EntomologiaExperimentalis etApplicata 74: 259-265, 1995. 259 @ 1995 KluwerAcademicPublishers. Printed in Belgium. Variation in diapause and sex ratio in the parasitoid Asobara tabida Alex R. Kraaijeveld* & Jacques J. M. van Alphen University of Leiden, Institute for Evolutionary and Ecological Sciences, Kaiserstraat 63 PO Box 9516, 2300 RA Leiden, The Netherlands *Present address: Imperial College, Silwood Park, Dept. of Biology, Ascot, Berks. SL5 7PY, UK Accepted: July 14, 1994 Key words: diapause, sex ratio, Asobara tabida, parasitoid, geographic variation Abstract Asobara tabida Nees (Hymenoptera: Braconidae) is a widespread parasitoid, attacking larvae of Drosophila (Diptera: Drosophilidae) species in fermenting substrates. In this species, geographic variation is found in the percentage of parasitoids entering diapause and in the sex ratio of emerging parasitoids. Percentage diapause appears to be influenced by host species (more parasitoids enter diapause in D. melanogaster Meigen than in D. subobscura Collin) and temperature. It is not correlated with any of the abiotic factors investigated, but is correlated with survival probability in D. melanogaster larvae and with the time of year in which the experiment was conducted (even though none of the parasitoids experienced natural day light). Sex ratio was only found to correlate with percentage diapause, suggesting that males enter diapause more frequently than females. It is concluded that A. tabida uses diapause to survive both unfavourable abiotic and biotic circumstances. Introduction Asobara tabida is a braconid parasitoid, attacking Drosophila larvae in fermenting substrates (e.g., rot- ting fruits or sap fluxes of bleeding trees). In Europe, this holarctic species is known from the Mediterranean to southern Scandinavia. Thus, it is likely that par- asitoids in different parts of the species' range live under different biotic and abiotic circumstances. One of the biotic circumstances that varies across the parasitoid's range is the relative abundance of lar- vae of the two most important host species. In the Mediterranean, D. melanogaster larvae are the main hosts available, whereas D. subobscura larvae are the main hosts in more northern parts of Europe. Larvae of D. melanogaster can encapsulate parasitoid eggs (Nappi, 1981; Rizki & Rizki, 1984), whereas larvae of D. subobscura lack this ability (Baker, 1979; Molle- ma, 1988; Kraaijeveld & van Alphen, 1993; Kraai- jeveld & van der Wel, 1994). Parasitoids from Mediter- ranean populations survive much better in sympatric D. melanogaster larvae than parasitoids from north- ern, western and central European populations (Kraai- jeveld & van der Wel, 1994): Mediterranean para- sitoids have a mechanism to prevent complete encapsu- lation of their eggs (Kraaijeveld & van Alphen, 1994). Parasitoid strains also differed in their host selection behaviour: Mediterranean parasitoids accept larvae of both host species for oviposition, whereas parasitoids from western and central Europe prefer D. subobscu- ra larvae over D. melanogaster larvae (Kraaijeveld, 1994; Kraaijeveld et al. unpubl.): when given a choice between the two host species, they accept D. subob- scura larvae and reject D. melanogaster larvae. Another biotic factor that may vary geographically is the number of females a patch (i.e., a rotting fruit or a sap flux) will attract. Preliminary observations indi- cate that mating in A. tabida takes place at the site of emergence (pers. obs. of emerging males which stay at the emergence site, apparently waiting for the females to emerge). Due to this local mating structure, the opti- mal sex allocation strategy for an ovipositing female depends on the expected number of females to visit a patch. This sex ratio will be more female-biased in areas where patches are rarely visited by more than one female than in areas where patches are visited by