6. Plant-insect interactions

The chemical composition of plants growing in a CO₂ enriched atmosphere can differ from that of plants growing at ambient CO₂ concentrations. This change in leaf quality may have possible implications for higher trophic levels feeding on altered food plants. We tested this hypothesis by studying two herbivores in this alpine system.

The alpine grasshopper (Miramella alpina) is the most abundant grasshopper in the treeline ecotone at Stillberg and prefers Vaccinium species as feeding plants. Plant-species specific reductions in grasshopper growth and decreased reproductive output was observed in response to elevated CO₂, suggesting altered trophic interactions in a future CO₂-rich world.

The larva of the grey larch bud moth (Zeiraphera diniana), a lepidopterous species (Tortricidae), is the most important leaf chewing insect herbivore of the larch-cembran pine forests in the central European Alps, causing large scale defoliations in these forests. We investigated the effects of elevated CO₂ and previous year defoliation on larch needle quality and larch bud moth performance. Larch bud moth larvae grew somewhat slower under elevated CO₂ compared to ambient CO₂ when trees remained undefoliated. If, however, trees were defoliated, this response was reversed, with a faster growth of larch bud moth on high CO₂-exposed trees than on control trees. Pupal weight was not affected by CO₂ or defoliation.