Variation in hatching time (phenology) might cause size differences within populations resulting in size-mediated priority effects (SMPEs) shaping intraspecific interactions. These phenology-driven effects potentially can be strengthened by seasonal time constraints caused by a short growth season, and depend on latitude. Here the single and combined effects of phenology and latitude-associated time constraints on SMPEs in larvae of an aquatic insect, the damselfly Lestes sponsa, are studied. We did so by rearing larvae in groups of 16 individuals with different phenology (hatching date) thereby imposing strong intraspecific competition, resulting in cannibalism. We thereby manipulated in a fully crossed way time constraints (combination of temperature and photoperiod: thermo-photoperiod) in larvae from low-latitude and more time constrained high-latitude populations, and examined effects on life history (survival, development, growth) and physiology (fat and protein contents, and phenoloxidase activity as a measure of immune function). Phenology, time constraints, and latitude of origin had strong effects on life history, but only the time constraint affected the physiology. We detected a SMPE for survival that, however, was not stronger under time constraints and was consistent in strength between latitudes. Phenology and time constraints interacted for development and growth in a direction suggesting adaptive responses to time constraints but these life history traits did not show SMPEs. We provided important insights in the study of SMPEs thereby showing these to be trait-dependent and not more pronounced under experimentally manipulated or latitude-associated time constraints. Our study thereby makes an important addition to geographic variation in SMPEs, a largely neglected topic.