Future atmospheric CO [sub 2] concentrations are predicted to increase, and directly affect host plant phenology, which, in turn, is assumed to mediate the performance of herbivorous insects indirectly as well as the abundance and epidemiology of plant diseases. In a 4-year field experiment, spring wheat (Triticum aestivum L. cv. Triso) and spring oilseed rape (Brassica napus L. cv. Campino) were grown using a mini- free-air CO [sub 2] enrichment (FACE) system, which consisted of a control (CON), an ambient treatment (AMB) and FACE treatments. The CON and AMB treatments did not receive additional CO [sub 2], whereas the FACE plots were moderately elevated by 150 μL L-1 CO [sub 2]. The impact of elevated CO [sub 2] was examined with regard to plant phenology, biomass, leaf nitrogen and carbon, abundance of insect pest species and their relative population growth by either direct counts or yellow sticky traps. Occurrence and damage of plants by pathogens on spring wheat and oilseed rape were directly assessed. Disease infestations on plants were not significantly different between ambient and elevated CO [sub 2] in any of the years. Plant phenology, aboveground biomass, foliar nitrogen and carbon concentrations were also not significantly affected by CO [sub 2] enrichment. In contrast, the abundance of some species of insects was significantly influenced by elevated CO [sub 2], showing either an increase or a decrease in infestation intensity. Future atmospheric carbon dioxide levels are likely to double in concentration, affecting agricultural crops, herbivorous insects and the possible progression of various plant diseases. The abundance of insects significantly changed under elevated CO [sub 2] in our study, whereas plant characteristics (phenology, aboveground biomass, foliar nitrogen and carbon contents) and development of fungal plant pathogens did not. [ABSTRACT FROM AUTHOR]