Adaptive value of host discrimination in parasitoids: when host defences are very costly

Host acceptance decision in parasitic wasps strongly depends on the parasitism status of the encountered host. In solitary species, a host only allows the development of a single parasitic larva and then, any oviposition in an already parasitised host leads to larval competition and loss of offspring. Females of many parasitoid species are able to discriminate between parasitised hosts and healthy ones. However, the host discrimination process may require more time than oviposition, exposing the wasp to high risks when the host has efficient defences. Consequently, depending on the degree of success of the host defence, the cost of host inspection for discrimination can outweigh the benefit of superparasitism avoidance. In the present paper, a theoretical approach was developed for determining how host defences may affect optimal host acceptance behaviour in parasitoids. The present model compares the lifetime reproductive success over the strategy used, discrimination and no-discrimination: a discriminating wasp sets a relatively greater value in its current oviposition, while a non-discriminating female sets a greater value in its own survival and future reproduction. The model predicts that depending on physiological state variables and environmental state variables, the optimal policy is not discriminating. Our results suggest that the low discriminating ability observed in some parasitic wasps could probably be an evolutionary response to host defences pressure.