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 predicted that insect herbivores will increase their feeding damage on C3 plants to a greater extent than on C4 plants (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 newly-molted sixth-instar larvae of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid) were allowed to feed upon the grasses' foliage.

What was learned
As expected, foliage protein concentration decreased by 20% in the C3 grass, but by only 1% in the C4 grass, when they were grown in CO2-enriched air; and, in the words of the authors, "to the extent that protein is the most limiting of the macronutrients examined, these changes represent a decline in the nutritional quality of the C3 grass."  However, and "contrary to our expectations," say Barbehenn et al., "neither caterpillar species significantly increased its consumption rate to compensate for the lower concentration of protein in [the] C3 grass," noting that "this result does not support the hypothesis that C3 plants will be subject to greater rates of herbivory relative to C4 plants in future [high-CO2] atmospheric conditions (Lincoln et al., 1984)."  In addition, and "despite significant changes in the nutritional quality of L. multiflorum under elevated CO2," they note that "no effect on the relative growth rate of either caterpillar species on either grass species resulted," and that there were "no significant differences in insect performance between CO2 levels."  By way of explanation of these results, they suggest that "post-ingestive mechanisms could provide a sufficient means of compensation for the lower nutritional quality of C3 plants grown under elevated CO2."

What it means
Contrary to early simplistic thought on the matter, in the estimation of Barbehenn et al., it is becoming ever more evident "there will not be a single pattern that characterizes all grass feeders" with respect to their feeding preferences and developmental responses in a world where certain C3 plants may experience foliar protein concentrations that are lower than those they exhibit today, nor will the various changes that may occur necessarily be detrimental to herbivore development or to the health and vigor of their host plants.

References
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.

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.