Performance of a generalist grasshopper on a C3 and a C4 grass:: compensation for the effects of elevated CO2 on plant nutritional quality

The increasing CO2 concentration in Earth's atmosphere is expected to cause a greater decline in the nutritional quality of C-3 than C-4 plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C-3 plants. These hypotheses were tested by growing the grasses Lolium multiflorum C-3) and Bouteloua curtipendula C-4) at ambient (370 ppm) and elevated (740 ppm) CO2 levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant x CO2 treatment combinations. As expected, the nutritional quality of the C-3 grass declined to a greater extent than did that of the C-4 grass at elevated CO2; protein levels declined in the C-3 grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C-3 grass grown under elevated CO2. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C-3 grass under elevated CO2. Consumption rates of the C-3 and C-4 grasses were also similar, demonstrating a lack of compensatory feeding on the C-4 grass. We also examined the relative efficiencies of nutrient utilization from a C-3 and C-4 grass by M. sanguinipes to test the basis for the C-4 plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C-4 grass than from the C-3 grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C-3 grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C-3 grass and the better growth performance of M. sanguinipes on this grass at both CO2 levels, we conclude that C-3 grasses are likely to remain better host plants than C-4 grasses in future CO2 conditions.