Elevated carbon dioxide impairs the performance of a specialized parasitoid of an aphid host feeding on Brassica plants J. Klaiber, A.J. Najar-Rodriguez ⇑ , E. Dialer, S. Dorn ETH Zurich, Institute of Agricultural Sciences/Applied Entomology, Schmelzbergstrasse 9/LFO, 8092 Zurich, Switzerland highlights  Effects of elevated CO 2 on a Brassica – aphid – parasitoid system were investigated.  Secondary metabolites induced by plant acclimation to elevated CO 2 were quantified.  Glucosinolates increased in Brassica and decreased in aphids under elevated CO 2 .  Aphid body mass remained approximately 20% lower under elevated CO 2 .  Parasitism efficiency by Diaeretiella rapae was impaired under elevated CO 2 . graphical abstract article info Article history: Received 1 November 2012 Accepted 3 March 2013 Available online 21 March 2013 Keywords: Diaeretiella rapae Carbon dioxide Brassica Glucosinolate Brevicoryne brassicae Parasitism rate abstract Elevated concentrations of atmospheric carbon dioxide (CO 2 ), a consequence of anthropogenic global change, may profoundly interfere with tritrophic interactions. Such effects with a focus on parasitoids as natural antagonists of herbivores have rarely been investigated. In particular, studies on effects of sec- ondary metabolites induced by crop plant acclimation to elevated CO 2 on higher trophic levels were yet missing. We used the system composed of Brassica plants, the aphid Brevicoryne brassicae and the endo- parasitoid Diaeretiella rapae, which is specialized on aphids feeding on brassicacean plants, to compare effects of elevated CO 2 (800 ppm) versus ambient CO 2 (400 ppm). Plants were exposed to the CO 2 concen- trations for up to 10 weeks, aphids for 2–3 generations, and parasitoids for 1 generation, to allow for acclimation. Concomitant bioassays with herbivore-infested plants and parasitoids showed a signifi- cantly lower proportion of hosts parasitized under elevated compared to ambient CO 2 after a 10-week plant exposure. Parasitoid progeny emerged earlier but offspring adults were shorter lived. Plant gluco- sinolate concentrations were higher under elevated compared to ambient CO 2 , whereas, contrary to expectation, aphid glucosinolate concentrations were significantly lower. Likewise aphid body mass remained approximately 20% lower under elevated compared to ambient CO 2 . Thus, elevated CO 2 seems to have enhanced plant direct defense by an increase of natural plant defense compounds, however, it led to a decrease in indirect defense, likely due to the reduced host size. Our results point, for the first time, to a conflict between bottom-up and top-down control under elevated CO 2 . Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Parasitoid performance may be altered in a tritrophic context by environmental factors predicted to change under the future cli- mate scenario (Roth and Lindroth, 1995; Sun et al., 2011b). The 1049-9644/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biocontrol.2013.03.006 ⇑ Corresponding author. Fax: +41 44 632 11 71. E-mail addresses: adriana.najar-rodriguez@ipw.agrl.ethz.ch, ajnajar@hotmail. com (A.J. Najar-Rodriguez). Biological Control 66 (2013) 49–55 Contents lists available at SciVerse ScienceDirect Biological Control journal homepage: www.elsevier.com/locate/ybcon