none If 1, 2, 4, and 8 female parasitoids of Lysiphlebus delhiensis were released per cage on 200 hosts (Rhopalosiphum maidis ) for 24 h, the number of hosts parasitised as well as of parasitoids emerged increased. However, the rate of multiplication (number of hosts parasitised/female) declined with the increase of parasitoid-host ratio. Increasing parasitoid density decreased the searching efficiency of the parasitoid. The fact that more parasitoids have lower rate of multiplication and search rate show the presence of mutual interference. Various aspects of the parasitoid's behaviour which could give rise to interference effects are discussed. Proportion of females in F sub(1) offspring decreased with increase of parasitoid-host ratio. The factors which could give rise such effect are explained. In relation to a given site, the data presented herein furnish an insight into the number of parasitoids needed to regulate the estimated aphid population. Life cycle and development; Development; Metamorphosis; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Ecology; Population dynamics; Population structure Lysiphlebus delhiensis (Braconidae); Emergence from pupa; Population density relations, hemipteran host; Hemipteran hosts; Rhopalosiphum maidis; Prevalence & bionomics, effect of population density; Prevalence; Rhopalosiphum maidis (Hemiptera) host; Population density; Development, bionomics & prevalence on hemipteran host relations; Population sex ratio; Parasite/hemipteran host ratio relations Rhopalosiphum maidis (Aphidoidea); Hymenopteran parasites; Lysiphlebus delhiensis; Parasite density effect on prevalence & parasite bionomics; Prevalence; Lysiphlebus delhiensis (Hymenoptera) parasite