none Glasshouse studies were conducted to ascertain the effect of population density on the development of Rose grain aphid (Metopolophium dirhodum Walker) and its parasitoid (Aphidius rhopalosiphi De Stephani Perez). The growth and development of the M. dirhodum and A. rhopalosiphi were greater when aphid population per plant was low. Percentage parasitism was also significantly higher at low aphid density (52.33% at 5.5 aphids per plant) than on maximum aphid density (28% at 30 aphids per plant). Biometrics; Reproduction; Reproductive productivity; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics Aphidius rhopalosiphi (Aphidiidae); Size; Hemipteran hosts; Metopolophium dirhodum; Host population density effect on prevalence & host & parasite performances Metopolophium dirhodum (Aphidoidea); Size; Hind tibia length; Fecundity; Hymenopteran parasites; Aphidius rhopalosiphi; Host population density effect on prevalence & host & parasite performances; Population density