The role of psyllids (Hemiptera: Psyllidae) and bell miners (Manorina melanophrys) in canopy dieback of Sydney blue gum (Eucalyptus saligna Sm)

The presence of the aggressive, colony-forming honeyeater, Manorina melanophrys (bell miner), in the canopies of unhealthy eucalypts has been well reported. There is, however, some debate as to the actual mechanisms producing these unhealthy trees. To investigate further some of the processes that may be contributing to this form of canopy dieback, two field trials were carried out in Olney State Forest, near Wyong, New South Wales. The study site contained Eucalyptus saligna (Sydney blue gum) with canopy dieback and was occupied by a large colony of bell miners. Close examination of the foliage revealed a large and diverse suite of phytophagous insects, including at least 16 species of psyllid (Hemiptera: Psyllidae). In the first trial, the use of bird exclusion cages over selected branches significantly improved leaf survival compared to leaves exposed to a relatively high density of bell miners. It is proposed thar colonization by bell miners may interfere with the efficacy of both other insectivorous birds (through aggressive interspecific territoriality) and the invertebrate predators and parasitoids. Interference with such regulatory factors may enable some phytophagous insect populations To rise to sustained damaging levels. In the second trial, an insecticide application combined. with reduced competition from the dense understorey and neighbouring trees was required to significantly improve trunk diameter and crown condition scores. After 12 months, neither treatment, by itself, significantly improved both growth measures. Possibly both treatments were required because the E. saligna trees were suffering from another source of stress (e.g. drought) in addition to the relatively high level of insect attack.