Rumex obtusifolius plants and three generations of the tri‐voltine leaf beetle Gastrophysa viridula were simultaneously exposed to elevated CO2 (600 ppm) to determine its effect on plant quality and insect performance. This exposure resulted in a reduction in leaf nitrogen, an increase in the C/N ratio and lower concentrations of oxalate in the leaves than in ambient air (350 ppm). Despite these changes in food quality, the effect of elevated CO2 on larvae of Gastrophysa viridula over three generations was minimal. However, the effect of CO2 did differ slightly between the generations of the insect. For the first generation, the results obtained were different from many of the published results in that elevated CO2 had no measurable effects on performance, except that third instar larvae showed compensatory feeding. Food quality, including leaf nitrogen content, declined over time in material grown in both ambient and elevated CO2. The results obtained for the second generation were similar to the first except that first instar larvae showed reduced relative growth rate in elevated CO2. Development time from hatching to pupation decreased over each generation, probably as a result of increasing temperatures. Measurements of adult performance showed that fecundity at the end of the second generation was reduced relative to the first, in line with the reduction in food quality. In addition at the end of the second generation, but not at the end of the first generation, adult females in elevated CO2 laid 30% fewer eggs per day and the eggs laid were 15% lighter than those in ambient conditions. These lighter eggs, coupled with no effect of elevated CO2 on growth during the third generation, meant that the larvae were consistently smaller in elevated CO2 during this generation. These results offer further insights into the effect that elevated CO2 will have on insect herbivores and provide a more detailed basis for population predictions.

Exposure of R. crispus and R. obtusifolius to elevated CO2 (600 ppm) resulted in an increased C:N ratio of leaf tissue and greater leaf areas. Larvae of P. nigritarsis mining leaves of R. obtusifolius during exposure produced significantly bigger mines in elevated than in ambient (350 ppm) conditions. There were no significant treatment effects on pupal weight although in both host species mean weight was greater in ambient than in elevated conditions. These results are consistent with the hypothesis that insect herbivores compensate for increased C:N ratios by increased food consumption. This response by herbivores may partially offset predicted increases in plant biomass in a future high CO2 environment.

No abstract

A population of the xylem‐feeding spittlebug, Neophilaenus lineatus, on blocks of natural vegetation transferred to large hemispherical chambers was studied over two generations with continuous exposure to elevated CO2 (600 ppm). The third generation was transferred from the blocks to potted Juncus squarrosus to enable measurements of fecundity. The principal food plant throughout was Juncus squarrosus. Survival of the nymphs was reduced by more than 20% in elevated CO2 relative to ambient (350 ppm) in both years of the main experiment. Elevated CO2 also delayed development by one or more nymphal instars in each year. Fecundity was not significantly affected. The C/N ratio of whole Juncus leaves was increased in elevated CO2 and the transpiration rates of the plants were reduced. These changes may have been responsible for the effect of elevated CO2 on spittlebug performance. However, other factors such as plant architecture and microclimate may also be important.

SUMMARYPlants of Rumex obtusifolius L. were grown in Solardomes under ambient and elevated (+ 250 /;mol mor') mole fractions of CO^ and were exposed to two levels of herbivory by Gastrophysa viridula Degeer larvae. The herbivory treatment lasted 1 month, thereafter half of the plants were harvested and over the following month during a period of regrowth physiological measurements were made on the remaining plants. At the termination of the herbivory treatment uninfested plants showed no damage, whereas the low-and high herbivore treatments caused 20-40 °o and 50-70 "o loss of leaf area as a proportion of total leaf area, respectively. The CO., treatment did not affect the degree of defoliation. Total leaf area was not significantly affected by either CO., or herbivory. Uninfested plants grown in elevated concentrations of CO, showed increased growth, root-to-shoot ratios (RS), rates of photosynthesis and reduced stomatal conductance compared with uninfested plants grown in ambient CO.^. A/Canalysis revealed that plants grown in elevated CO, showed reductions in I c^^^. For plants grown in ambient CO, the high herbivory treatment led to increased rates of photosynthesis and decreased rates of dark respiration per unit leaf area, and caused increases in stomatal conductance and RS. For plants grown in elevated CO., the high herbivory treatment increased plant biomass and RS. The increases in RS in response to elevated CO, and herbivory appeared to be additive. Defoliation did not reduce the degree of photosynthetic down-regulation caused by growth in elevated concentrations of CO,, but appeared to reduce the rate of ontogenic decline in photosynthesis in ambient CO,.