Background
Increases in the air's CO2 content typically lead to greater decreases in the concentrations of nitrogen and, therefore, protein in the foliage of C3 as compared to C4 grasses (Wand et al., 1999).  As a result, in the words of Barbehenn et al., "it has been hypothesized that herbivores will disproportionately increase their feeding damage on C3 plants to compensate for the larger changes in C3 plants in elevated CO2 (Lincoln et al., 1984, 1986; Lambers, 1993)."

What was done
To test this hypothesis, the authors grew Lolium multiflorum Lam. (Italian ryegrass, a common C3 pasture grass) and Bouteloua curtipendula (Michx.) Torr. (sideoats gramma, a native C4 rangeland grass) in chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which grasshopper (Melanoplus sanguinipes) nymphs that had been reared to the fourth instar stage were allowed to feed upon the grasses' foliage.

What was learned
As expected, foliage protein concentration decreased much more in the C3 grass than in the C4 grass (22% vs. 7%) when the grasses were grown in CO2-enriched air.  However, and "contrary to the hypothesis that insect herbivores will increase their feeding rates disproportionately in C3 plants under elevated atmospheric CO2," Barbehenn et al. report that "M. sanguinipes did not significantly increase its consumption rate when feeding on the C3 grass grown under elevated CO2," suggesting that this observation implies that "post-ingestive mechanisms enable these grasshoppers to compensate for variable nutritional quality in their host plants" and noting that some of these post-ingestive responses may include "changes in gut size, food residence time, digestive enzyme levels, and nutrient metabolism (Simpson and Simpson, 1990; Bernays and Simpson, 1990; Hinks et al., 1991; Zanotto et al., 1993; Yang and Joern, 1994a,b)."  In addition, their data indicate that, if anything, M. sanguinipes growth rates were increased, perhaps by as much as 12%, when they fed upon the C3 foliage that had been produced in the CO2-enriched, as compared to the ambient-treatment, air.

What it means
Just as was found in the study of Barbehenn et al. (2004a), the CO2-induced decrease in leaf protein concentration observed in this study did not induce an increase in foliage consumption in the C3 plant studied, nor did it reduce the growth rate of the herbivore studied.  With respect to this finding, the authors state that "although compensatory feeding was commonly observed in early studies [of this subject], the absence of compensatory feeding on C3 plants grown under elevated CO2 has since been observed frequently among herbivorous insects (Bezemer and Jones, 1998)," which suggests that the latter response may ultimately be found to be the more common of the two.

References
Barbehenn, R.V., Karowe, D.N. and Spickard, A.  2004a.  Effects of elevated atmospheric CO2 on the nutritional ecology of C3 and C4 grass-feeding caterpillars.  Oecologia 140: 86-95.

Bernays, E.A. and Simpson, S.J.  1990.  Nutrition.  In: Chapman, R.F. and Joern, A. (Eds.).  Biology of Grasshoppers.  Wiley, New York, NY, pp. 105-127.

Bezemer, T.M. and Jones, T.H.  1998.  Plant-insect herbivore interactions in elevated atmospheric CO2: quantitative analyses and guild effects.  Oikos 82: 212-222.

Hinks, C.R., Cheeseman, M.T., Erlandson, M.A., Olfert, O. and Westcott, N.D.  1991.  The effects of kochia, wheat and oats on digestive proteinases and the protein economy of adult grasshoppers, Malanoplus sanguinipes.  Journal of Insect Physiology 37: 417-430.

Lambers, H.  1993.  Rising CO2, secondary plant metabolism, plant-herbivore interactions and litter decomposition.  Theoretical considerations.  Vegetatio 104/105: 263-271.

Lincoln, D.E., Sionit, N. and Strain, B.R.  1984.  Growth and feeding response of Pseudoplusia includens (Lepidoptera: Noctuidae) to host plants grown in controlled carbon dioxide atmospheres.  Environmental Entomology 13: 1527-1530.

Lincoln, D.E., Couvet, D. and Sionit, N.  1986.  Responses of an insect herbivore to host plants grown in carbon dioxide enriched atmospheres.  Oecologia 69: 556-560.

Simpson, S.J. and Simpson, C.L.  1990.  The mechanisms of nutritional compensation by phytophagous insects.  In: Bernays, E.A. (Ed.).  Insect-Plant Interactions, Vol. 2.  CRC Press, Boca Raton, FL, pp. 111-160.

Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S.  1999.  Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions.  Global Change Biology 5: 723-741.

Yang, Y. and Joern, A.  1994a.  Gut size changes in relation to variable food quality and body size in grasshoppers.  Functional Ecology 8: 36-45.

Yang, Y. and Joern, A.  1994b.  Influence of diet quality, developmental stage, and temperature on food residence time in the grasshopper Melanoplus differentialis.  Physiological Zoology 67: 598-616.

Zanotto, F.P., Simpson, S.J. and Raubenheimer, D.  1993.  The regulation of growth by locusts through post-ingestive compensation for variation in the levels of dietary protein and carbohydrate.  Physiological Entomology 18: 425-434.