Behavioral, Ecological, and Morphological Data Suggest a Close Relationship Between the Ant <i>Colobopsis truncata</i> and the Gall Wasp <i>Aphelonyx cerricola</i>

Wasps of the family Cynipidae are known to induce galls of a species-specific morphology, which during senescence provide a refuge for secondary insect fauna, especially ants. Here, we studied for the first time the colonization of galls of Aphelonyx cerricola by two ant species, Colobopsis truncata and Crematogaster scutellaris. Field sampling across one urban and one natural site in Italy showed that 61% of A. cerricola galls were colonized by these ants, though essentially (87%) by Co. truncata. These species exclusively colonized partially empty or completely empty galls, predominantly the larger ones composed of fewer subunits. By inspecting the European distribution of Co. truncata and Cr. scutellaris, we found that Co. truncata tends to be closer to the host plant of A. cerricola, Quercus cerris, than Cr. scutellaris. Climatic niche modeling indicated a greater niche overlap between Co. truncata and the host plant Q. cerris compared to Cr. scutellaris. Additionally, we discovered that Co. truncata queens have mandibles with a morphology likely less optimized for excavating compared to workers, with lower zinc enrichment levels than workers. This suggests that colony founding in Co. truncata may require less excavating activity compared to Cr. scutellaris, possibly explaining Co. truncata preference for empty galls and the higher colonization rate of non-emptying galls of other cynipids by Cr. scutellaris. Altogether, our data point to a close relationship between the ant Co. truncata and A. cerricola, and further investigations may shed light on its possible benefits for both species.