Size and age thresholds for pupation are important life history traits of insects. They are the ultimate consequences of the underlying physiological mechanism that optimize resource allocation. Such thresholds may have a plastic response under time-varying environmental conditions, developmental polymorphism (i.e., plasticity in the number of instars before pupation) being a common strategy adopted by insects to overcome this challenging situation. In this study, we systematically explore the variables related with both age and size thresholds for pupation and developmental polymorphism in the browntail moth, Euproctis chrysorrhoea (Lepidoptera: Lymantriidae), by rearing a group of caterpillars under conditions that either emulate larval diapause or prevent it. As an innovative approach, we evaluated the importance of predictor variables by means of generalized linear modeling in a multi-model inference framework. Our results show that (i) rearing conditions affect fitness, (ii) rearing conditions, size of hatchlings, size and age at maturity and sex are related to the number of instars before pupation, and (iii) there are both age and size thresholds for pupation which differ between sexes and between larvae reared under different conditions. Results are discussed in the context of lepidopteran plasticity in life history traits and its relationship with optimal molting strategies.