Tortoise beetles ( Cassida and related genera) are a large cosmopolitan group that includes several pests of agricultural crops and natural enemies of weeds but their biology and ecology remain poorly known. Using a set of environmental chambers, we address simultaneous effects of temperature and photoperiod on immature development and adult body mass in two European species, C . rubiginosa and C . stigmatica . Consistent with its broader distribution range, the former species is less susceptible to low rearing temperatures, develops faster and has a larger body mass than the latter. However, C . rubiginosa seems to be less adapted to late-season conditions as a short-day photoperiod accelerates its immature development to a lesser extent than it does in C . stigmatica , which nevertheless results in greater larval mortality and slightly but significantly smaller adults. By contrast, in C . stigmatica , which is more likely to encounter late-season conditions due to its slower life cycle, short-day acceleration of development is achieved at no cost to survivorship and final body mass. The experiment with C . stigmatica was repeated during two consecutive years with different methods and the main results proved to be well reproducible. In addition, laboratory results for C . rubiginosa agree with field data from literature.