Understanding and managing the role of bell miners (Manorina melanophrys) in forest dieback: A review of the ecological and management evidence

Abstract

<p>Severe impacts of herbivorous and xylophagous insects are a frequent characteristic of tree canopy diebacks globally. Insects play many roles in forest ecosystems, although they are often not explicitly included in ecosystem models used to predict forest change or guide management. Bell Miner Associated Dieback (BMAD) is a syndrome of eucalypt-canopy dieback associated with irruptions of lerp-building Psyllidae (Hemiptera) in south-eastern Australian forests. Research on BMAD has tended to focus on the role of a despotic honeyeater, the bell miner, Manorina melanophrys, rather than the management of psyllids. We reviewed the literature about the causes of BMAD, the role of bell miners, and the success of different management interventions designed to mitigate BMAD impacts. Excessive damage to eucalypt canopies by psyllids is the proximate cause of BMAD, with bell miners helping to maintain and expand psyllid infestations by excluding insectivorous birds. However, the link between bell miners and psyllid outbreaks is not entirely obligate, as both may be found in isolation. Various approaches have been used to manage BMAD, including removal of bell miners and understorey management using controlled burns and clearing, but these interventions have not been universally effective, suggesting that additional site-specific factors are critical drivers of bell miner occurrence and psyllid outbreaks. Based on a synthesis of past findings, we present a conceptual model incorporating all known or postulated factors that lead to BMAD. We also suggest three main strategic approaches to management: shrub reduction, bell miner removal, and landscape modification. However, many questions remain about the main drivers of bell miner occurrence and psyllid irruptions in eastern Australian forests, particularly in relation to the importance of forest and understorey type, environmental conditions, forest management, fire, and increasingly severe weather extremes. In addition to better understanding these factors, future studies should investigate the role of insect canopy species, including the parasitoids and hyperparasitoids of psyllids, and their interactions in different forest and understorey types under a range of environmental and management conditions and fire histories. We propose that the BMAD syndrome be broadened to include psyllid-mediated dieback in south-eastern Australian forests generally, both in areas with and without bell miners, to assist in adjusting management responses as needed to balance forest health and biodiversity outcomes with multiple forest uses on a site-by-site basis. We should not expect a one-size-fits-all solution to this complex multi-factorial dieback syndrome, and further field experiments and careful assessment of the impact of future management interventions (and controls) are needed.</p>