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In this paper, we compare the host selection behaviours of two parasitoids, Aphidius rhopalosiphi and Aphidius picipes, in order to analyse whether behavioural adaptations to the defensive behaviour of their host (the grain aphid Sitobion avenae) could, in part, be responsible for the simultaneous presence of both species in cereal fields.The oviposition behaviour of A. picipes differed from that of A. rhopalosiphi by including a number of 'fluttering wings' sequences followed by immobility. It resulted in a 44 times longer host-handling time for A. picipes than for A. rhopalosiphi. Hosts attacked by A. picipes exhibited fewer defensive behaviours than hosts attacked by A. rhopalosiphi. A. picipes and A. rhopalosiphi rejected respectively 0% and 53% of unparasitized hosts presenting cornicle secretions, one of the defensive means of aphids. Furthermore, A. picipes females rejected 100% of the hosts that were already parasitized, whereas A. rhopalosiphi was previously described to reject only 20 to 40% of such hosts. Such differences could be explained by the way the two species deal with the aphid defensive behaviour.Field analyses, showed that A. rhopalosiphi was already present in wheat fields in early April whereas A. picipes appeared later and only achieved a low level of parasitism. However, when both species were present simultaneously, they shared the same resource.