Social insects are at risk from a diverse range of
parasites. The antibiotic-producing metapleural gland is an
ancestral trait in ants which is thought to be one of their
primary mechanisms of resistance. However, the metapleural
gland has been lost secondarily in three ant genera,
which include weaver ants that are characterised by the
remarkable construction of their nests using larval silk.
Silken nests may have allowed reduced investment in costly
disease resistance mechanisms like the metapleural gland if
the silk has antimicrobial properties, as in other insects, or is
a hygienic substrate. Here we examine this hypothesis in the
weaver ant Polyrhachis dives. We found no evidence of a
beneficial effect of silk. The presence of silk did not improve
the already high resistance of ants to the entomopathogenic
fungus Metarhizium, the ants only rarely interacted with the
silk regardless of whether they were exposed to Metarhizium
or not, and silk also did not inhibit the in vitro germination or
growth of Metarhizium. Furthermore, silk was found in vitro
to be heavily contaminated with the facultative entomopathogenic
fungus Aspergillus flavus, and many more ants
sporulated with this fungus when kept with silk in vivo than
when they were kept without silk. Further work is needed to
examine the effects of silk on other parasites and of silk from
other weaver ants. However, the results in combination suggest
that silk in P. dives is unlikely to provide protection
against parasites and that it is also not a hygienic substrate.
Alternative explanations may therefore be needed for the loss
of the metapleural gland in weaver ants.