The functional roles of psyllid abundance and assemblage on bird-associated forest defoliation

Abstract

Fluctuations in the abundance of insect herbivores and the severity of damage they cause to their hosts are key factors affecting tree health and trophic interactions. Bell miner associated dieback (BMAD) is a trophic cascade involving eucalypt trees, psyllids and bell miners. Bell miners are a territorial species of bird that reduce insectivorous bird diversity, thereby increasing psyllid abundances. The role of psyllids in BMAD is debated, primarily because psyllid host specificity, genus, defoliation potential and/or lerp composition have been conflated or unexplored. This study documented psyllid communities and canopy structure of four eucalypt hosts (Eucalyptus bridgesiana, E. caliginosa, E. propinqua and E. siderophloia) in BMAD and non-BMAD-affected forests in northern New South Wales. Psyllid abundances were significantly higher in BMAD-affected forests. However, psyllid assemblages did not differ between forest types which contrasts with an influential hypothesis concerning bell miner farming of psyllids that produce sugary lerps (Glycaspis species). Importantly, psyllid communities differed among species of eucalypt host. Hosts supporting higher abundances of Cardiaspina tended to have a stronger negative relationship between canopy health and psyllid abundances than other host trees. Cardiaspina were also found on older leaves and associated with more leaf damage than other psyllids. Unlike most other psyllid genera, Cardiaspina initiate premature foliar senescence leading to defoliation which ultimately changes the age structure of leaves of the canopies of affected trees. Our study supports the functional linkage between psyllid assemblages, their relative abundances and defoliation potential to bottom-up mechanisms behind a bird-associated trophic cascade.

The Hidden Secrets of Psylloidea: Biology, Behavior, Symbionts, and Ecology

“…Forests colonized by lerp-consuming bell miners frequently experience co-occurring psyllid outbreaks not only of Glycaspis, but also of more damaging leaf-feeding psyllids that do not form lerps, such as senescence-inducing Cardiaspina spp. (121). These outbreaks, when severe, cause dieback of multiple Eucalyptus species (for a review, see 68).…”

Host assessment and feeding behaviour of adult female Glycaspis brimblecombei on preferred and non‐preferred eucalypts

“…Glycaspis brimblecombei is a sugary lerp‐forming psyllid in a megadiverse genus, which comprises 137 described species (Hollis, 2004). Nymphs produce sugary lerps (once described as ‘crystal palaces’ by Wooster, 1880) providing an important source of food for a diverse array of vertebrates and insects (Steinbauer et al, 2015; Smith et al, 2021). The species has been introduced to the USA and many other countries.…”