Climate changing for bug battle in Western Sydney
Climate and environmental change could be giving some insects the advantage over trees – resulting in vast tracts of woodlands with mature eucalypts losing leaves, destroying ecosystems and turning leafy suburbs brown.
Dr Markus Riegler and his team from the University of Western Sydney are looking to expand their recently published research which found the complex relationship between eucalypt eating insects and their host trees are particularly affected by climate change, to examine a devastating die back of gum trees in Western Sydney.
A psyllid outbreak in Western Sydney has attacked hundreds of hectares of mature eucalypts in parks, reserves and backyards from Blacktown to the foot of the Blue Mountains. The affected trees are all of the species Eucalyptus moluccana (commonly known as Grey Box), a dominant tree species of the endangered Cumberland Plain Woodland Community.
The numbers of psyllids of the genus Cardiaspina, small sap sucking insects, have exploded on E. moluccana in the past couple of years causing leaves to discolour and trees to appear dead.
“Whole suburbs are affected,” says Dr Riegler, from the UWS Hawkesbury Institute for the Environment. “As spring brings a fresh flush of green to other plants, the affected eucalypts remain a stark brown stain across the landscape.”
The Cumberland Plain Woodland communities have already been heavily impacted by urban development and only about 5 percent remain in the Sydney region.
Dr Riegler believes recent extreme dry and wet climate cycles in combination with warmer temperatures may have contributed to the psyllid population explosion and weakened trees’ abilities to fend off the attack.
“The psyllid outbreak in Western Sydney could be an example of the unexpected consequences of climate change on native ecosystems, but until we do more research on this insect we won’t know.”
Last month, Dr Riegler and colleagues published unique research in Oecologia which measured the interactive effects of simultaneous changes in atmospheric CO2 concentrations and temperature on insect development and behaviour over the entire life cycle of tree-feeding insects.
“Eucalypt eating insects may be particularly sensitive to climate-driven shifts in plant chemistry, as eucalypt foliage is naturally low in nitrogen-containing compounds such as amino acids and proteins –key nutrients required by insects,” says Dr Riegler, an expert on insect biology.
“Increased carbon dioxide in the atmosphere increases the carbohydrate content in leaves which dilutes the available nitrogen in leaves. The insects have to eat more leaves to gain the same nitrogen which places greater stress on the tree.”
Dr Riegler’s experiment, funded by an Australian Research Council grant, exposed Doratifera quadriguttata, a moth whose larvae feed on eucalypts to different temperatures and concentrations of carbon dioxide. The larvae were housed in glasshouse chambers which also contained eucalypt seedlings under different CO2 and temperature conditions.
“Both the insects and trees lived in time capsules which mimicked climatic conditions past, present and future,” says Dr Riegler.
Overall the insects were found to perform best on foliage grown in the pre-industrial environment where CO2 levels were lower. The elevated CO2 environment triggered more feeding by insects as the leaves were less nutritious. However, the researchers found higher temperatures interact with and dampen the negative effects of elevated CO2 by changing the developmental times of the insects. So for this insect at least, rising temperatures combined with elevated CO2 may reduce its impact on the trees.
“This is the first time we’ve been able to establish interactive climate change effects on insects that feed on tree foliage,” says Dr Riegler.
“Studies like ours open the door to a better understanding of the complex impact of climate on the interaction of insects and plants. Overall we still know very little about interactive responses of multiple species and trophic levels in ecosystems to climate change.”
Dr Riegler says running similar studies on the psyllid outbreak in Western Sydney may reveal the cause of the sudden shift in advantage to the insect and provide avenues for a solution.
“It’s important we don’t leap at temporary solutions such as chopping down all affected trees. Not all mature trees are dead, and they remain an essential part of the local ecosystem for birds, small mammals such as possums and other, beneficial, insects. If we suddenly clear Western Sydney of large trees the ecosystem could be irreparably damaged – a local impact that’s potentially far greater than climate change.
“Planting seedlings of other local eucalypt species may be an option. This would help to establish a healthy tree canopy that should then eventually be reverted back to the typically Grey Box dominated Cumberland Plain Woodland communities,” he says.
31 January 2013