Oecologia (2009) 160:267–277 Fig. 1 Mean shoot biomass (a) and mean root biomass (b) at harvest. Closed bars without aphids, open bars with aphids. c Aphid population growth rates per day, d proportion of aphid populations with negative growth rates. DiVerent letters above bars indicate a signiWcant diVerence between arbuscular mycorrhizal fungal (AMF) treat- ments, asterisks indicate a sig- niWcant diVerence between aphid-infested and -uninfested plants within the AMF treat- ments (P < 0.05) according to orthogonal contrasts, whiskers standard error Source of variation Initial plant size Block Aphid presence AMF AMF vs. control Intra vs. moss Aphid presence £ AMF Residuals df 1 9 1 2 1 1 2 162 Table 1 Results of ANCOVA on shoot and root biomass of Phleum pratense at harvest Shoot biomass Root biomass MS 0.09 0.10 0.43 0.12 0.21 0.03 0.01 0.03 F value 3.21 3.34 14.57 3.91 6.94 0.86 0.47 P value 0.08 <0.001* <0.001* 0.02* 0.009* 0.36 0.62 MS 0.63 0.92 0.61 1.04 2.07 0.01 0.53 0.15 F value 4.13 6.06 3.99 6.84 13.60 0.10 3.48 * SigniWcant at P < 0.05 AMF, Arbuscular mycorrhizal fungi; ANCOVA, analysis of covariance Orthogonal contrasts are calculated for AMF treatments (see Materials and methods for details) control, G. intraradices, and G. mosseae treatments, respectively (Fig. 1a). The ANCOVA showed a signiWcant eVect of the interaction between AMF and aphids on root biomass. This reXects the increase in root biomass (+12.8%) in the G. mosseae treatment under aphid presence (Fig. 1b), whereas root biomass was strongly reduced under aphid presence in the control (¡32.3%) and G. intraradices (¡38.3%) treatments, respectively. Plant C, N, and P contents are given in Table 2. The val- ues varied slightly between the fungal treatments, but ANOVA showed no signiWcant diVerences. Aphid population development Aphid population growth rates were signiWcantly smaller in the AMF-inoculated plants (growth rate per day § SE: 2.5 § 0.8 and 3.5 § 1.2 for G. intraradices and G. mosseae, respectively) compared to control plants (6.2 § 1.6, Fig. 1c; Table 3). No diVerence between the eVect of the two AMF species was found in aphid population growth rates. According to the analysis of deviance, aphid popula- tions with negative growth rates were also more frequent on mycorrhizal plants (42 and 38% for G. intraradices and G. mosseae, respectively) than on non-mycorrhizal plants (24%; Fig. 1d; Table 3). Parasitoid presence had no eVect on aphid numbers at harvest. We detected winged aphids on two pots in the non- mycorrhizal control only, with four and one winged aphid, respectively. Parasitoid responses Emerging parasitoid wasps were signiWcantly heavier when they developed on plants of the two AMF treatments (mean 271 P value 0.04* <0.001* 0.05* 0.001* <0.001* 0.76 0.03* 123