Oecologia (2009) 160:267–277 on wheat (Triticum aestivum L.). Rhopalosiphum padi has been shown to be compatible with Phleum pratense (Orlob 1961) and is widely used in greenhouse experiments (Ponder et al. 2000; Vestergård et al. 2004). We chose the parasitoid wasp species Aphidius rhopalosiphi (DeStefani-Perez), which is a natural enemy of R. padi (e.g. Gonzáles et al. 1999) commonly occurring throughout Northern Europe (Muratori et al. 2004). Wasps were bought as mummies (Katz Biotech AG) and allowed to hatch and mate. After 2 days, the wasps were anaesthetized with CO2 and sorted according to sex. Single female wasps between 2 and 4 days old were then introduced into the parasitoid treatments. Experimental set-up The experiment was set up in ten blocks in the greenhouse in a full randomized block design. Three mycorrhizal treat- ments (non-mycorrhizal control, inoculation with G. intra- radices or with G. mosseae) were combined with the three insect treatments (no insects added, aphids added, or aphids and female parasitoid added). These nine treatments were replicated 20 times, resulting in 180 pots in total. Two plants from each of the nine treatments were randomly assigned to each block. Each 1-l pot (height 13.5 cm, diameter approx. 10 cm) was Wlled with 2 cm expanded slate and 1 cm washed sand for drainage. Pots were then Wlled with sterile substrate, with the mycorrhizal inoculum placed 1 cm below the sur- face. Each third of the pots received 15 g inoculum of either G. intraradices, G. mosseae or an autoclaved mixture of both (non-mycorrhizal control). To establish a natural microbial community, we irrigated all pots with 10 ml soil suspensions from the Weld soil Wltered through a Whatman Wlter paper No. 4 with pore sizes of 20–25 m (Whatman International, Kent, UK) to exclude AM propagules from the suspension (Schroeder and Janos 2004). A bulk seed collection of Phleum pratense was germinated in sterile sub- strate. After 2 weeks, one seedling was planted into each pot and its height recorded as initial plant size. Temperatures in the greenhouse ranged from 18°C (14-h day) to 13°C (10-h night) with additional light provided by 400 W lamps. Plants were watered three times a week with tap water. The plants were cut 2 cm above the soil surface 15 and 21 weeks after planting to mimic the mowing regime of the grassland the plants originated from. This time period also provided the plants with enough time to establish mycorrhi- zal symbiosis. One week after the second cut, Wve R. padi instars (3–5 days old) were added to the respective treat- ments (120 pots) using a Wne brush. All pots were encaged in air-permeable cellophane bags (width 185 mm, length 390 mm). Twenty-Wve days after the aphids had been intro- duced, single females of Aphidius rhopalosiphi were intro- duced into the parasitoid treatments (60 pots) and allowed 269 to parasitize aphids for 12 h during daytime, after which they were removed from the cellophane bags. Plants were harvested 2 weeks after introduction of the parasitoids (i.e., 39 days after aphid introduction), when visible mummies had developed. The shoots were cut at the soil surface, and aphids and mummies were carefully separated from plant material. Plant measurements Plant roots were washed free of soil, and a 2-g root aliquot from each pot was stored in formaldehyde–acetic acid (FAA: aqueous solution of 6.0% formaldehyde, 2.3% gla- cial acetic acid, 45.8% ethanol, all v/v). Root subsamples stored in FAA from Wve pots of each mycorrhizal and con- trol treatment were stained in lactophenol blue solution according to Phillips and Hayman (1970), with modiWca- tions of Schmitz et al. (1991). We studied 300 stained root segments under a light microscope at 200£ magniWcation using the line intersect method (Brundrett et al. 1996) and detected mean mycorrhizal colonization rates of 42 and 21% in the G. intraradices and G. mosseae treatments, respectively. The AMF structures were absent in the control treatment. Above- and belowground plant material was dried at 60°C for 48 h and then weighed. Phosphorus concentra- tions and total N and C content from Wve plants in the mycorrhizal and control treatments were also determined using plant material ground in a ball mill. The P concentra- tions were analyzed with a CIROS ICP spectrometer (SPECTRO Analytical Instruments, Kleve, Germany) fol- lowing combustion of the subsamples at 550°C and dissolu- tion of the ash with 4 N nitric acid. Total N and C contents were measured with an Elementar Vario EL element ana- lyzer (Elementar Analysengeräte, Hanau, Germany). Aphid and parasitoid measurements The numbers of aphids per plant were determined 11 days after the aphids had been added to the plant system in order to monitor the establishment of their populations. No counts were carried out thereafter during the experiment to avoid aphid disturbance (Godfray 1994). Aphids, winged aphids and mummies were counted at the end of the experi- ment (day 39). Mummies were placed singly into gelatine capsules and put in a growth chamber (16/8-h light/dark photoperiod, with 22:12°C day/night temperatures, 50% relative humidity) until emergence. Capsules were checked three times per day. Freshly hatched wasps were immedi- ately frozen and their developmental time recorded until all wasps had emerged 1 week after the end of the experiment (day 46). Wasps were sexed, dried at 60°C for 24 h, and weighed. For each mummy, we determined whether the 123