Forest eucalypt dieback associated with over-abundant psyllids and Bell Miners (often referred to as 'Bell miner associated dieback' or BMAD) was listed as a key threatening process on 31 October 2008.
Both psyllids (small sap-sucking insects that secrete a sugary shield, called a lerp), and bell miners, large honeyeater birds (Manorina melanophrys), are native and occur naturally in moderate abundance in localised areas. Bell miners feed on the psyllids and their lerps, and may even ‘farm’ them, possibly selecting large lerps and preferentially feeding on the lerp rather than the insect itself.
Bell miners are aggressive and extremely effective at excluding other bird species from their territories. This can lead to psyllid populations increasing to the extent that they cause substantial canopy damage.
BMAD is a form of tree canopy dieback affecting extensive areas of eucalypt forest. This form of dieback can be diagnosed by the presence of over-abundant populations of psyllids, and often large numbers of bell miners are also present. As bell miner colonies tend to be relatively small, and colonies tend to move when the canopy is heavily defoliated, they may not always be evident at locations affected by BMAD.
This form of dieback is particularly prevalent in north-eastern NSW, but also occurs south to Victoria. It has been estimated that 2.5 million ha of forest in NSW has the potential to be affected.
Dieback is initially expressed through leaf loss from the tips of twigs and branches, and can result in defoliation of whole trees and stands of trees. It may include cycles of defoliation and regrowth, with fluctuating abundances of bell miners and psyllids, but if these factors persist it can lead to tree-death over extensive areas.
Over-abundant psyllid populations and bell miner colonies tend to start in sites with high soil moisture and suitable tree species where the tree canopy has been reduced and a dense understorey, often of lantana, is present. Such conditions arise as a consequence of landscape-level disturbance of forest ecosystems, and broad-scale canopy dieback associated with psyllids and bell miners usually occurs in disturbed landscapes, and involves interactions between habitat fragmentation, logging, nutrient enrichment, altered fire regimes and weed-invasion.
The forest types most susceptible to BMAD are those dominated by Dunn’s White Gum (Eucalyptus dunnii), Sydney Blue Gum (E. saligna), Flooded Gum (E. grandis), Grey Ironbark (E. siderophloia), Narrow-leaved White Mahogany (E. acmenoides), Grey Gum (E. punctata) and Grey Ironbark (E. paniculata).
Other eucalypt species such as E. moluccana, Corymbia maculata and C. variegata may be affected, usually after a substantial decline in the most susceptible species.
Most of the information on the number and range of species that are threatened by BMAD comes from the Kyogle area where 39 species of threatened animals and nine species of threatened plants occur in forests experiencing canopy dieback.
Threatened species recorded within the dieback-affected area surrounding Kyogle include: Stephen’s Banded Snake (Hoplocephalus stephensii), Fleay’s Barred Frog (Mixophyes fleayi), Glossy Black-cockatoo (Calyptorhynchus lathami), Eastern Bristlebird (Dasyornis brachypterus), koala (Phascolarctos cinereus), Rock-face Bluebell (Wahlenbergia scopulicola), and Rainforest Cassia (Senna acclinis).
BMAD has also been identified as a threat to the White Gum Moist Forest in the NSW North Coast Bioregion endangered ecological community, and is considered to be a significant threat to remnants of the Blue Gum High Forest critically endangered ecological community. See the Scientific Committee’s final determination for the full list of threatened species potentially affected by BMAD.
At present, no single cause explains this form of dieback, and expert opinion varies considerably as to which factors are causes of dieback and which factors are secondary effects. It is not yet known if the presence of psyllids and bell miners actually causes the dieback, or are secondary consequences.
Broad-scale research and adaptive management are required to understand how best to manage BMAD, to prevent its expansion throughout forests of eastern NSW.