Ant nesting site selection mediated by insects frass

Facilitation is an ecological interaction in which the presence of one species (e.g., ecosystem engineers) alters the environment in a way that enhances growth, survival or reproduction of a neighboring species. Wood-boring insects are considered facilitators for cavities-nesting ants, which experience intense intra and interspecific competition for these sites. But how do ants find these cavities? Here, we suggest that ants could be attracted by chemical/odorous cues emitted by the frass of wood-boring insects. Using captive colonies of Cephalotes pusillus (Klug 1824) in controlled conditions, we carried out a bioassay to test the following hypotheses: (i) the frass from beetles are more attractive to ants than those from caterpillars (Cossidae: Lepidoptera), since boring beetles are more common and produce more cavities for ant nest on the studied plant, Caryocar brasiliensis (Caryocaraceae). If this first hypothesis is true, (ii) in relation to the cavity substrate type, the insect frass will be an attraction for workers from captive colonies (colonies, hereafter): both from compromised colonies (colonies without nest and in vulnerable condition) and intact colonies in tubes and (iii) and workers from compromised colonies colonize tubes with frass more quickly than those from intact colonies. The three hypotheses were confirmed, and Cephalotes pusillus ants prefer beetle frass, moving more quickly (five times faster) into tubes with frass when in vulnerable conditions. Our findings revealed that frass from beetles is a trigger to workers of C. pusillus locate and then nesting on branches of C. brasiliensis in Brazilian Cerrado. We suggest that the selection for a nesting cavity by arboreal ants is not random, but guided by chemical/odorous cues.