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.

The Influence of Avian Biodiversity and a Weedy Understorey on Canopy Arthropod Assembly

“…The interaction between forest management and bird foraging is important e.g. [12] [26] [62], but little is known about this relationship and it needs further examination [18].…”

“…This cooperatively breeding species feeds as part of a colony, consuming primarily insects from the foliage and bark of eucalypts [12] [14] [50] [51]. Colonies inhabit open eucalypt forests and woodlands with a thick understorey in broad gullies of foothills, usually at edges of rainforest areas [13].…”

“…Chewing insects have been found to be the most damaging insects in eucalypts [41], and one of feeding guilds that bell miner chicks and adults consume [42] [43]. However, psyllid nymphs, that produce a sugaryprotective coating and feed on the sap of leaves, have been implicated in increased levels of damage in trees, particularly in association with bell miners [12] [44]. This relationship is an extreme example of creating exclusion zones as it has been associated with tree death, known as bell miner associated dieback [11] [45].…”

“…Evidence suggests that bell miner colonies defend areas containing high levels of damaging insect populations, particularly psyllids, from other insectivorous birds, indirectly leading to the death of canopy trees [12] [14] [39] [40]. Chewing insects have been found to be the most damaging insects in eucalypts [41], and one of feeding guilds that bell miner chicks and adults consume [42] [43].…”

“…Bell miners are despotic, excluding all other avian taxa from an area [11], to possibly defend a food source and remove all other insectivorous predators [12]. Additionally, it has been suggested that this species uses thick or weedy understories for nesting [13].…”