Oecologia (2009) 160:267–277 decreased development time, which is also reXected by the signiWcant inXuence of the ratio between sexes within pots on parasitoid dry weight (Table 4). Despite this sex-speciWc diVerence in the G. mosseae treatment, the observed diVer- ences in parasitoid dry weight and development time between mycorrhizal and control treatments were not due to changes in sex ratio, as this variable was relatively constant in all AMF treatments (proportion of females 44.8 § 7.1%). Alternatively, it might be the case that male and female para- sitoids can use the resources provided by G. mosseae infec- tion in diVerent ways. This hypothesis needs to be addressed in further studies. Parasitoid developmental time was correlated with aphid density, i.e., parasitoids developed faster when more aphids were available for oviposition. One possible explanation for this relationship is that at higher aphid densities, the parasit- oids encounter more aphids of diVerent larval instars and hence are able to carry out more ovipositions in more suit- able aphid stages. In the pea aphid (Acyrthosiphon pisum), oviposition in intermediate instars reduced the developmen- tal time of Aphidius ervi relative to ovipositions in younger instars or adults (Sequeira and Mackauer 1992). While we did not directly study parasitoid oviposition, such selection behavior is conceivable. More generally, there is little infor- mation about density dependence in parasitoid host selection behavior and the consequences for oVspring Wtness. Sequeira and Mackauer (1992) showed that the values of weight and developmental time covary and are furthermore highly dependent on the age of the parasitized aphids. This association was not present in our study, as the proportion of aphids that died as adults was the same in the two AMF inoculation treatments and the control, indicating that aphids were parasitized at comparable larval stages in all fungal treatments. In accordance to the study by Gange et al. (2003), we found changes in the rates of parasitism, expressed as a signiWcant higher proportion of parasitized aphids in the G. intraradices treatment (Fig. 3). Gange et al. (2003) par- tially attributed their observed mycorrhizal eVects on the rates of parasitism to a decreased parasitoid searching eYciency due to changes in plant architecture. However, the limited space under the cellophane bags in our experi- ment surely interfered with this eVect. Additional eVects, such as the induction of volatiles inXuencing parasitoid activity, are also likely, as these can be AMF species-speciWc (Bezemer and van Dam 2005). Interactions of belowground organisms with plant roots resulting in contrasting reactions on aboveground aphids and parasitoids were also reported by Bezemer et al. (2005). They attributed increasing parasitoid performance to a visually observed increase in aphid size, although they did not explicitly quantify this parameter. Another possible explanation for the observed eVects on aphid and parasitoid 275 level would be a decrease in the aphid immune answer against parasitoid eggs on mycorrhizal plants, which could have led to an increase in parasitoid performance (W. Völkl, personal communication; see also Godfray 1994). However, all of these hypothetical mechanisms do not seem to follow a linear relation, as aphid population growth rates were highest on control plants, intermediate on plants from the G. mosseae treatment, and worst on G. intraradices- inoculated plants. In contrast, parasitoid weight and development time were best on G. mosseae-inoculated plants, worst on control plants, and intermediate with G. intraradices. Conclusion Our results show that three interacting trophic levels are signiWcantly aVected by both the presence and the species identity of AMF. Bottom–up eVects of AMF inXuenced plants, aphids, and their parasitoids diVerently, with a posi- tive impact on plants and parasitoids and a negative impact on aphids. However, changes in plant nutrient contents C, N, and P) were not driving the observed performance alter- ations, as these values were equal between mycorrhizal and non-mycorrhizal plants. Therefore, food choice experi- ments (Prince et al. 2004) and stable isotope probing (Langellotto et al. 2006) would be useful approaches for monitoring changes in preferences and nutrient Xuxes. The observed changes in the trophic interactions due to AMF inoculation emphasize that belowground interactions can have strong implications for aboveground food webs (van der Putten et al. 2001). Models of trophic interactions (Hoover and Newman 2004; van der Putten et al. 2004) should include the impact of a symbiosis as widespread as arbuscular mycorrhiza (Treseder and Cross 2006). Acknowledgments This study was part of BIOLOG-Europe, a bio- diversity scientiWc program funded by the German Federal Ministry of Education and Research (BMBF), Grant no. 01LC0013. The authors would like to thank W. Völkl for suggestions concerning aphid and parasitoid handling. Help and support in the greenhouse by all mem- bers of the mycology group, Department of Soil Ecology, is gratefully acknowledged. We are in debt to Dirk Krüger for correcting the man- uscript. We would also like to thank the Editor in Chief Christian Kör- ner, the Handling Editor, and three anonymous reviewers for helpful comments on earlier versions of the manuscript. The experiments com- ply with current German laws. References Bennett AE, Alers-Garcia J, Bever JD (2006) Three-way interactions among mutualistic mycorrhizal fungi, plants, and plant enemies: hypotheses and synthesis. Am Nat 167:141–152 Bezemer TM, van Dam NM (2005) Linking aboveground and below- ground interactions via induced plant defenses. Trends Ecol Evol 20:617–624 123