Elevated atmospheric CO2 affects the chemical quality of Brassica plants and the growth rate of the specialist, Plutella xylostella, but not the generalist, Spodoptera littoralis

Cabbage, Brassica oleracea subsp. capitata (cv. Lennox and Rinda), and oilseed rape, Brassica rapa subsp. oleifera (cv. Valo and Tuli), plants were grown under ambient CO2 (360 ppm) or elevated CO2 (720 ppm) at 23/18 degreesC and under a photoperiod of 22/2 h light (250 mumol m(-2) s(-1))/dark regime for up to 5 weeks. Afterward, the performance of the crucifer specialist Plutella xylostella (Lepidoptera: Plutellidae) and the generalist Spodoptera littoralis (Lepidoptera: Noctuidae) on those plants was studied. The mean relative growth rate (RGR) of P. xylostella larvae, feeding on both cultivars of oilseed rape or on the Lennox cultivar of cabbage leaves grown at an elevated CO2 concentration, was significantly reduced as compared to ambient CO2. A negative larval growth rate at elevated CO2 was observed for P. xylostella on both oilseed rape cultivars, but the growth rate was reduced but positive on cabbage. Conversely, the RGR of S. littoralis on either plant species was not affected by CO2 treatment but was lower on cabbage cv. Rinda than on cv. Lennox. The mortality of the larvae was not affected by CO2 treatment either. At the same time, elevated CO2 significantly decreased the concentrations of leaf phytochemical constituents in oilseed rape, i.e., total phenolics and total nitrogen, but not in cabbage. The effect of elevated CO2 on the leaf glucosinolate concentrations of both plant species was marginal. In addition, the observed significant changes in individual glucosinolate concentrations of oilseed rape leaves were not consistent among cultivars. However, our results demonstrate for the first time quite strong effects of CO2 enrichment on the larval performance of P. xylostella, which is an important pest of Brassica plants around the world. Further studies are still required to increase our understanding of why elevated CO2 differently affects the performance of specialist and generalist insect herbivores on Brassica plants.