Female parasitoids forage for host resources essential to the development of their offspring, so patch exploitation decisions have a direct influence on their fitness. This paper analyses the patch exploitation behaviour and patch-leaving decisions of the parasitoid wasp Trissolcus basalis (Hymenoptera: Scelionidae), when searching alone on patches of host eggs in the laboratory. Oviposition behaviour was examined in detail and the temporal and sequential structure of patch exploitation was analysed. Time inhomogeneities representing major behavioural change points during patch visits were identified, and the behavioural sequence within homogeneous periods was summarised. As the patch neared full depletion, wasps switched from a repetitive cycle of host examination and oviposition to a ‘leaving routine’, in which they alternated between searching on and off the patch, before abruptly leaving it. Despite the absence of competitors, females remained on the patch for up to 5 h after initiating the leaving routine, and periodically interrupted it to engage in bouts of active patch defence. Such pre-emptive patch defence was more pronounced when the patch was larger and the resource value therefore greater. This unique patch-leaving strategy is interpreted as an adaptation to high levels of resource competition and the consequent risk of losing offspring through superparasitism by competitors.