No abstract

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.

Heather plants (Calluna vulgaris (L.) Hull) were grown for two years (April 1994–October 1996) under ambient or enhanced ultraviolet‐B radiation (UVB: 280–315 nm), provided as a modulated treatment simulating a 15% ozone depletion (seasonally adjusted). Populations of the psyllid (Strophingia ericae (Curtis)) were measured before treatment and at yearly intervals thereafter. Before treatment there was no significant difference in the psyllid populations between treatments, or between the experimental and source populations. Enhanced UVB progressively produced a reduction in S. ericae populations compared with controls over 27 months. Analyses of C, N, total water soluble phenolics, total free amino acids and measurements of leaf angles and distances between leaflets demonstrated no effects of UVB treatment. However, concentrations of the amino acid isoleucine were lower (28%) in C. vulgaris exposed to the enhanced UVB treatment. Over the duration of the experiment the psyllid population structure at Lancaster changed from that typical of the upland site of origin (two‐year cycle with overlapping cohorts) to a one‐year life cycle typical of lowland sites, but this was independent of UVB treatments. Reduced isoleucine might explain the negative effects of elevated UVB on psyllid population numbers, but the precise effects of UVB on host chemistry and morphology are unknown. The problem of interpreting herbivore responses to enhanced UVB treatments in the field is discussed.

Three species of root aphid (Pachypappa vesicalis Koch, Pachypappa tremulae L. and Prociphilus (Stagona) xylostei De Geer) were found feeding on Norway spruce [Picea abies (L.) Karsten] that were exposed to elevated concentrations of O3, and/or SO2 in a field manipulation experiment at Liphook, England. The experiment exposed conifer trees to controlled concentrations of these gases between 1987 and 1990. Root‐aphid populations feeding on Norway spruce were monitored in autumn 1990, the third year of fumigation and in autumn 1991, 10 months after fumigation had ceased. In 1990 the numbers of aphids in the high‐SO2, high‐SO2+ O3 and O3 treatments were significantly less than in ambient plots. Although after fumigation had ceased in 1991 the ambient plots still had higher numbers than the treatment plots, most of these differences were no longer statistically significant. In all plots sampled, total numbers of root aphids were higher in 1991 than in 1990. These results indicate that the numbers of root aphids in the experiment were negatively related to pollution levels. This relationship is the same as that shown in controlled laboratory experiments and is the converse of that generally reported in experiments using shoot‐feeding aphids.