north. Within this area there was a central region that was severely affected, with most of the trees either dead or very close.

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Map of Monaro region showing road survey sites (grey circles indicate no Ribbon Gum present, coloured circles indicating dieback severity), study sites (labelled 1-8), and dieback extent (outer shaded area) and severely affected area (inner shaded area). Map: Clive Hilliker.

The victim – Manna Gum 

From the road survey it was clear that the dieback was very species specific, affecting only the Manna Gums (E. viminalis) and to a lesser extent Candlebark (E. rubida). Manna Gum is the dominant species in most of the affected areas, and usually occurs in pockets on rocky outcrops and hills surrounded by grazing land. Other co-occurring species were relatively unaffected, including Apple Box (E. bridgesiana), Snow Gum (E. pauciflora) and Black Sallee (E. stellulata). The edges of the dieback seem to be defined by a change in species composition, where the Manna Gum is no longer dominant.

The cause of death  – Dieback 

So what is dieback? The word dieback usually refers to the gradual decline in health of a stand of trees, which can lead to premature death. The first sign of dieback is usually canopy thinning which starts at the branch tips, followed by defoliation, epicormic growth (from the trunk and branches) and dead branches, eventually leading to tree death.

Declining tree health can be caused directly by things like salinity, pollution or disease, but dieback usually refers to a more complicated problem, often due to a combination of factors (Manion 1981). Some suggested causes include agricultural practices (grazing, improved pastures, fertilisation, clearing), altered fire regimes, and climatic effects (warming, extreme events e.g. flood, drought).

Dieback is often associated with insect attack, which can be the main cause or the result of an underlying problem. The epicormic growth produced by the trees is particularly palatable to herbivores, which creates a feedback effect of repeated defoliation and regrowth until the trees eventually exhaust their resources.

The murder weapon – The ‘Evil Weevil’ 

In this case, the dieback has been associated with the eucalyptus weevil (Gonipterus sp.). These small, brown beetles and their bright green larvae have been found in large numbers on the affected trees, and all the remaining foliage shows characteristic damage. Manna Gum is the weevil’s preferred host, and selected trees that have been treated with stem-injected pesticides have shown a significant improvement in health.

Weevil

The eucalyptus weevil is native to eastern Australia and Tasmania, but has become a pest when introduced to Eucalyptus plantations in Western Australia and overseas. This is probably due of the absence of a native parasite which normally helps control the population, and has been used successfully as biological control in some cases. Mapondera et al. (2012) recently identified that there are actually several genetically distinct species of eucalyptus weevil that are indistinguishable from the outside (cryptic), but occur in different climatic regions and have different parasites. Could it be that a weevil from a different region has been introduced or migrated to the Monaro, leaving its parasite behind?

I had the weevil identified by Dr. Oberprieler, an expert on weevils at the CSIRO. In fact, it turned out that it was the local species of eucalyptus weevil, known as Gonipterus sp. no.2 (as yet undescribed). This meant that the weevil was no longer a suspect but merely the weapon, being wielded by a hidden hand – an underlying cause still to be found…

In next week’s installment we will investigate each of the suspects in this intriguing ‘murder on the Monaro‘. 

References:

Manion, P., 1981. Tree Disease Concepts. Englewood Cliffs, New Jersey: Prentice-Hall Inc.

Mapondera, T., Burgess, T., Matsuki, M. & Oberprieler, R., 2012. Identification and molecular phylogenetics of the cryptic species of the Gonipterus scutellatus complex (Coleoptera: Curculionidae: Gonipterini). Australian Journal of Entomology

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