none Drosophila melanogaster; Drosophila subobscura; HYMENOPTERAN-PARASITES; Asobara tabida; Adaptive variation in host selection behaviour Asobara tabida; FORAGING-; Dipteran host selection; EVOLUTIONARY-ADAPTATION; BEHAVIOURAL-VARIATION; Adaptive variation in dipteran host selection; DIPTERAN-HOSTS; Drosophila melanogaster & D. subobscura; Adaptive variation in host selection behaviour; SURVIVAL-; TEMPERATURE-; Dipteran host selection relationships, adaptive variation significance 1. The braconid Asobara tabida is a widespread parasitoid of Drosophila larvae in fermenting substrates. 2. Across its geographic range, survival probability in D. subobscura larvae is high and fairly constant; survival probability in D. melanogaster larvae shows much variation. 3. When females of different strains are offered a choice between larvae of the two host species, the degree of preference for D. subobscura larvae is linked to the difference in survival probability in the two host species. 4. Host selection behaviour of the females matches the predictions made by an optimal foraging model. 5. Survival probability in D. melanogaster also depends on temperature and host selection behaviour appears to be influenced by temperature as well. 6. It is concluded that host selection behaviour of A. tabida is a trait with phenotypic plasticity on top of genetic differences.