The success of social insect societies is often attributed to an efficient reproductive division of labor between queen and worker castes. At the group level, social insect colonies must decide both the timing and amount of resources to allocate to each caste. Queen production is typically restricted by factors such as season or the presence of fertile queens in colonies. In the absence of such inhibition, the relative production of new queens versus workers varies between and is regulated by colonies. Here, we investigate social regulation of caste in the ant Monomorium pharaonis through a series of experiments manipulating the environment in which developing larvae are reared. The number of new queens produced depended strongly on diet as well as the number of late-instar worker larvae in the colony at the time of caste regulation. Given that these late-instar larvae are known to process solid protein for the rest of the colony, their stimulatory effect on queen production is likely a result of their contribution to a colony’s nutrient assimilation. Our results emphasize the need to consider late-instar larvae as critical social players and effectors of colony regulatory dynamics.