Effects of resource heterogeneity in trees upon insect herbivory

The aim of this thesis is to research into adaptabilities of herbivorous insects to host individuals. For that, the resource heterogeneity for herbivores, physiological adaptations of larvae during their developmental period and the formation of locally adapted demes were investigated. Resource heterogeneity and their temporal predictability are essential prerequisites for local adaptations. In this thesis, leaf tissue quality, herbivore attack in the field and leaf utilization by larvae of a polyphagous herbivore (Spodoptera littoralis, Lepidoptera) in bioassays were estimated in lime (Tilia cordata), ash (Fraxinus excelsior), sycamore (Acer pseudoplatanus) and common oak (Quercus robur) across two years. The leaf tissue quality, measured as C/N-ratio and water content, differed among as well as within individual trees in all four species. Levels of leaf damage by herbivores in the field differed among tree individuals in lime, oak and sycamore, but not in ash. Further, it varied in all four tree species within tree individuals between the upper and the lower tree layer. The utilization of leaf tissue by larvae of S. littoralis differed among and within tree individuals in all investigated tree species. Therefore, the resource heterogeneity as prerequisite for local adaptations of herbivores is fulfilled in each of the tree species. However, the pattern of resource quality for herbivorous insects among host individuals changed in lime and oak already across two years. In ash and sycamore, the relative ranking of tree individuals in terms of insect performance remained constant, thus satisfying the second prerequisite for long-term local adaptations, the predictability of host quality. Therefore, ash and sycamore individuals may be more suitable hosts for forming adapted demes of herbivores than lime or oak. However, two consecutive years may represent rather an indication of long-term variability and are unlikely to show the full range of variations. An efficient physiological adaptation of larvae to the specific leaf tissue quality that they encounter during their developmental period may facilitate gradual adaptations of insects over many generations. In this study, reciprocal transfer experiments with larvae of the gypsy moth, Lymantria dispar (Lepidoptera), showed that the oak individuals differed in the quality of their leaves and that phenotypic variation in growth on different oak individuals existed among families of larvae. Both are requirements for local adaptations to host individuals. However, the transfer of the larvae to novel host individuals did not reveal a significant effect on their relative growth rate. Hence, there was no evidence for physiological adaptation to individual hosts during larval development. This was the first study on this specific form of adaptation; further studies are needed to draw a general conclusion. Even if physiological adaptations of larvae to host individuals are not verifiable, long-term genetic adaptations may take place. The adaptive deme formation hypothesis of Edmunds & Alstad (1978) assumes that over generations herbivorous insects form ecologically and genetically distinct groups within species – the demes. The hypothesis has been discussed controversially and experimental tests have been equivocal. In this thesis, the adaptive deme formation hypothesis was tested with two leaf-chewing species of sawflies, Tomostethus nigritus and Macrophya punctumalbum (Hymenoptera) on ash (Fraxinus excelsior). Ash has been previously shown to be a suitable host for forming adapted demes of herbivores. In reciprocal transfer experiments with the larvae on ash individuals, there was no effect of the transfer: the relative growth rate did not differ significantly between transferred and non-transferred larvae. Therefore, there was no evidence for local adaptation of the herbivores to individual host plants and leaf chewers may not be more likely to adapt to individual hosts than piercing-sucking herbivores. To summarise, the variation of resource heterogeneity for herbivorous insects and its temporal predictability differs among plant species. Therefore, the probability that insects form locally adapted demes might depend on the specific plant-insect system. Together with the contradicting results from transfer experiments, this indicates that generalisations about fine-scale adaptations are premature. It is suggested that local adaptation may fine-tune herbivore populations to host individuals under very specific conditions. However, it may not play a general role in the evolution of herbivorous insects.