The potential of parasitoids for aphid control during summer has been well documented. Few results are available on the impact of parasitoid populations on aphid hosts during autumn and winter and on the dynamics of their interactions during this period. The population development of Sitobion avenae, in Belgium, is analysed, from October to April, in the presence and absence of the parasitoid Aphidius rhopalosiphi. In the presence of parasitoids in winter, aphid populations decreased markedly and remained low at the beginning of spring. Induction of winter diapause in A. rhopalosiphi was observed during November at a mean temperature of 6.3 [degree]C and a decreasing photoperiod from 9.5-8.5 h of day light. A large range of A. rhopalosiphi mummy colourations, between dark and light, was noticed. This range of colouration did not allow a clear-cut distinction between diapausing and non-diapausing individuals of A. rhopalosiphi. The influence of seasonal weather and particularly temperature conditions on parasitoid mortality, strategy for overwintering and aphid population dynamics are discussed. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Land zones; Palaearctic region; Eurasia; Europe Aphidius rhopalosiphi (Aphidiidae); Hemipteran hosts; Sitobion avenae; Seaonal dynamics of host population on wheat, relations with parasites; Belgium; Louvain-la-Neuv; Hemipteran host seasonal dynamics Sitobion avenae (Aphidoidea); Hymenopteran parasites; Aphidius rhopalosiphi; Seaonal dynamics of host population on wheat, relations with parasites; Plant hosts; Wheat; Population dynamics; Belgium; Louvain-la-Neuve; Seasonal dynamics on wheat, relations with hymenopteran parasites none