none DROSOPHILIDAE-; HYMENOPTERAN-PARASITES; Asobara rufescens; Host suitability, geographical variation, Europe Asobara rufescens; VARIATION-; Survival probabilities in dipteran hosts; BEHAVIOURAL-VARIATION; DIPTERAN-HOSTS; Drosophilidae; Host suitability; SURVIVAL-; Dipteran host suitability; HABITAT-PREFERENCE; Microhabitat preference; PALAEARCTIC-REGION; Europe; Microhabitat preference & dipteran host suitability, geographical variation In the Netherlands, Asobara rufescens (Forster) (Hymenoptera: Braconidae) is a parasitoid of drosophilid larvae in decaying plant material. In several places in the Mediterranean, parasitoids looking very similar to A. rufescens were collected on fermenting substrates. A hybridization experiment showed that the parasitoids were indeed A. rufescens. In an olfactometer Portuguese A. rufescens do not have a preference for either the odor of yeast or decaying leaves, while their Dutch conspecifics prefer the odor of decaying leaves. The survival probability of Portuguese A. rufescens in Drosophila melanogaster Meigen (Diptera: Drosophilidae), a species typical for fermenting substrates, is much higher than the survival probability of Dutch A. rufescens in this host species. It is hypothesized that decaying plant material may be unsuitable for drosophilid larvae during part of the year in the Mediterranean, forcing A. rufescens there to broaden its microhabitat choice. The use of fermenting substrates brings A. rufescens in contact with its sibling A. tabida Nees, a species typical for fermenting substrates in most of Europe. Portuguese A. rufescens appear to be genetically isolated from A. tabida. In the Netherlands, where the two species occupy different microhabitats, there is only a premating barrier.