Decision-makers can be described as economically rational (making consistent choices), or economically irrational (making choices that vary with the options available). As the extent to which animals can and do make rational versus irrational decisions remains unclear, we tested the decision-making strategies of female Nasonia vitripennis parasitic wasps in two behavioural contexts: oviposition and foraging. In our first experiment, to determine whether oviposition preferences changed depending on the options available, we presented females with a high and a medium-quality blow fly host to parasitize, and gave some females an additional low or very low quality ‘decoy’ host. Presence of decoy options did not affect females’ oviposition choices, either in willingness to parasitize a host or the number of offspring laid. In our second experiment, we tested the effects of a low-quality decoy option on foraging preference for a high and a medium-quality sucrose concentration option. Here, presence of the low-quality decoy enhanced female preference for the high-quality option. Females therefore made economically rational decisions when ovipositing and economically irrational decisions when foraging. This difference in decision outcomes suggests that the cost/benefit ratio of making one type of decision over another may differ with the behavioural task.