Larvae promote a decline in the harvesting of myrmecochorous seeds by Myrmica rubra ants

Abstract

Non-granivorous ant species that contribute to seed dispersal (myrmecochory sensu stricto) are assumed to benefit from the larval consumption of elaiosomes, the lipid-rich appendages of myrmecochorous seeds. It is, however, questionable whether this ant–plant interaction is truly mutualistic, since some ant partners do not show a clear-cut fitness gain and may cease the harvesting of myrmecochorous seeds over time. In particular, the role of ant larvae as potential consumers of seed elaiosomes is not yet fully understood. In this study, we investigated how the number of larvae influences the foraging responses of the red ant Myrmica rubra to Viola odorata myrmecochorous seeds and to Drosophila melanogaster prey. Ant workers retrieved less elaiosome-bearing seeds than prey and the harvesting of seeds remained low, even in the presence of larvae. Furthermore, while the ant colonies continued to collect prey over successive foraging events, the retrieval of myrmecochorous seeds decreased, most particularly in colonies containing a large number of larvae. Brood exposure to diaspores also declined, with very few larvae coming into contact with seeds. Our results suggest that the role of larvae as elaiosome consumers should be reconsidered and that larvae may provide workers with cues about the poor palatability of V. odorata diaspores, thereby altering the stability of this ant–plant interaction.