Interactions between willows and insect herbivores under enhanced ultraviolet-B radiation

We studied the effects of elevated ultraviolet-B radiation on interactions between insect herbivores and their host plants by exposing two species of phytochemically different willows, Salix myrsinifolia and S. phylicifolia, to a modulated increase in ultraviolet radiation in an outdoor experiment and monitoring the colonisation of insect herbivores on these willows. We examined the effect of increased ultraviolet-B (UV-B) radiation on (1) the quality of willow leaves, (2) the distribution and abundance of insect herbivores feeding on these willows, (3) the resulting amount of damage, and (4) the performance of insect larvae feeding on the exposed plant tissue. Six clones of each of the two willow species were grown in eight blocks for 12 weeks in the UV-B irradiation field. The clones were exposed to a constant 50% increase in UV-B radiation (simulating 20-25% ozone depletion), to a small increase in UV-A radiation or to ambient solar irradiation. We allowed colonisation on the willows by naturally occurring insects, but also introduced adults of a leaf beetle, Phratora vitellinae, a specialist herbivore on S. myrsinifolia. Increased UV-B radiation did not affect any of the measured indices of plant quality. However, numbers of P. vitellinae on S. myrsinifolia were higher in plants with UV-B treatment compared with UV-A and shade controls. In laboratory tests, growth of the second-instar larva of P. vitellinae was not affected by UV-B treatment of S. myrsinifolia, but was retarded on UV-B treated leaves of S. phylicifolia. In addition, naturally occurring insect herbivores were more abundant on willows exposed to elevated UV-B radiation compared to those grown under control treatments. In spite of the increased abundance of insect herbivores, willows treated with elevated UV-B did not suffer more herbivore damage than willows exposed to ambient solar radiation (shade control). The observed effects of UV-B on herbivore abundance, feeding and growth varied significantly due to spatial variation in environment quality, as indicated by the UV-treatment x block interaction. The results suggest that (1) environmental variation modifies the effects of UV-B radiation on plant-insect interactions and (2) specialist herbivores might be more sensitive to chemical changes in their secondary host plants (S. phylicifolia) than to changes in their primary hosts (S. myrsinifolia).