Parasitoids and beetles of decaying wood: the role of fungi and volatiles

Abstract

The saproxylic habitat harbours a remarkable diversity of Coleóptera and associated parasitoids. Owing to the inaccessibility of this habitat and the often concealed nature of its associated organisms, we know very little about most saproxylic species. Many species are fundamental ecosystem components and have the potential of being economically important pests or biocontrol agents. Likewise, vast numbers of saproxylic Coleóptera and parasitoids are under threat. To date, the reasons and manner in which saproxylic organisms partition themselves within the saproxylic habitat is inadequately understood. Thus, the reasons behind saproxylic diversity and the reason why so many species are under threat remains unknown. Furthermore, our understanding of saproxylic pests and biocontrol agents is minimal. There is thus a pressing need for continued and thorough investigations of this unique habitat, in particular, the chemical ecology of saproxylic Coleóptera and their parasitoids is clearly a subject of considerable scientific and practical interest. This thesis investigates the role of fungi and volatiles in saproxylic systems, in particular, it questions their importance in the chemical ecology of saproxylic beetles and parasitoids. In the first study of its kind, Chapter 2 samples volatile organic compound (VOC) emissions from felled beech logs during their initial 30 months of decay. Volatile emissions were found to be in accordance with amounts detectable to insects suggesting that, in a natural decay situation, saproxylic volatiles are readily available as insect semiochemicals. Emissions of particular VOCs were found to differ significantly through time, indicating that saproxylic insects can potentially use semiochemical information to partition themselves according to decay stage. Potential sources of the detected compounds include wood, fungi and other saproxylic organisms. Fungal volatiles are speculated to have a particularly important involvement in these volatile emissions because fungal decay dynamics are shown to coincide with the release of particular volatiles.Chapter 3 expands upon concepts developed in Chapter 2 by investigating the role of fungi in the volatile emissions of decaying beech. Volatiles were sampled from beech logs inoculated with basidiomycetes (Stereum hirsutum, Coriolus versicolor, Phanerochaete velutina, Hypholoma fasiculare, Bjerkandera adusta and various combinations thereof). Fungal community structure was found to significantly affect the volatile profile of decaying wood, with specific VOCs associated with particular fungal treatments. The study concludes that volatiles of fungal origin are a strong potential source of semiochemicals for insects utilising and partitioning themselves within the saproxylic habitat and their natural enemies. Chapter 4 describes an insect attraction experiment and develops the findings of the previous studies in an entomological context. Wood-decaying fungi of the same species and combinations used in Chapter 3 (fungi known to have an effect on volatile emissions of decaying wood) were applied as bait in insect traps. Various beetles and parasitoids exhibited preferences for particular treatments. The dataset is notable for the absence of wood-dwelling insects suggesting that stimuli from fungi alone are insufficient in attracting such species.In light of the conclusions of Chapter 4, Chapter 5 explores the role of saproxylic fungi in a real wood-decay context. Beetles were reared from beech logs inoculated with the same fungal treatments as used in preceding chapters. Various beetle species exhibited significant associations with particular fungal treatments indicating that fungal community structure of decaying wood has marked effects on beetle species composition. Despite the fact that the same fungal species were used as Chapter 4, the dataset is notable for having a vastly different species composition to that of this preceding study (including the presence of numerous wood-living species) confirming that decaying wood is a rich source of stimuli and fungi alone tend to be an insufficient information source. This thesis concludes that saproxylic fungi have a marked influence on beetle communities and volatile emissions of coarse woody debris. Saproxylic fungi are also likely to be strong mediators of the ecology of parasitoids and other saproxylic insects groups.