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.