268 Introduction Multitrophic interactions between above- and belowground organisms are powerful forces shaping the structure and diversity of natural communities (van der Putten et al. 2001). For example, belowground herbivores can inXuence aboveground herbivores via a shared host plant and vice versa (van Dam et al. 2003; Wurst and van der Putten 2007). One interaction that has been found to aVect the per- formance of both above- and belowground organisms is the symbiosis between plants and arbuscular mycorrhizal fungi (AMF; Bennett et al. 2006; Bezemer and van Dam 2005; Gehring et al. 2002). The infection of plants by AMF aVects the interactions of the former with root pathogenic fungi (Newsham et al. 1995), Collembola (Gange 2000), saprotrophic fungi (Tiunov and Scheu 2005), above- and belowground herbivores (Gange 2001; Goverde et al. 2000), and parasitic plants (Stein et al. 2009). Aphids, as one guild of herbivores directly feeding on plant phloem, can be inXuenced by AMF colonizing the roots of their host plants (e.g., Gange et al. 1999; Guerri- eri et al. 2004; Wurst et al. 2004), but the direction of the eVects have varied between diVerent experiments. While Gange and West (1994) and Gange et al. (1999) found a positive inXuence of AMF on weight and fecundity of two Myzus species reared on Plantago lanceolata, negative AMF eVects were reported with Chaitophorus populicola reared on Populus angustifolia £ P. fremontii (Gehring and Whitham 2002) and Macrosiphum euphorbiae reared on Lycopersicon esculentum (Guerrieri et al. 2004). One possible explanation for this inconsistency in results may be the variability of arbuscular mycorrhizal symbiosis itself, which ranges from mutualism to parasitism depend- ing on various biotic and abiotic factors (Johnson 1993; Klironomos 2003). In addition, infection by diVerent AMF species can have diVerent eVects on several plant traits, such as biomass or nutrient capture (van der Heij- den et al. 1998). There are also indications that AMF infection of plants can have cascading eVects in the food chain up to higher trophic levels (Gange et al. 2003). For example, there is evidence that AMF symbioses with plants can aVect both the rate of aphid parasitism by para- sitoid wasps (Gange et al. 2003) and parasitoid prefer- ence, where aphid infested non-mycorrhizal plants are as attractive to parasitoid wasps as uninfested mycorrhizal plants (Guerrieri et al. 2004). However, both of these studies did not directly assess parasitoid performance, although it is likely that the strong eVects of AMF reported on primary producers (plants) and primary con- sumers (herbivores) cascade upwards in the food chain and thus also aVect several traits in predator or parasitoid performance, such as food consumption or reproductive output (Bezemer et al. 2005). 123 Oecologia (2009) 160:267–277 The objective of this study was to test AMF species eVects on the tri-trophic interaction of a typical grassland plant species (Phleum pratense), its insect herbivore Rho- palosiphum padi L., and the parasitoid Aphidius rhopalos- iphi. In a greenhouse experiment, the grass was inoculated with either one of the two AMF species, Glomus intrara- dices or G. mosseae, and the results compared to a non- mycorrhizal control. These three treatments were combined with three insect treatments: (1) plants only (no insects), (2) plant + aphid, and (3) plant + aphid + parasitoid. We hypothesized that: 1) the association with AMF improves plant biomass and nutrient capture; 2) there is an increase in food quality which beneWts aphid reproduction and supports larger aphid populations on mycorrhizal plants; 3) larger aphid populations allow female parasitoids to choose more suitable aphids for parasitization, which leads to an increase in parasitoid weight and a decrease in parasitoid development time; and that 4) the two AMF species have diVerent eVects on the tri- trophic interaction. Materials and methods Plant, aphid, and parasitoid material Plant seeds and soil were collected from a hay meadow in the Franconian Forest in Central Germany (11°26 44 E/ 50°23 04 N). We collected seeds from Phleum pratense L. (timothy grass) in the summer and autumn of 2006. Phleum pratense is common in European grasslands and an impor- tant grass for hay production. The substrate used in the experiment consisted of 50% sieved soil (1 cm) collected from the top 10 cm of the Weld site and 50% washed silica sand. The substrate was heated for 48 h at 200°C to kill soil organisms, including AMF. Pre-experimental soil analyses showed soil nutrient contents of 0.48% organic carbon (C), 0.1% nitrogen (N), and 36.85 g g¡1 plant available phos- phorus (P) at a pH (H2O) of 6.6. Inoculum of two AMF species, Glomus intraradices N.C. Schenck & G.S. Sm. isolate BEG140 and G. mosseae (T.H. Nicolson & Gerd.) Gerd. & Trappe isolate BEG25, were purchased as two separate mixtures of spores and mycorrhizal roots from a commercial supplier (SYMbio-M, Lannkroun, Czech Republic). Both isolates have been widely used in greenhouse experiments, and both species are commonly found in grasslands (Hempel et al. 2007; Rosendahl and Stukenbrock 2004). Aphids (R. padi L., cherry oat aphid) were purchased from Katz Biotech AG (Bayreuth, Germany) and propagated