The aphid life cycle contains a series of parthenogenetic, viviparous generations, followed usually by a sexual generation that produces resistant overwintering eggs. Since the decision to produce sexually-reproducing offspring ends the period of rapid growth of the clone, it should be postponed as late as is compatible with successful oviposition. The time of leaf fall determines the latest possible time of oviposition, and is itself determined mainly by daylength. The time required for the development of the final generations of aphid depends on temperature. The decision to end the sequence of parthenogenetic generations should thus depend on temperature and photoperiod. This paper calculates the optimal combination of daylength and temperature for the cueing of this decision in Rhopalosiphum padi. It is shown that the experimental data of Dixon and Glen (1971) are in agreement with the predictions, and that, in the field, the time of production of male R. padi does vary adaptively with July temperature.