Larvae of Myrmica rubra can develop into queens only if they have passed through a period of halted development called diapause. These 'queen-potential' larvae develop from eggs laid during the early summer that reach a mass of at least 1mg by the 3(rd) instar stage in the autumn. Development then ceases and normally restarts only after chilling for a minimum of three weeks. The larvae then develop into queens or workers depending, among other things, on their treatment by the workers. In this paper we show that the period of diapause can be extended by prolonged cold temperature treatment and that this extension of diapause increases the production of queen pupae but not worker pupae. The extended diapause may allow a greater number of larvae to develop more fully their wing and ovariole buds before pupation. These results suggest that the duration of cold winter temperatures may have a significant bearing on colony demography and sex-allocation ratios.