Weaving resistance: silk and disease resistance in the weaver ant <Emphasis Type="Italic">Polyrhachis dives</Emphasis>

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.