Signalling is necessary for the maintenance of inter-specific mutualisms but is vulnerable to exploitation by eavesdropping. While eavesdropping of intra-specific signals has been studied extensively, such exploitation of inter-specific signals has not been widely documented. The juvenile stages of the Australian lycaenid butterfly, Jalmenus evagoras, form an obligate association with several species of attendant ants, including Iridomyrmex mayri. Ants protect the caterpillars and pupae, and in return are rewarded with nutritious secretions. Female and male adult butterflies use ants as signals for oviposition and mate searching respectively. Our experiments reveal that two natural enemies of J. evagoras, araneid spiders and braconid parasitoid wasps, exploit ant signals as cues for increasing their foraging and oviposition success respectively. Intriguingly, selection through eavesdropping is unlikely to modify the ant signal.