We develop a three-stage stochastic individual-based model for the phenology of the parthenogenetic mayfly Neocloeon triangulifer (Ephemeroptera: Baetidae). Mayflies are poikilothermic and develop at a rate dependenton the temperature of their aquatic environment. The model was built using development data for each aquatic stage from controlled laboratory studies to determine the variation in development rates present in the populationand the development rate response curve to temperature. Variable temperature experiments simulating diel temperature changes were used to validate the model. The thermal input data for the model is based onaverage daily water temperature recordings from White Clay Creek from January 2007 to 2013. The model predicts a multivoltine life cycle with a mode of 3 generations per year that agrees well with field observations.An optimally timed quiescent period for larvae (triggered by day length) is shown to enhance synchronization of adult emergence. Any future stream temperature rise is predicted to increase the number of generations per yearand desynchronize adult emergence, and these changes could lead to significant population decline in species that reproduce sexually.