Effects of elevated CO2 and temperature on development and consumption rates of Octotoma championi and O. scabripennis feeding on Lantana camara

We carried out a factorial experiment to explore the effect of doubled CO₂ concentration and a 3°C temperature increase on the development of a complete generation of the beetles Octotoma championi Baly and O. scabripennis Guérin-Méneville (Coleoptera: Chrysomelidae). These species are biological control agents of Lantana camara L. (Verbenaceae), with a leaf-mining larval phase and free-living, leaf-chewing adults. Plants grown at elevated CO₂ had enhanced above-ground biomass, thicker leaves, reduced nitrogen concentration, and increased C:N ratios. Under the high temperature treatment, plants grown at ambient CO₂ suffered wilting and premature leaf loss, despite daily watering; this effect was ameliorated at elevated CO₂. The wilting of plants in the ambient CO ₂/high temperature treatment reduced the emergence success of the beetles, particularly O. championi. Development time was accelerated by approximately 10-13 days at the higher temperature, but was not affected by CO₂. Neither CO₂ nor temperature affected adult beetle weight. Consumption rates of free-living beetles were not affected by either CO₂ or temperature. By contrast, in the short-term trials using excised foliage, beetles given no choice between ambient and elevated CO ₂-grown foliage, consumed more from ambient plants. When beetles were offered a choice between foliage grown at the two CO₂ levels, O. championi did not display a significant preference but O. scabripennis consumed more ambient CO₂-grown foliage when feeding at the lower temperature. This study indicates that under future conditions of higher temperatures, amelioration of water stress in host plants growing in elevated CO₂ may benefit some endophagous insects by reducing premature leaf loss. Under some circumstances, this benefit may outweigh the deleterious effects of lower leaf nitrogen. Our results also indicate that foliage consumption under elevated CO₂ by mobile, adult insects on whole plants may not be significantly increased, as was previously indicated by short-term experiments using excised foliage.