none Quantitative effects of the patchy distribution of the English green aphid S. avenae on its relation with the parasitoid Aphidius rhopalosiphi are studied by using a simulation model. The model computations are compared with field data. Different hypotheses on the parasitoid-host relation are tested. Due to the functional response and aggregation of the parasitoids, clustering increases the effect of parasitoids on population development of S. avenae but the parasitoid does not prevent population upsurge even when it shows clear aggregation to host clusters. Behaviour; Social behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology Aphidius rhopalosiphi (Braconidae); Hemipteran hosts; Sitobion avenae; Host clustering effect on host parasite relationships; Host parasite relationships; Sitobion avenae (Hemiptera) host clustering effects, model Sitobion avenae (Aphidoidea); Aggregating behaviour; Hymenopteran parasites; Aphidius rhopalosiphi; Host clustering effect on population dynamics & host parasite relations; Host parasite relationships; Host clustering effects on Aphidius rhopalosiphi (Hymenoptera) parasite; Population dynamics; Clustering effect on relations with hymenopteran parasite; Distribution within habitat; Clustering effect on population dynamics & hymenopteran parasites, model