The objective of this study was to determine how increasing atmospheric CO2 change plant tissue quality in four native grassland grass species (Agrostis stolonifera, Anthoxanthum odoratum, Festuca rubra, Poa pratensis) which are all larval food-plants of Coenonympha pamphilus (Lepidoptera, Satyridae). We assessed the effect of these changes on the performance and larval food-plant preference of C. pamphilus in a greenhouse experiment. Furthermore, we tested the interactive effects of elevated CO2 and soil nutritional availability in F. rubra and its effect an larval development of C. pamphilus. In general, elevated CO2 decreased leaf water concentration, nitrogen concentration and specific leaf area (SLA), while leaf starch concentration was increased in all grass species. A species-specific reaction to elevated CO2 was only found for foliar starch concentration. P. pratensis did not increase its starch concentration under elevated CO2 conditions, whereas the other three species did. Fertilisation, investigated only for F. rubra, increased leaf nitrogen concentration and amplified the CO2-induced decrease in leaf nitrogen. Development time of C. pamphilus was on the average prolonged by two days under elevated CO2 and the prolongation differed from 0.7 to 5.3 days among food-plant species. Pupal fresh weight differed marginally between CO2 treatments. Fertilisation of the larval food-plant F. rubra shortened development time by one day and significantly increased pupal and adult fresh weights. C. pamphilus larvae showed a clear food-plant preference among grass species at the age of 36 h or older. Additionally, a change of food-plant preference under elevated CO2 was found. Larvae at ambient CO2 preferred Agrostis stolonifera and F. rubra, while under elevated CO2Anthoxanthum odoratum and P. pratensis were preferred. The present study demonstrates that larval development of C. pamphilus is affected by food-plant species and CO2 induced changes in foliar chemistry. Although we found some species-specific reactions to elevated CO2 for foliar chemistry, no such CO2 by species interaction was found for insect development. The change in food-plant preference of larvae under elevated CO2 implies potential changes in selection pressure for grass species and might therefore affect evolutionary processes.
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