Phenotypic plasticity involves adaptive responses to predictable environmental fluctuations and may promote evolutionary change. We studied the regulation of phenotypic plasticity in an important agricultural pollinator, the solitary alfalfa leafcutting bee (Megachile rotundata F.). Specifically, we investigated how larval nutrition affects M. rotundata diapause plasticity and how diapause plasticity affects adult female reproductive behavior. Field surveys and laboratory manipulations of aspects of larval diet demonstrated nutritional regulation of M. rotundata diapause plasticity. Manipulation of larval diet quality through the addition of royal jelly, the caste-determining substance of the honey bee Apis mellifera L., increased the probability of diapause in M. rotundata. We also found that larval nutrition and diapause status affected M. rotundata adult female reproductive behavior. Nutritional effects on larval diapause that also impact adult fitness have intriguing implications for the evolution of developmental plasticity in bees. In particular, as the solitary lifestyle of M. rotundata is considered to be the ancestral condition in bees, nutritionally regulated plasticity may have been an ancestral condition in all bees that facilitated the evolution of other forms of phenotypic plasticity, such as the castes of social bees.