Placing traits into novel evolutionary contexts may profoundly alter their functional roles. Here, we investigated whether the elaiosome, a lipid‐rich appendage located on seeds, retained its role as a seed dispersal trait promoting mutualisms with insectivorous ants following human‐mediated introduction of the elaiosome‐bearing Carduus nutans into the Argentinean Caldenal. This system is located within the Neotropical region, an alleged myrmecochory cold spot. Specifically, we first tested the assumption that the elaiosome mediates the interaction between C. nutans and the native ant Pheidole bergi . Then, we explored the hypothesis that, instead of a mutualism, the elaiosome promotes an antagonism between these species because P. bergi predates on both insects and seeds. Finally, we assessed the possibility that the elaiosome is rare in our system, as predicted from its location within the Neotropics. By manipulating the presence/absence of C. nutans ' elaiosomes, we demonstrated that P. bergi strongly prefers to collect seeds with versus without C. nutans ' elaiosomes, indicating that the elaiosome indeed mediates the interaction between these species. While we detected no direct signs of predation on nor alteration of viability in seeds recovered from P. bergi 's refuse dumps, 80% of offered C. nutans seeds remained inside P. bergi colonies, where they are likely consumed by ants, buried too deep for emergence or destroyed by pathogens. Importantly, by quantifying the outcome of the C. nutans – P. bergi interaction, we showed that this relationship is strongly antagonistic. Finally, by sampling taxa most likely to have elaiosomes, we identified eight native species with that trait, preliminary confirming that elaiosome‐bearing species are uncommon in the Caldenal. Taken together, our findings suggest that the elaiosome promotes an antagonism that deters invasion in a cold spot of myrmecochore diversity. The functions of phenotypic traits can thus vary according to the ecological and evolutionary contexts in which they operate.