Parasitoids flip a coin before deciding to superparasitize

Abstract

1. Host acceptance decision in parasitic wasps depends strongly on the parasitism status of the encountered host. In solitary species, a host allows the development of only a single parasitic larva, and then any oviposition in an already parasitized host leads to larval competition and to potential loss of offspring. The ability of parasitoids to discriminate between parasitized and healthy hosts is well documented. Despite this, parasitized hosts are still accepted by the foraging wasps, an occurrence termed superparasitism. 2. In the last decades, theoretical studies have suggested that under certain circumstances superparasitism can be optimal. Generally, the superparasitism theory predicts that the optimal host acceptance decision should follow a zero-one rule: in response to both environmental and physiological state characteristics, a given female should switch from acceptance to rejection of parasitized hosts. 3. However, some experiments have shown that parasitoids may decide to accept parasitized hosts with an intermediate probability. 4. A model was developed in order to explain this partial preference in parasitoids. 5. More generally, the present study demonstrates that when the consequences of one decision cannot be predicted perfectly, the optimal decision rules could be stochastic rather than deterministic. Accordingly, behavioural stochasticity need not necessarily result from errors in decision-making or from an incapacity to assess the actual environmental state but could instead have evolved as the optimal response to the uncertainty of future environmental state.