Reviewer 2's Review

“The present paper describes experimental evidence that the corn leaf aphid Rhopalosiphum maidis has an increased fitness on different transgenic Bt-lines of Zea mays in comparison to the respective near-isogenic lines. Furthermore, the authors found that females of the parasitoid Cotesia margeniventris, parasitizing herbivorous caterpillars and using aphid-produced honeydew as a source of nutrients, have an increased longevity and fecundity in the presence of R. maidis on Bt-maize compared to R. maidis on the respective near-isogenic maize lines. The authors determined the amino acid composition of phloem sap from Bt- and near-isogenic maize and found some apparently subtle differences which they conclude may in part explain the improved aphid performance on Bt-maize. To find an explanation for the increased parasitoid fitness, the authors analyzed the sugar composition of honeydew from aphids on Bt and near-isogenic maize. They did not find any significant differences in the composition of sugars leading them to conclude that it is the increased availability of honeydew rather than qualitative differences that was responsible for the observed effect on the parasitoids.
The ms. is generally well written and the results are original since they add another facet to the study of effects of genetically modified plants on non-target organisms. Surprisingly, the natural enemy (parasitoid) of the target organism (caterpillar) benefits from an increased performance of another herbivore (aphid) on Bt-maize which finally should have a positive effect on caterpillar reduction. Thus, plant transformation might have both direct (by the Bt-toxin) and indirect (by the parasitoid) effects on the target organism. I am sure that these findings are interesting for a broad readership. As for the mechanisms that might have caused the observed effects, however, the authors have to argue a little bit more conservative and provide more experimental details, respectively.”

N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.