The ability of host discrimination allows insect parasitoids to avoid superparasitism (oviposition in a previously attacked host). However, superparasitism can sometimes be adaptive, so attempts to identify host discrimination must be made under appropriate ecological conditions. We tested the ability of the parasitoid wasp Trissolcus basalis to discriminate between self- and conspecific-parasitised hosts (conspecific discrimination) under ecologically realistic conditions, in which conspecific discrimination should be adaptive. Data were analysed using a Monte Carlo simulation model that permitted testing of several different ways in which conspecific discrimination could be achieved. We obtained the novel result that females avoided self-superparasitism on a patch consisting of a mixture of self- and conspecific-parasitised hosts, but that this avoidance was not due to true conspecific discrimination. Instead, it was due to short- term discrimination between newly and previously parasitised hosts. Two likely mechanisms for such discrimination are proposed: a short-lived host-derived volatile; and the presence of two or more chemical components in the marking pheromone.