274 plant community varied independently with the identity of the inoculated AMF species. Although aphid presence had a consistently negative impact on shoot biomass, this reduction was only signiW- cant in the control and G. intraradices-inoculated plants, indicating a higher tolerance to aphid feeding in the G. mosseae-inoculated plants (Fig. 1a). In contrast, an inconsistent pattern was observed in root biomass. Plants inoculated with G. mosseae increased their root biomass under aphid presence, whereas lowered biomass was detected in control and G. intraradices-inoculated plants under aphid herbivory (Fig. 1b). Such interactive eVects of AMF species and aphids have been reported previously (Gange and West 1994) and may reXect diVerences in nutri- ent allocation within plants under aphid attack (Vestergård et al. 2004). AMF eVects on aphids The negative eVect of AMF inoculation on aphid popula- tion growth rates found in our experiment (Fig. 1c, d) con- tradicts some results of other studies on AMF-aphid interactions (e.g., Gange et al. 1999; Gange and West 1994). However, negative interactions have also been reported by other authors (Gehring and Whitham 2002; Guerrieri et al. 2004; Wurst et al. 2004). In most of the respective publications, AMF eVects on aphids were mea- sured using the reproductive Wtness of individual females (e.g. Gange et al. 1999; Wurst et al. 2004). In our study, however, we measured aphid response to AMF in terms of population growth rather than in terms of individual Wtness. Although individual Wtness and population growth rate may be correlated (Ponder et al. 2000), we would like to empha- size that we can draw conclusions only for AMF eVects on the population growth of aphids. Three potential mechanisms limiting aphid growth in our experiment must be considered: aphid crowding, nutrient limitation, and plant defense compounds. Winged morphs, a good indicator of aphid crowding (Hodgson 2001), were rarely detected (in two pots only). Nutrient limitation is also unlikely, as aphid population growth rates (Fig. 1c) and plant nutrient contents at harvest (Table 2) show no correlation. Inoculation with G. intraradices induced the highest decrease in aphid population growth rates, but the respective plants contained as much N as control plants and tended to contain even more P than plants of the two other fungal treatments. This lack of correlation between N con- tents and aphid performance is in accordance with a Weld study by Gange and West (1994), who hypothesized that changes in aphid numbers were more related to a changed leaf morphology (phloem location and size) in mycorrhizal plants than to diVerences in N content. In contrast, previous studies (Bezemer et al. 2005; Ponder et al. 2000) reported 123 Oecologia (2009) 160:267–277 decreased aphid population sizes in parallel with decreased foliar N concentrations using the same aphid species (R. padi) as was used in our study. Another explanation for the high proportion of aphid populations with negative growth rates on mycorrhizal plants might be the presence of defence compounds in the phloem of Phleum pratense, indicating increased plant resistance against aphids induced by AMF (Pozo and Azcón-Aguilar 2007). Bezemer et al. (2005) have recently shown that R. padi might be sensitive to phenolic compounds encountered in the phloem, which may be synthesized at higher rate upon mycorrhizal inocu- lation (Zhu and Yao 2004). However, elicitation of defence compounds by repeated cutting of Phleum pratense is not likely, as plant response mechanisms triggered by wound- ing and by phloem feeding insects (i.e. aphids) follow diVerent signaling pathways (Pozo and Azcón-Aguilar 2007). The missing eVect of parasitoid presence on aphid num- bers is not surprising as the proportion of parasitized aphids was generally low (<16%) and aphid populations encoun- tered parasitoids only once during a 12-h period. AMF eVects on parasitoids First studies concerning the eVects of AMF on parasitoid wasps focused on parasitoid preferences. Gange et al. (2003) provided data on AMF species-dependent variations in the rates of parasitism of the ichneumon Diglyphus isaea parasit- izing the leaf mining Xy Chromatomyia syngenesiae. Guerri- eri et al. (2004) showed that non-mycorrhizal tomato plants infested with aphids were as attractive to Aphidius ervi as mycorrhizal, non-infested plants. In our study, we revealed that the performance of parasitoid wasps is also inXuenced by the presence and identity of AMF, as Aphidius rhopalos- iphi got heavier and developed faster when their host R. padi was reared on mycorrhizal plants (Fig. 2). Changes in weight at eclosion and developmental time are highly correlated to several Wtness traits, such as longevity, number of hosts attacked (in case of females), and number of matings achieved (in case of males; Godfray 1994). These changes were rather uniform for male and female parasitoids in the case of the G. intraradices treatment, but they varied sub- stantially between sexes in the G. mosseae treatment. While we cannot discern the underlying reasons for the sex-speciWc diVerence, one possibility is that females adjusted their behavior when plants were infested by G. mosseae such that fertilized eggs resulting in females were laid in hosts that diVered in size from those of unfertilized eggs. Alternatively, the observed sex-speciWc pattern in the G. mosseae treatment may indicate that larger female parasitoids enjoy a propor- tionally greater increase in Wtness than larger males (Godfray 1994). Therefore, female parasitoids may have invested additional resources in an increased weight rather than