What is BMAD?

The BMAD Cycle


Bell Miner Associated Dieback (BMAD) is a form of eucalypt dieback associated with unnaturally dense mid-storey vegetation (2-6 m high), overabundant native bell miners ( Manorina melanophrys ) and psyllid insects. Affected trees undergo repeated cycles of defoliation, eventually resulting in death of the trees and the forest. BMAD causes major changes to the structure and composition of affected forest and diminishes habitat and biodiversity values.



BMAD is found along the east coast of Australia, from Victoria to southeast Queensland and has substantial environmental, economic and social impacts. Because of the risk to biodiversity, BMAD is listed as a key threatening process in New South Wales, under the Biodiversity Conservation Act 2016. Without intervention forests may become irreparably damaged robbing future generations of this important resource.


Read about how we got here.


Take a look at the steps below to get an understanding of how the BMAD cycle works.


Or download our What is BMAD? fact sheet



  1. At first you have, healthy forest

In a healthy forest, the canopy is intact and there is natural structural complexity. Light levels in the mid-storey and understory layer are low and these layers are relatively sparse. It is difficult for weeds to invade. The habitat is not suitable for bell miners and the insect populations are kept in check by abundant insectivorous birds and other animals.


2. If a forest is disturbed, it opens up the canopy

Under the right conditions, the BMAD cycle starts when a healthy forest containing BMAD susceptible eucalypt species is disturbed by activities such as high intensity fire or timber harvesting resulting in thinning or loss of the canopy. Light penetration to the forest floor increases significantly in both wet and dry forest types.


3. Woody weeds invade the understory

High light levels on the forest floor in the absence of activities to suppress understorey growth (e.g. intermittent grazing, low-intensity fire) lead to the growth of an unnaturally dense layer of vegetation in the mid storey (generally between 2 and 6m).


On the NSW North Coast, this growth is often the invasive weed lantana (Lantana camara), but it can also be other species such as native water vine (Cissus sp.).


4. Bell miners might move in

The unnaturally dense mid-storey layer, particularly if composed of lantana, and sparse eucalypt canopy provide ideal habitat for bell miners. Bell miners prefer to nest in the dense undergrowth forming colonies in contiguous areas of vegetation.


In colony formation, Bell Miners are aggressive and force other animals to leave the area, including insectivorous birds who compete for the bell miners food supply. The sparse canopy gives the bell miner an advantage, using the trees as outposts to protect their nesting territory from intruders.


5. Psyllid numbers increase

Pysllid is a term to describe a large group of naturally occurring leaf sap-sucking insects. Bell miners feed on a sugary coating produced by psyllid insects (known as 'lerp') found on eucalypt leaves, but generally don't eat the psyllid insect itself. With no other insectivorous birds around, psyllids increase greatly in number.


6. Psyllids damage leaves and cause defoliation
Psyllids damage leaves by sucking sap and causing sooty mould. The tree sheds damaged leaves in response. An overabundance of psyllids can cause the tree to lose all of its leaves, stopping it from photosynthesising and replacing its food reserves. The tree begins to use its internal energy reserve to replace lost leaves.


7. Repeated cycles, dieback and death

Unfortunately, the tree's new shoots are high in nutrients and attract even more psyllids, so the problem continues.


The tree enters repeated cycles of defoliation and regrowth, which weakens and eventually kills the tree. Whole areas of forest can be affected. 


More info below...

Elements of BMAD

Bell miners

The bell miner is a native honeyeater that is found along the east coast of Australia from the outskirts of Melbourne to southeast Queensland.  They are a small bird, olive-green in colour, with a yellow beak and a red-orange patch behind the eye.


Because of their colour, Bell miners are difficult to spot and are more often heard than seen. Their distinctive ‘tink’ call is usually the first thing you’ll notice and is the simplest way to recognise them. 


Bell miners live in colonies of up to 200 birds. Like their cousins, the noisy miner (Manorina melanocephala), bell miners are aggressive and highly territorial. Bell miners exclude all other birds from their territories. 


Bell miner colonies are relatively sedentary and occupy a discrete area. Colonies are found in wet and dry eucalypt forest, preferring moist forest in areas close to a water source, such as a creek or more often a dam. Colonies usually occur in moist foothills and gullies and on south and east-facing slopes. The presence of bell miners is positively correlated with the topographic wetness index (locations receiving higher rainfall but are not found at high altitudes (above 500m asl). 


Bell miners prefer forests with a dense understorey layer for nesting and a sparse canopy, which allows them to cooperatively defend their territories. Their diet consists mostly of lerp, the sugary casing excreted by psyllids, sap-sucking insects that feed on eucalypt trees. Bell miners usually only eat the lerp leaving the insect behind. 


This foraging behaviour combined with the exclusion of other insectivores causes psyllid populations to explode in bell miner territories, impacting die-back susceptible eucalypt species.

Psyllids

Psyllids are small, sap-sucking insects that are related to aphids and whiteflies. They are diverse with over 400 species native to Australia. Psyllids have multiple life stages with sedentary nymphs and mobile, flying adults. 


When psyllids feed, they produce excess sugar which is excreted. About half of the Australian psyllid species use this sugar exudate to create lerp, a protective casing surrounding the psyllid.  


Psyllids are highly host specific and some species only feed on young leaves (less than 6 months old) while others specialise old leaves. On the same tree, psyllids feeding on the old leaves are very likely to be different from those feeding on the young leaves. 


The psyllids involved in BMAD are thought to be primarily Glycaspis species. They specialise in young foliage and produce sugary lerp. The sugary lerps of Glycaspis are favoured by many bird species including bell miners. Glycaspis has adapted to having its lerp removed and can readily produce a new lerp if it becomes lost. 


Psyllids damage tree leaves by sucking sap from the leaves and encouraging the growth of sooty mould. In response, trees drop damaged leaves and replace them with new growth. 

Impacts on eucalypts and dieback

Bell miner colonies exclude other birds that eat psyllids resulting in a heavy psyllid load inside a bell miner territory. Some eucalypt species naturally have higher loads of psyllids than others and some species are more vulnerable to BMAD. 


Heavy psyllid loads result in significant leaf damage to eucalypts. In response, the tree drops its leaves prematurely and cannot photosynthesize. The tree must then produce new leaves to replace the old ones using its internal carbohydrate stores. 


The new leaves are more nutritious and are favoured by Glycaspis, the pysllid type implicated in BMAD. As psyllids infest the new leaves the tree goes through repeated cycles of defoliation. This forces the tree to deplete its internal reserves of starch, making it more vulnerable to other pests,  diseases and climate stresses such as drought. 


Not able to photosynthesise to any great degree during this time, the tree may eventually exhausts its carbohydrate reserves after repeated defoliation events and dies. 


As more trees defoliate and die, there is more light penetration and more understorey growth  increasing habitat suitability for the bell miner colony. However, eventually all the BMAD susceptible overstorey trees will die and the area will no longer be suitable as bell miners habitat and the colony will move on. 


The remaining dense undergrowth limits   recruitment or regeneration of new trees. At this point, the forest has transitioned into a new, degraded state with significantly less biodiversity. The forest will not be able to return to a healthy state without significant management intervention at this point. 


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