Life-history traits correlated to reproduction are important in deciding adaptability of insects. Lagria hirta L. has a short phytophagous adult phase in summer and a very long saprophagous larval phase extending from autumn until spring. This study revealed that its reproduction was modified by external factors and by complex interactions involving different life-history traits. Oviposition occurred only at 15–25°C, LD (L16: D8) or SD (L8: D16). Within this temperature range, preoviposition time, fecundity and adult life span were affected (P ≤ 0.05). Photoperiod was found significantly to affect fecundity, viability of eggs and preoviposition time (P ≤ 0.04). Unmated females can lay eggs, but the eggs are sterile. The survival of eggs is also influenced by photoperiod (P < 0.001). In this study, the subtle variations were examined in the last instar’s duration, pupal weight, fecundity and adult life span. In terms of reproduction, two correlations were apparent: larval short last instar durations result in heavier pupae and eclose to longer-lived adults; conversely, larvae with longer last instars produce lighter pupae which, if female, tend to lay fewer eggs. These correlations were statistically significant (P ≤ 0.05). These trends represent a trade-off in resource allocation: long final instar duration may consume more energy that would be invested in reproduction. This is heterogeneous (phenotypically or genotypically) among the individuals of L. hirta.