Effect of elevated CO₂ on interactions between the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae) and the common milkweed, Asclepias syriaca

We measured the effect of elevated CO₂ on populations of the western flower thrips, Frankliniella occidentalis and on the amount of leaf damage inflicted by the thrips to one of its host plants, the common milkweed, Asclepias syriaca. Plants grown at elevated CO₂ had significantly greater aboveground biomass and C:N ratios, and significantly reduced percentage nitrogen. The number of thrips per plant was not affected by CO₂ treatment, but the density of thrips (numbers per gram aboveground biomass), was significantly reduced at high CO₂. Consumption by thrips, expressed as the amount of damaged leaf area per capita, was significantly greater at high CO₂, and the amount of leaf area damaged by thrips was increased by 33%. However overall leaf area at elevated CO₂ increased by 62%, more than compensating for the increase in thrips consumption. The net outcome was that plants at elevated CO₂ had 3.6 times more undamaged leaf area available for photosynthesis than plants at ambient CO₂, even though they had only 1.6 times the overall amount of leaf area. This study highlights the need for measuring the effects of herbivory at the whole-plant level and also the importance of taking herbivory into account when predicting plant responses to elevated CO₂.