EFFECTS OF ENHANCED UVB ON POPULATIONS OF THE PHLOEM FEEDING INSECT STROPHINGIA-ERICAE (HOMOPTERA, PSYLLOIDEA) ON HEATHER (CALLUNA-VULGARIS)

Heather plants (Calluna vulgaris (L.) Hull) were grown for two years ( April 1994-October 1996) under ambient or enhanced ultraviolet-B radia tion (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 y early intervals thereafter. Before treatment there was no significant difference in the psyllid populations between treatments, or between t he experimental and source populations. Enhanced UVB progressively pro duced a reduction in S. ericae populations compared with controls over 27 months. Analyses of C, N, total water soluble phenolics, total fre e amino acids and measurements of leaf angles and distances between le aflets demonstrated no effects of UVB treatment. However, concentratio ns of the amino acid isoleucine were lower (28%) in C. vulgaris expose d to the enhanced UVB treatment. Over the duration of the experiment t he 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 inde pendent of UVB treatments. Reduced isoleucine might explain the negati ve effects of elevated UVB on psyllid population numbers, but the prec ise effects of UVB on host chemistry and morphology are unknown. The p roblem of interpreting herbivore responses to enhanced UVB treatments in the field is discussed.