Australian longicorn borers are known worldwide for their ability to aggressively infest eucalypt plantations. In Australia, too, longicorn outbreaks have struck plantations in Tasmania, Victoria and Queensland. By contrast, outbreaks of longicorns are rare in native eucalypt forests. Why? Well, it’s not clear, but the current theory is that to be successful, longicorn populations must develop in forests that are persistently drought-stressed. In plantations, a single species is often planted over a varying landscape that is, in some cases, too dry. Native eucalypt forests, however, generally comprise numerous species leading to a patterned landscape of species that is more resilient to stress.
But what happens when the native forest landscape is made up of only one species?
We now know that snow-gum dieback is widespread throughout Victoria, NSW and the ACT. We also know that the phenomenon is the product of infestation by ‘ring-barker’ longicorns (Phoracantha mastersi). That the affected landscape is made up of only a single species (Eucalyptus pauciflora), albeit one with numerous subspecies, suggests that knowledge from mon-specific plantations may be useful in understanding snow-gum dieback.
So what is it about drought that is so attractive to longicorn borers? Overseas, it appears drying bark plays an important role in affecting the survival of newly hatched insects of the so-called ‘eucalypt long-horn’ (Phoracantha semipunctata). That is, the larvae appear to drown in wet bark. Great, so in a year in which it just keeps raining snow-gum dieback should stop, right?
Unfortunately, our snow gums now hold several generations of insects, some only 12 months old, others 24 months and others still are ready to emerge as adults. That is, we might need three consecutive good years to knock them off. The fact is, we don’t know how long it might take. More fundamentally, we don’t know whether drying bark is the driving factor. That is, we don’t know whether the trees have been drought stressed over the last decade, whether the bark is actually drying down during the period in which newly hatched larvae are active, nor whether ring-barker borers are sensitive to bark moisture content in snow gum.
That is, we need to understand the behaviour of the insect, variation bark moisture content and the factors that affect it, and long-term evidence of tree moisture status.
Obviously, we have a long road ahead.
This year sees our team back out in the field, surveying sites, watching beetles, running experiments and collecting samples. One experiment has been running since April of 2021. That work focuses on monitoring high-resolution variations in bark thickness and growth. Using automated tree dendrometers deployed across six sites from the Perisher Valley to Charlotte Pass, we’re measuring trees every five minutes. The result is a daily variation that appears lust like pulse on ECG—except in this case a single beat lasts a whole day.
So if you’re off track this summer, maybe walking up to Mt Stillwell, roaming the woodlands of Betts Camp and Spencers Creek, or taking an evening stroll while staying at a lodge in Perisher, don’t be surprised if you spy a researcher in an ever-reliable Mont sun hat attaching a laptop to a tree. I’m just checking its pulse.
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Earlier this month over the Easter long weekend I headed back to rediscover my love for a familiar country, the Ngarigo high country. With good weather forecast and a wonderful full moon, I thought it was high time I returned to the beautiful hills of Kosciuszko National Park.