Non-lethal foraging by bell miners on a herbivorous insect: Potential implications for forest health

Abstract

Tree health is often negatively linked with the localized abundance of parasitic invertebrates. One group, the sap-sucking psyllid insects (Homoptera: Psyllidae) are well known for their negative impact upon vegetation, an impact that often culminates in the defoliation and even death of hosts. In Australia, psyllid-infested forest in poor health is also frequently occupied by a native honeyeater, the bell miner ('Manorina melanophrys'; Meliphagidae), so much so that the phenomenon has been dubbed 'bell miner-associated dieback' (BMAD). Bell miners are thought to be the causative agent behind BMAD, in part because the species may selectively forage only upon the outer covering (lerp) exuded by psyllid nymphs, leaving the insect underneath to continue parasitizing hosts. As bell miners also aggressively exclude all other avian psyllid predators from occupied areas, these behavioural traits may favour increases in psyllid populations. We examined bell miner foraging behaviour to determine if non-lethal foraging upon psyllid nymphs occurred more often than in a congener, the noisy miner ('M. melanocephala'; Meliphagidae). This was indeed the case, with bell miners significantly more likely to remove only the lerp covering during feeding, leaving the insect intact underneath. This arose from bell miners using their tongue to pry off the lerp cases, whereas noisy miners used their mandibles to snap at both the lerp and insect underneath. Furthermore, psyllids left behind following a bell miner foraging event were significantly more likely to be viable and regrow a lerp covering than those exposed by noisy miners. Together, this behaviour supports the theory that non-lethal foraging behaviour of bell miners may contribute to high psyllid abundance, consistent with the mechanisms by which BMAD is thought to develop.