Plant species may be subject to periodic plagues of predators or diseases. Widespread dieback and death of mature trees has occurred on a large scale in many NSW woodland areas, particularly on the Tablelands. Factors implicated in dieback include ageing of isolated remnant trees and little natural replacement due to grazing of seedlings, combined with a buildup of leaf-eating insects and increased levels of pasture nutrients that favour the larval stages of these insects.
Impacts of plagues and diseases are most obvious on tree canopy species because of their size and importance, but have doubtless affected other smaller species, but which have not been recorded. Signs of soil-borne disease such as Phytophthora have not been noted in the woodland, but it is unlikely that many of the native species will be susceptible.
Crown canopy dieback of eucalypts
Crown canopy dieback of eucalypts is periodically noted in Cumberland Plain Woodland remnants in western Sydney, but has not been associated with widespread tree death, perhaps as regeneration of trees in many areas has occurred so there are many young trees. Such periods of leaf attack rarely last more than one or two seasons.
Bell Miner associated dieback
A more localised form of eucalypt decline is Bell Miner associated dieback. Bell Miners, Manorina melanophrys, are insectivorous, territorial birds that live in large colonies covering several hectares. Eucalypt stands colonised by Bell Miners often exhibit symptoms of crown decline, and colonisation by Bell Miners correlates with high densities of leaf damaging psyllids in tree crowns. Bell Miners have colonised the woodland at the Australian Botanic Garden, Mount Annan in the last ten years. Experimental treatment of psyllids by arborists at the Australian Botanic Garden aims to reduce defoliation of Eucalypts.