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Elevated atmospheric CO2 may affect the performance of specialist (Plutella xylostella) and generalist (Spodoptera littoralis) on Brassica plants

G. V. P. Reddy, A.-M. Manninen and J. K. Holopainen

Department of Ecology and Environmental Science, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland

Abstract

The general hypothesis is that herbivores are adversely affected by changes in plant chemical quality at elevated atmospheric CO2 and that they are likely to become less abundant. Most of the chewing insects generally perform less well at elevated CO2 than at ambient conditions. However, specialist and generalist insect herbivores show differential response to changes in secondary metabolite concentration of host plant. We studied how elevated CO2 affects the performance of the generalist Spodoptera littoralis on two oilseed rape (Brassica rapa subsp. oleifera DC) varieties, Tuli and Valo, and the glucosinolate concentrations in the leaves. Presently we are studying the impact of elevated CO2 on crucifer specialist, diamondback moth (Plutella xylostella) on the two varieties of oilseed rape and cabbage (Brassica oleracea L. subsp. capitata) plants.

Our results indicated that the generalist S. littoralis thrived less on oilseed rape plants grown under elevated CO2 according to our hypothesis. As a result, the relative growth rate and the total food consumption of the larvae were reduced on elevated CO2 plants compared with that on the plants grown under ambient conditions. The results of chemical analysis indicated that the leaves of variety Valo oilseed rape, grown under CO2 exposure had low concentrations of 4-hydroxyglucobrassicin and neoglucobrassicin. However, the leaves of the variety Tuli had considerably high concentrations of two individual glucosinolates, gluconasturtin and neoglucobrassicin, and also the total amount of glucosinolate was increased in CO2 exposed Tuli plants. Observed changes in glucosinolates after CO2 exposure may be associated with the performance of the generalist herbivore and may have effects also on diamondback moth. We elucidate on the effects of elevated CO2 on the composition of phenolic compounds and glucosinolates in leaves of oilseed rape and cabbage that may explain the differences in performance observed.

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