The ant, Aphaenogaster picea, benefits from plant elaiosomes when insect prey is scarce.

Myrmecochory is a facultative, mutualistic interaction in which ants receive a protein-rich food reward (elaiosome) in return for dispersing plant seeds. In North American northeastern hardwood forests, Aphaenogaster ants are the primary genus dispersing myrmecochorous plants. In these forests, myrmecochores occur in plant guilds of understory spring ephemerals or seasonal greens. This mutualism has been demonstrated for Aphaenogaster rudis (Emery) and individual plant species, but it has not been demonstrated for other Aphaenogaster species or guilds of myrmecochores as they naturally occur. Aphaenogaster picea (Wheeler) colonies were fed three treatments over 5 mo: 1) a mixture of only elaiosomes from an entire plant guild, 2) a diet of only insect protein and 3) a combination diet of both elaiosomes and insect protein. This experiment investigated two potential hypotheses through which elaiosomes can benefit ants: 1) elaiosome proteins can substitute for protein nutritional requirements when ants are prey-limited, and 2) elaiosome nutrition can supplement insect protein when prey is ample. First, a mixture of elaiosomes from four myrmecochorous plant species provided to A. picea colonies was sufficient to maintain worker production, larval growth, and fat stores when no other food was available. A. picea colonies consuming elaiosomes as their only protein source could be sustained for a growing season (5 mo). Second, colonies fed both elaiosomes and protein did not yield more productive colonies than a control diet of just insect protein. These results support the hypothesis that myrmecochory is indeed a facultative mutualism in which ants take advantage of the protein content of elaiosomes when it is favorable, but when they are not limited by insect prey they do not gain any additional benefit from elaiosomes.