Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii: nutritional mutualism and implications for pathogen spread

Myrtle rust fungus, Austropuccinia psidii (Basidiomycota: Pucciniales), infects more than 500 species in the Myrtaceae family and is a significant threat to keystone plant taxa, particularly in Australia. Urediniospores are primarily dispersed by wind, but there are several reports of animal-mediated dispersal events. Recent observations of Apis mellifera (Western honey bees) collecting urediniospores of A. psidii raise the possibility of a novel mutualistic interaction between these two alien species. Here, we test two key conditions required for such a mutualism: (1) that spores remain viable after entering hives, and (2) that they provide nutritional benefit to the pollinator. We show that A. psidii spores remain viable for at least nine days within honey bee colonies and match or exceed the nutritional quality of known high-protein pollens. Trials using artificial rearing of bee larvae further demonstrate that A. psidii spores support normal development and survival in honey bee brood. These findings suggest that spore foraging may not be an aberration but a viable foraging strategy for honey bees. Our results support the hypothesis of a mutualism and dispersal of these plant pathogens, whereby nutritional benefits to A. mellifera facilitate the long-distance dispersal and epidemiology of A. psidii . This has implications for models of pathogen spread, especially in regions where managed hives are regularly transported over long distances in agricultural pollination services landscapes. Recognising pollinators as potential vectors of invasive plant pathogens is essential for developing more effective biosecurity and conservation strategies.