Slowing them down will make them lose: a role for attine ant crop fungus in defending pupae against infections?

Fungus-growing ants (Attini) have evolved an obligate dependency upon a basidiomycete fungus that they cultivate as their food. Less well known is that the crop fungus is also used by many attine species to cover their eggs, larvae and pupae.
The adaptive functional significance of this brood covering is poorly understood. One hypothesis to account for this behaviour is that it is part of the pathogen protection portfolio when many thousands of sister workers live in close proximity and larvae and pupae are not protected by cells, as in bees and wasps, and are immobile.
We performed behavioural observations on brood covering in the leaf-cutting ant Acromyrmex echinatior and we experimentally manipulated mycelial cover on pupae and exposed them to the entomopathogenic fungus Metarhizium brunneum to test for a role in pathogen resistance.
Our results show that active mycelial brood covering by workers is a behaviourally plastic trait that varies temporally, and across life stages and c...