Performance of a generalist grasshopper on a C 3 and a C 4 grass: compensation for the effects of elevated CO 2 on plant nutritional quality

Abstract

The increasing CO 2 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) CO 2 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 × CO 2 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 CO 2 ; 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 CO 2 . Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C 3 grass under elevated CO 2 . 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 CO 2 levels, we conclude that C 3 grasses are likely to remain better host plants than C 4 grasses in future CO 2 conditions.