Direct and indirect competitive effects of foliage feeding guilds on the performance of the birch leaf-miner Eriocrania

1. Competitive interactions between Eriocrania spp. Zeller (Lepidoptera: Eriocraniidae) leaf-miners and other folivorous guilds (free-living Lepidoptera and phloem feeding aphids) of silver birch (Betula pendula Roth) were examined experimentally. 2. Measurements of the foliage quality were taken from the experimental trees in order to discover whether variation in nitrogen and secondary compounds (specifically total phenolics), had an effect on the strength of any competitive interactions between the guilds. 3. There was evidence for indirect interactions within the leaf-mining guild mediated by the host plant: miners in aggregations had double the mortality of single miners. Interactions with leaf-chewing Lepidoptera were limited to interference and resource removal since only damage to the leaf being mined had an effect on the miner performance. There were no interactions between Eriocrania and aphids, despite the presence of aphids reducing the quantity of foliage available by 33%. 4. The strength of the competitive interactions, either within or between guilds, was not modified by the quality of the host trees. However, biomass of emerging larvae was negatively related to increased phenolic content of host trees and positively related to increased nitrogen content. Furthermore, larvae emerged later from trees where foliage had higher phenolic content. 5. The longevity of feeding events by competitive folivore guilds and the proximity of the feeding to the miners were identified as the key factors influencing the effects of foliage damage on the performance of Eriocrania.