Abstract1Populations ofMicrolophium carnosumon patches of perennial stinging nettle,Urtica dioicaincreased rapidly during April and May to reach their peaks in June. The decline in numbers was equally rapid and very small populations persisted into autumn; parthenogenetic over‐wintering was recorded.2Suboptimal ambient temperature and mortality due to natural enemies contributed mainly to the post‐peak development of aphid populations.3The combined effects of intraspecific competition and a deterioration in the food quality of the host plant appeared to be the major factors determining the temporal pattern of aphid abundance.4Variation in the size of populations between the three sampled sites was correlated with differences in the food quality of nettles as indicated by aphid mean relative growth rate.5Each nettle patch has a particular ‘carrying capacity’ for aphids, within which a biennial fluctuation between relatively large and small aphid populations appeared to be emerging at most of the sites investigated. A natural or aphid‐induced cycle in host plant quality, or alternatively, the persistent effects of intraspecific competition over several generations, may explain this fl