The term 'dieback' refers to the decline in health and, in severe cases, death of tree crowns. Dieback can refer to natural forest dynamics that lead to long-term species succession or the outcome of dysfunction associated with disturbance or periodic stress. Because of the complexity of forested ecosystems, the ultimate drivers of dieback can be difficult to determine.
Snow-gum dieback refers to the death of snow-gum species as a consequence of infestation by a wood-boring longicorn beetle. Larvae, feeding on the outer layers of wood and inner layers of bark, ring-bark affected trees. The canopy of affected trees gradually declines in health and dies. In most instances infestation ends with the complete death of the tree, and in the most severe cases, the entire stand. Although snow-gum dieback is known to have occurred sporadically throughout the latter part of the twentieth century, the current outbreak appears to eclipse the extent of earlier outbreaks.
What is snow-gum dieback?
Where is dieback?
Current observations indicate that snow-gum dieback is largely restricted to areas above 1600 m above sea level (asl). A particularly severe outbreak is underway on the Kiandra high plains (~1400 m asl).
While, dieback has been recorded in Kosciuszko (NSW) , Brindabella (NSW), Namadgi (ACT) and Alpine National (Vic.) National Parks (see the map below), the full extent of the outbreak remains unknown. You can help address this major knowledge gap by reporting your own observations of snow-gum dieback. Head to our Help us page, and start recording. The map below summarises verified observations.
The early stages of snow-gum dieback can be hard to identify. The later stages are unmistakable.
Infestations are first indicated by tiny entrance hole a round 1 mm wide. They're very hard to spot and easily confused with the holes left by small shoots. Over time, small (3-4 mm wide), clean-cut circular holes appear. These holes are spaced ~3-5 cm apart and are arranged horizontally or vertically. The holes allow frass (chewed-up wood) to be pushed out of the feeding gallery under the bark. Frass with the appearance of sawdust might be present around the holes. The holes may also be leaking a deep reddish exudate known as kino.
As the feeding of larvae progresses overlying bark dries, puckers and cracks. A subtle depression in the bark may also indicate the exact position and size of the underlying feeding gallery. As feeding intensifies patches of bark die completely, splitting open to reveal the feeding galleries themselves. These galleries are evident as deep horizontal scars across the stem and are an unmistakable sign of snow-gum dieback. In the most advanced infestations, slabs of bark fall off affected trees leaving the scars associated with feeding larvae fully exposed. These scars remain obvious on affected trees decades after the tree died.
Click through the gallery below for examples of the early indicators, later signs and the insects themselves.