Tritrophic interactions between Populus tremula, leaf beetles and their natural enemies - from the field to the laboratory

Abstract

A drastic increase of global energy consumption is expected for the following years. Due to the increasing demand for renewable energy, much of the current research focuses on short-rotation forestry with high productive Salix and Populus species as a potential energy source. In this context the European aspen (Populus tremula) gained importance for biomass production, because of its ability to reach considerable growth rates even on poor soils and under unfavourable habitat conditions. The European aspen is a pioneer species and is one of the most widespread tree species in boreal and temperate forests. On natural sites aspen create habitats for a rich arthropod community including many endangered species. Current knowledge in plant-insect interactions of willows and poplars is based on studies, which are mostly concentrated on only two trophic levels and differences in leaf chemistry affecting phytophagous insects. The aim of the first part of this thesis was to characterize the differences between closely related aspen (P. tremula) full-sib families and to examine the influence of their morphological and chemical traits on herbivorous insects and their natural enemies in real-world multitrophic interactions. Thereto two common garden experiments were established with closely related aspen full-sib families (6 - 51 % genetic distance). As hypothesized the aspen full-sib families differed in morphological traits such as tree height and leaf number as well as in leaf chemical traits such as phenolic glycosides. In the first common garden experiment a negative relationship was found between phenolic glycosides and chewing herbivorous insects (e.g. Phratora vulgatissima). In the second common garden experiment specialist leaf beetles and their larvae were influenced by different morphological tree traits. The imagines were positively related to the number of leaves (i.e. the resource abundance), whereas leaf beetle larvae were negatively related to tree height, which is in contrast to Lawton s plant size hypothesis. Herbivore-induced plant volatiles differed between spring and summer, but did not change with aspen genetics or herbivore load. Generalist predatory arthropods were indirectly affected by genetic aspen differences as their abundance closely followed the abundance of leaf beetle adults and larvae. They were also positively directly related to emitted herbivore-induced plant volatiles. In addition, after the colonization of one half of the experimental field in spring our results revealed a spatio-temporal shift to the second field half in summer, presumably due to avoidance of tree leaves with induced responses because of the spring infestation. The second part of the thesis is based on the knowledge that predatory arthropods often use infochemical cues for host finding and host location and that multitrophic interactions are often shaped by such infochemicals. The aim of the two studies of the second part was to investigate the olfaction ability of generalist and specialist predatory insects of volatiles emitted by their prey and the prey s host plants and the effects of these volatiles to the predators. This was tested with a field study and laboratory experiments (olfactometer tests and elctroantennography (EAG)). In contrast to recent studies our results showed that generalist predatory beetles such as Harmonia axyridis were attracted by salicylaldehyde, the defense compound of salicin-using leaf beetle larvae. The results further suggest that H. axyridis imagines can learn to use salicylaldehyde as an attractant. The results of EAG experiments indicate that specialist predatory eumenid wasps are able to perceive salicylaldehyde and herbivore-induced plant volatiles and that the olfaction ability reflects the degree of prey specialisation of the different wasp species.We conclude that direct and indirect effects shape aspen-insect interactions. Aspen influence herbivorous insects directly via plant morphological, chemical and genetic traits as well as indirectly via their spatial distribution and temporal changes. Predators are likewise directly influenced by aspen via emitted herbivore-induced plant volatiles and indirectly via herbivore abundances on the trees. Furthermore, infochemicals emitted by their prey affect generalist and specialist predators in the tritrophic system consisting of Populus tremula, specialist leaf beetles, their larvae and their predators.