Abstract Understanding the speed of and the type of mechanisms that species use to adapt to rapid change is a central question in evolutionary biology. Classically, the two mechanisms denoted in the literature that allow individuals to address these environmental changes are either phenotypic plasticity or rapid evolutionary changes. However, phenotypic plasticity itself can evolve rapidly. In this study, we investigated the genetic differentiation between marginal and core populations of a high-trophic level insect, Leptopilina boulardi , a Drosophila parasitoid, which has exhibited a very rapid progression northward of its geographical range. Several life history traits have been investigated in different populations according to four fluctuating thermal regimes that mimic the thermal conditions in the field. We found that at low developmental temperature, the two northern marginal populations that have to face a colder winter, survive longer than the two core populations. In addition, the northernmost populations exhibit a higher potential fecundity, a higher starvation resistance and a larger amount of energy at low temperatures. These significant genetic differentiations with genotype-by-environment interactions show that a rapid genetic differentiation of the shape of thermal reaction norms is possible when populations have to cope with new environments.