Bell-miner-associated dieback at the tree crown scale: a multi-trophic process

Summary

This paper examines some of the factors and processes which interact within eucalypt stands affected by bell-miner-associated dieback. A key symptom of this form of dieback is sustained foliar damage from herbivorous insects, in particular psyllids. Repeated cycles of defoliation/refoliation result in branch death and crown contraction. Weakened trees become more susceptible to secondary stressors such as wood borers, soil fungal pathogens and abnormal levels of soil moisture. Psyllid outbreaks can occur through a reduction in the efficacy of their natural enemy complex, and the provision of sufficient favourable foliage to sustain high insect populations. The young foliage preferred for feeding and oviposition by most eucalypt insect herbivores can occur when soil moisture and nutrients and canopy irradiation are non-limiting. Bell miner colonies require high insect densities in their food source and they reinforce this situation through their feeding behaviour and territorial defence. The study suggests the effect of bell miners on the populations of herbivorous insects, and hence the amount of insect-damaged foliage, is density dependent. Bell miners also require dense understorey, about 2–5 m high, for nesting sites, and the presence of surface water. Any management practice that reduces bell miner density will in turn reduce the density of insect herbivores and hence foliar damage in the eucalypt tree crowns. Then, if the trees have not become too debilitated, crown recovery is likely to occur.

Keywords:

• forest management
• dieback
• herbivores
• bell miner
• Manorina melanophrys
• psyllids
• Eucalyptus
• Australia