Background
The plant-insect system studied in this experiment - Birdfoot Deer Vetch (Lotus corniculatus L.) and the Common Blue Butterfly (Polyommatus icarus) - had earlier been studied in both a growth-chamber and a greenhouse experiment (Goverde et al., 1999; Bazin et al., 2002); and the authors report that the elevated CO2 concentrations of those experiments caused L. corniculatus to increase its biomass and accumulate starch, while P. icarus feeding on the CO2-enriched plants "showed higher rates of consumption, conversion efficiency and consequently accelerated growth."

What was done
In the current study, plants were grown from seed for three months in tubes recessed into the ground under natural conditions in a nutrient-poor calcareous grassland, where an extra 232 ppm of CO2 was supplied to them via a screen-aided CO2 control (SACC) system (Leadley et al., 1997, 1999) and insect larvae were allowed to feed on the plants (half of which received extra phosphorus fertilizer) for the final month of the experiment.

What was learned
The 63% increase in atmospheric CO2 concentration increased the total dry weight of plants growing on the unfertilized soil by 21.5% and that of the plants growing on the phosphorus-enriched soil by 36.3%.  However, the elevated CO2 treatment had no effect on pupal and adult insect mass.  Nevertheless, Goverde et al. report there were "genotype-specific responses in the development time of P. icarus to elevated CO2 conditions," with larvae originating from different mothers developing better under either elevated CO2 or ambient CO2, while for still others the CO2 concentration had no effect on development.

On an entirely different note, condensed tannins in the foliage of plants growing on the nutrient-poor calcareous soil were increased by 23.7% in the CO2-enriched air.

What it means
The results of this study and its two predecessors suggest that the ongoing rise in the air's CO2 content will likely be positive for L. corniculatus plants, while it ranges from positive to nil for the insect herbivore P. icarus that feeds upon them.  In addition, the authors suggest that rising levels of CO2 might be "a selective factor, affecting both plant and herbivore populations and their interaction," and, therefore, that "genotype-specific responses must be considered because this will affect the outcome of elevated CO2 for plant-herbivore interactions."  It is currently unclear, however, what the range of those potential outcomes might be.

Last of all, since the presence of condensed tannins in foliage eaten by ruminants tends to decrease their emissions of methane, one might also expect the CO2-induced increases in the concentrations of these substances in the leaves of L. corniculatus may help to reduce the atmospheric concentration of this powerful greenhouse gas in a CO2-enriched world of the future.

References
Bazin, A., Goverde, M., Erhardt, A. and Shykoff, J.A.  2002.  Influence of atmospheric CO2 enrichment on induced defense and growth compensation after herbivore damage in Lotus corniculatus.  Ecological Entomology 27: 271-278.

Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A.  1999.  Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype.  Functional Ecology 13: 801-810.

Leadley, P.W., Niklaus, P., Stocker, R. and Korner, C.  1997.  Screen-aided CO2 control (SACC): a middle-ground between FACE and open-top chamber.  Acta Oecologia 18: 207-219.

Leadley, P.W., Niklaus, P.A., Stocker, R. and Korner, C.  1999.  A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland.  Oecologia 118: 39-49.