The early stage of wood decay : wood/fungus interaction and its attraction to xylophagous coleoptera, especially cerambycids and their hymenopteran parasitoids

Abstract

Loss of dead wood habitat and biodiversity has led to numerous excellent conservation based ecological and management studies. However, the structure of the xylophagous food web remains unclear for saproxylic xylophagous beetles and their parasitoid wasps, but fungi are believed to play an important role, both in direct and indirect nutritional acquisition and production of volatile chemicals. In the first study of its kind this thesis explores the importance of volatiles in host-searching behaviour and role of fungi in oviposition choice and larval nutrition, using three endophytic fungi, Stereum hirsutum, Stereum gausapatum and Eutypa spinosa, and two tree species Fagus sylvatica and Quercus robur, in early stage decay. Newly designed traps, excluding wood visual stimuli, provided the first evidence of wood-fungal volatile attraction by saproxylic wood-boring beetles, including primary and secondary xylophages, and their parasitoid wasps. Expanding on these results, the importance of fungi was further demonstrated through oviposition choice in field logs by two polyphagous cerambycid species. Ovipositing within or 20 mm from their preferred fungal species plug, females chose abiotic conditions suitable for sustaining fungal growth, suggesting a larval nutritional benefit. Further investigation, using one of the cerambycid species above, corroborated, via olfactometry, that volatiles from the preferred wood-fungal combination were attractants to gravid females, and induced ovipositor probing within the olfactometer. In addition, GC-EAG found six volatile compounds provoked an antennal response, including a monoterpene, two sesquiterpenes, an alkane and a ketone. Most importantly olfactometry and GC-EAG showed gravid females were unresponsive to wood or favored fungus presented singly. The result are discussed herein. In conjunction with the volatile only field trap experiment, a novel method to promote fungal only volatiles, by autoclaving the wood prior to sub-culturing, was trialed. GC-MS comparative analysis showed disparity between agar sub-cultured, autoclaved wood and natural wood with fungi, volatiles. This suggests that genes encoding cellulases and hemicellulases are regulated according to carbohydrate source, nutritional availability, oxygen and metal ions, supporting the variations observed. These results are discussed herein with regard to xylophagous beetles and their parasitoid wasps. The principal findings of this thesis are that wood-fungal interaction volatiles of the ephemeral early decay stage provide important olfactory host resource cues for saproxylic xylophagous beetles and their parasitoid wasps. Fungi, and particular, endophytic fungi, were shown to be a key component of the early stage xylophagous food web through volatile production, and as nutritional support evidenced by oviposition choice, olfactometry, GC-EAG and GC-MS analysis. This thesis has opened a new, exciting and important area of research with the potential to radically improve conservation management and employ a more holistic approach. These insights bring together the disparate research conducted individually on wood decay fungi, saproxylic xylophagous beetles and their parasitoid wasps