There is evidence for the effect Bell Miners have on psyllids and thus
indirectly on the health of trees (canopy dieback). For example:
1. Clarke & Schedvin (1999). Removal of bell miners resulted in: short-term
increase in avian diversity, immeditate influx of insectivorous birds,
decrease in abundance of Glycaspis spp. psyllids (eaten by other bird
species). Upon recolonisation psyllid abundance recovered. However, the cull
of bell miners failed to lead to recovery of the trees despite reduced
psyllids and miners over a period of 10 months, the authors suggest that
Phytophthora cinnamoni may have also contributed to the ill-health of the
trees.
2. Stone (1996). One field trial excluded miners from selected branches of a
eucalypt species - the leaves of these branches had greater survival than
leaves exposed to bell miners. Bell miners may interfere with regulatory
factors that control phytophagous insect populations (i.e. other
insectivorous birds and invertebrate predators/parasitoids). Again the
authors suggest that another factor may have also contributed to the
ill-health of the trees (e.g drought).
Having supported the "bad side" of bell miners they are nonetheless a native
PROTECTED species and as such individuals cannot legally make efforts to
control or remove colonies (through culling/trapping/translocation etc)
without the appropriate permission.
One last point: If we advocate controlling bell miners because of the
problems they apparently cause (or at least exacerbate), where do we stop?
If we take this attitude we could control any other native species that we
determine are causing a problem that doesn't SEEM to be natural (e.g Noisy
Miner, Pied Currawong). But what is natural? and how do we define whether
something is a problem?. From the perspective of the focal species involved
(e.g. bell miner, noisy miner, pied currawong) the process is natural.
However, the effect of the process is probably worsened by human activities
(e.g. creation of optimal habitat, fragmentation of habitat).
An important question yet to be sufficiently answered is: are bell miners
actually the primary cause or just one of the factors involved in canopy
death of trees? Other anthropogenic/naturally occurring factors may be
involved that have either lead to the behaviour of bell miners being
detrimental to the trees or combine to cause the problem - both the papers
cited here indicate that other factors may have been involved.
1. Clarke Michael F & Schedvin Natasha. Removal of bell miners Manorina
melanophrys from Eucalyptus radiata forest and its effect on avian
diversity, psyllids and tree health. Biological Conservation. 88(1). April,
1999. 111-120.
2. Stone Christine. The role of psyllids (Hemiptera: Psyllidae) and bell
miners (Manorina melanophrys) in canopy dieback of Sydney blue gum
(Eucalyptus saligna Sm.). Australian Journal of Ecology. 21(4). 1996.
450-458.
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