270 aphids were adult at mummiWcation using the shape of the cauda as a criterion (Minks and Harrewijn 1987). Data analysis Calculations were carried out with JMP ver. 7 (SAS Insti- tute, Cary, USA), with a few exceptions (given below). One plant inoculated with G. mosseae and one control plant died during the experiment and were excluded from the analysis. As mortality caused our data to be unbalanced, we used type III sum of squares (Shaw and Mitchell-Olds 1993). Data on initial plant size, plant dry weights, and numbers of aphids were log transformed, and proportion data (sex ratio and proportion of adults among mummies) were arcsine- square root transformed to achieve normal distribution. The combined eVects of mycorrhizal treatments and aphids on shoot and root biomass were analyzed in separate analyses of co-variance (ANCOVA). Initial plant size was used as a covariate; block, aphid presence, AMF, and the interaction of the latter two were used as main eVects in both analyses. Additionally, using orthogonal contrasts, we tested the fol- lowing two initial hypotheses: (1) plants perform better with AMF than without—“control versus AMF” and (2) plants are diVerently aVected by the two AMF isolates— “G. intraradices versus G. mosseae”. These contrasts were also calculated on the level of aphids and parasitoids (see below). In addition, we compared root and shoot biomass of aphid-infested and uninfested plants within each mycor- rhizal treatment using orthogonal contrasts. Plant C, N, and P content data were compared in a one-way analysis of var- iance (ANOVA) between the fungal and control treatments. To test for possible eVects of mycorrhizal treatments on aphid population establishment (i.e., the number of aphids detected 11 days after adding), we used an ANCOVA with initial plant size as the covariate and block and AMF treat- ment as main eVects. According to this analysis, initial aphid population establishment was independent of mycor- rhizal treatments (F2,107 = 2.33; P = 0.10). It is conceivable that some of the released aphids were not able to localize adequate feeding sites on the plants in time and thus died due to starvation. Therefore, we used the result of our counting exercise 11 days after the addition of aphids to the plant system as a starting point and excluded all pots in the aphid treatments showing no aphids at this time point from further analysis. Aphid population growth rates per day were calculated between day 11 and 39 (harvest) for each pot. The impact of AMF treatments on aphid population growth rates was analyzed using an ANCOVA. We included initial plant size as the covariate, and block, para- sitoid presence, and AMF were used as main eVects. As the population growth rates were negative in one third of all pots, we analyzed the AMF treatment eVect on the propor- tion of pots with this negative growth pattern. We used an 123 Oecologia (2009) 160:267–277 analysis of deviance with quasi F-statistics, binomial error distribution, and logit link function with the same covariate and main eVects as for aphid population growth rates. This model Wts our data reasonably well, as indicated by the goodness-of-Wt statistics (model deviance = 85.0, df = 75, P = 0.2). We calculated the average parasitoid dry weight and the average development time for each pot. The impact of AMF inoculation on parasitoid dry mass and development time was then assessed using ANCOVA. To account for the highly variable number of mummies in each pot, we used this number as a weighting factor in the ANCOVA. Blocks were poorly replicated due to the extinction of aphid popu- lations on some plants and therefore excluded from the analyses. We used the number of aphids present on the respective plant at harvest as the covariate and parasitoid sex ratio together with AMF as the main factors in the anal- ysis. Aphid numbers and parasitoid sex ratio are very likely to have an inXuence on the dry weight and developmental time of parasitoids; as in larger aphid populations, oviposi- tions can be made in more suitable aphid stages, and male parasitoids are usually smaller and develop faster (Sequeira and Mackauer 1992). We also used ANCOVAs to sepa- rately analyze the impact of the diVerent mycorrhizal inoc- ulations on sex ratio and the number of adults among parasitized aphids, using the number of mummies as a weighing factor, the number of aphids as the covariate, and the AMF treatment as the main eVect. To test for the impact of AMF inoculation on rates of parasitism, we used major axis (MA) regression (SMATR ver. 2.0, Falster et al. 2006). Major axis regression is an appropriate method for evaluating the association between variables that have been measured with error, and where error variances are unknown, but expected to be within similar dimensions (Sokal and Rohlf 2003). With the algo- rithms given in SMATR we also compared intercept and slope between the MA regression of each mycorrhiza treat- ment to test for changes in the rates of parasitism and aphid density-dependent reactions of parasitoids, respectively. Results Plant responses Shoot biomass at harvest increased due to AMF inoculation (Fig. 1a), with orthogonal contrasts showing signiWcant diVerences compared to control plants; however, there were no diVerences between the two AMF species (Table 1). A similar pattern was found for root biomass (Fig. 1b), which showed an even stronger mycorrhizal eVect (Table 1). Aphids had a signiWcant negative impact on shoot biomass, with a reduction of 14.2, 10.3, and 5.2% in the