Effects of elevated atmospheric CO 2 on the nutritional ecology of C 3 and C 4 grass-feeding caterpillars

Abstract

It is plausible that the nutritional quality of C3 plants will decline more under elevated atmospheric CO2 than will the nutritional quality of C4 plants, causing herbivorous insects to increase their feeding on C3 plants relative to C4 plants. We tested this hypothesis with a C3 and C4 grass and two caterpillar species with different diet breadths. Lolium multiflorum (C3) and Bouteloua curtipendula (C4) were grown in outdoor open top chambers at ambient (370 ppm) or elevated (740 ppm) CO2. Bioassays compared the performance and digestive efficiencies of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid). As expected, the nutritional quality of L. multiflorum changed to a greater extent than did that of B. curtipendula when grown in elevated CO2; levels of protein (considered growth limiting) declined in the C3 grass, while levels of carbohydrates (sugar, starch and fructan) increased. However, neither insect species increased its feeding rate on the C3 grass to compensate for its lower nutritional quality when grown in an elevated CO2 atmosphere. Consumption rates of P. unipuncta and S. frugiperda were higher on the C3 grass than the C4 grass, the opposite of the result expected for a compensatory response to the lower nutritional quality of the C4 grass. Although our results do not support the hypothesis that grass-specialist insects compensate for lower nutritional quality by increasing their consumption rates more than do generalist insects, the performance of the specialist was greater than that of the generalist on each grass species and at both CO2 levels. Mechanisms other than compensatory feeding, such as increased nutrient assimilation efficiency, appear to determine the relative performance of these herbivores. Our results also provide further evidence against the hypothesis that C4 grasses would be avoided by insect herbivores because a large fraction of their nutrients is unavailable to herbivores. Instead, our results are consistent with the hypothesis that C4 grasses are poorer host plants primarily because of their lower nutrient levels, higher fiber levels, and greater toughness.

Phenological and Temperature Controls on the Temporal Non-Structural Carbohydrate Dynamics of Populus grandidentata and Quercus rubra

“…Higher tissue NSC concentrations also may affect the exchange of C and nutrients between plants and both herbivores and symbionts. Elevated CO 2 experiments show that rising NSC concentrations promote ectomycorrhizal fungi growth and colonization [49,50], and the dilution of foliar nutrients available to herbivores [47,48,51,52]. A rise in tissue NSC also may positively or negatively change the concentration of secondary compounds known to thwart herbivores [47,48,53].…”

Limited impact of elevated levels of polyphenol oxidase on tree-feeding caterpillars: assessing individual plant defenses with transgenic poplar

“…Ascorbic acid was extracted from fresh leaf samples in 5% w/v metaphosphoric acid (containing 1 mM EDTA) and analyzed with reverse-phase HPLC (Barbehenn 2003). Other nutrients were measured in samples that had been lyophilized and ground to a fine powder in a dental amalgamator (Barbehenn et al 2004). Water was measured as the difference between the fresh and dry weights of leaf disks (70°C; 2-3 days).…”

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

“…Open top chambers (0.5 m 3 ) were constructed of PVC tubing, covered on four sides and a frustrum with clear polyester film (Drake et al 1989;Karowe et al 1997). Grasses were grown either in 370 ppm or 740 ppm CO 2 (n=20/ treatment) from early June to early August, 2001 (Barbehenn et al 2004a). …”

“…Seeds were examined for endophytic fungi, which were absent, and seedlings were grown in a greenhouse for approximately 3 weeks (Barbehenn et al 2004a). Seedlings were transplanted to tall 3-l pots containing an 80:20 (v/v) mixture of potting soil and sand.…”