Oecologia (2009) 160:267–277 Table 4 Results of ANCOVA on mean parasitoid dry mass and development time as means per pot * SigniWcant at P < 0.05 Orthogonal contrasts are calculated for AMF treatments (see Materials and methods for details) Source of variation Number of aphids Sex ratio AMF AMF vs. control Intra vs. moss Residuals df 1 1 2 1 1 21 Dry weight MS 357.3 4316.9 2726.6 5401.8 28.4 819.8 0.44 5.27 3.33 6.59 0.03 F value P value 0.52 0.03* 0.05* 0.02* 0.85 Development time MS 28.84 1.99 19.64 33.78 4.84 3.16 F value 9.14 0.63 6.22 10.70 1.53 273 P value 0.007* 0.44 0.008* 0.004* 0.23 Fig. 3 Scatterplot of the number of unparasitized against the number of parasitized aphids per pot for control (circles), G. intraradices (squares), and G. mosseae (diamonds) treatments. Major axis regres- sion: control (solid line, y = 0.785 £ ¡0.787, R2 = 0.89), G. intrara- dices (dashed line, y = 0.942 £ ¡0.789, R2 = 0.82), G. mosseae (dotted line, y = 0.913 £ ¡1.097, R2 = 0.93) signiWcant diVerence in aphid density-dependent reactions of parasitoids between the fungal treatments (expressed as slope, P = 0.64). The rates of parasitism decreased with increasing number of aphids (slope < 1). Discussion In contrast to our initial hypotheses, positive eVects of AMF inoculation on performance could only be observed at two trophic levels. One is the level of primary producers (Phleum pratense), which beneWtted from the association in terms of an increase in biomass, and the other level is that of the parasitoids (Aphidius rhopalosiphi), which showed increased weight at eclosion and shorter developmental time on mycorrhizal plants (Figs. 1, 2). Population growth rates of aphids (R. padi) as primary consumers decreased on plants inoculated with AMF. Although we did not detect signiWcant diVerences between the two inoculated AMF species concerning aphid population growth rates, such diVerences were clearly present in terms of plant biomass under aphid attack and in the rates of parasitism. These diVerences may reXect direct physiological eVects of the two AMF species, but they may also result from the observed diVerences in mean colonization rate between G. intraradices and G. mosseae (42 and 21%, respectively). The diVerent mycorrhization rates, in turn, may be an arte- fact of our experiment, but we suggest that they instead reXect innate diVerences between AMF species, as shown by Hart and Reader (2002), because we used the same amount of inoculum for both AMF species. Nevertheless, the results of our experiment do not allow the indirect eVects of diVerent colonization rates to be disentangled from the direct physiological eVects of AMF species, and we also have to consider with caution any conclusion regarding species-speciWc eVects on higher trophic levels. It has to be considered that the AMF inoculum (a com- mercial cultivar) and plant seeds used in this study share no common ecological background and conceivably are not adapted to each other (Fitter et al. 2005). Klironomos (2003) showed that the combination of non-adapted AMF and plants can narrow the range of host plant responses. Yet, our study demonstrates potential eVects of diVerent AMF species on plants and higher trophic levels, rather than revealing the actual outcome of these interactions under natural conditions. However, a positive impact of the two AMF isolates on plant biomass was present. Highly adapted AMF can be expected to provide even more bene- Wts to their host plants (Helgason et al. 2007); as such, com- parable or even stronger eVects on higher trophic levels may be expected under more natural conditions. Arbuscular mycorrhizal fungi and aphid eVects on plants The positive eVect of AMF inoculation on plant biomass was also present at the two interim clippings (data not shown). In the case of G. mosseae, this positive eVect was clearly not reXected in foliar N and P contents (Table 2), as these values tended to be lower than in the control. Such species-speciWc eVects of AMF on several plant variables are also in accordance with previous studies (see Jansa et al. 2008; Maherali and Klironomos 2007; van der Heijden et al. 1998), which showed that biomass and nutrient capture of a 123