As with all organisms, ant colonies coexist with a wide
diversity of opportunistic microbes that can be parasitic, such
as the Aspergillus fungus found in this experiment and in ant colonies studied previously (Pereira and Stimac 1997;
Schmid-Hempel 1998; Hughes et al. 2004b; Poulsen et al.
2006; Lacerda et al. 2010; Fountain and Hughes 2011). Adult
ants appear to suffer relatively little from these opportunistic
parasites even when their production of antimicrobial secretions
is prevented (Poulsen et al. 2002; Graystock and Hughes
2011), presumably due to their well-developed immune system
and grooming behavior.We also found this to be the case
here for brood and nestmaterialwhen the antibiotic-producing
glands of nurses were functioning. However, when the antimicrobial
secretions of nurses were lacking, most brood
succumbed to infection by opportunistic Aspergillus fungi
and most nest material became overgrown. It cannot be excluded
that some of the fungal growth may have been opportunistic
growth on larvae that died from another cause.
However, even if this is the case, then the results nevertheless
demonstrate the importance of antimicrobial secretions from
nursing workers for sanitizing the cuticles of larvae. It,
therefore, appears that the antimicrobial secretions of
adult ants are essential to protect the vulnerable brood
against opportunistic parasites and to prevent nest material
becoming overgrown by contaminant fungi. It may
indeed be the case that the protection of larvae against
ubiquitous opportunistic microbes is of greater importance
for ant fitness than protection against more specialist
parasites such as Metarhizium, which tend to be rarer, and
may potentially have driven the evolution of antimicrobial
secretions in ants.