Studies On The Host Selection Mechanism Of Two Bark Beetles

Bark beetles are a group of the most important forest insect pests which cause significant economic and ecological losses when the population reaches epidemic level. The pioneer bark beetles can kill the healthy trees as the direct threats to the forest. Although the secondary bark beetles mainly attack weaken or dying trees, they can also attack healthy trees at high population densities.The objects of the dissertation mainly focus on two conifer bark beetles, one is the Asian larch bark beetle, Ips subelongatus Motsch (Coleoptera:Curculionidae:Scolytinae), which is more aggressive species. As one of the pioneer insect pests in larch plantation, the Asian larch bark beetle often outbreaks and attacks healthy larch trees thereby causing significant tree mortality in this region; The other object is a secondary beetle, Phloeosinus aubei Perris (Coleoptera:Curculionidae:Scolytinae), which may infest Cupressus and Juniperus species. Its major host in China is Oriental Arborvitae, Platycladus orientalis (L.) Franco and Chinese Juniper, Juniperus chinensis L. As a destructive insect pest of juniper, it usually attacks weaken or newly planted trees.Bark beetles have complicated semiochemical communication system. With the plant volatiles as dominants for host selection, bark beetles also produce aggregation pheromones to call conspecies to have mass attack. Meanwhile, non-host volatiles (NHV) play an important role for host selection as well. In this dissertation, on the basis of extraction and analysis on pheromones, host volatiles (HV) and NHV, the efficacy of pheromones, HVs and NHVs on the two bark beetles were studied for manipulating their aggregation behavior by the methods of the electro-physiological or field test. And their host selection mechanism were furtherly discussed. The adult Asian larch bark beetles may aggregate in large amounts with aggregation pheromones. However, the field bioassay results in two provinces (Inner Mongolia and Jilin) of northeastern China suggest that there is a strong geographical variation in aggregation pheromone response of I. subelongatus in northeastern China. Behavioral responses of I. subelongatus Motsch. to three potential aggregation pheromone components, ipsenol [racemic or (-)-enantiomer], ipsdienol [racemic or (+)-enantiomer] and 3-methyl-3-buten-l-ol, were tested in Inner Mongolia and Jilin Province. Our field bioassays in Inner Mongolia (Larix principis-rupprechtii Mayr. plantation) clearly showed that ipsenol, either racemic or 97%-(-)-enantiomer, was the only compound that significantly attracted both sexes of I. subelongatus, while all other compounds (singly or in combinations) were unattractive. There were no two or three-way synergistic interactions. In Jilin Province [L. gmelini (Rupr.) Rupr. plantation], however, all the individual compounds tested were inactive, while a combination of ipsenol and ipsdienol [racemates or enantiomerically pure natural enantiomers] showed a significant attraction to both sexes of I. subelongatus, indicating a two-way synergistic interaction between these two major components. Addition of 3-methyl-3-buten-l-ol to these active binary blend(s) did not have any effects on trap catches, suggesting that ipsenol and ipsdienol are the synergistic aggregation pheromone components of I.subelongatus in Jilin province.Both sexes of adult I. subelongatus are strongly attracted by Larix logs. Analysis on aeration of larch logs by Gas Chromatography-Mass Spectrometry (GC-MS) showed that the major volatiles of larch logs were monoterpenes such as a-pinene,β-pinene andβ-phellandrene. Although bioassay with Electro-Antenna-Graphy Recording (EAG) detected the electrophysiological response of I. subelongatus to a-pinene, no behavioral responses were observed in the field bioassay for attraction of I. subelongatus to the host materials or host monoterpene mixture.In the field bioassay of I. subelongatus to non-host volatiles (NHV) with aggregation pheromones as positive control, the combinations of trans-conophthorin +verbenone, 1-hexanol+ trans-conophthorin +verbenone and 1-hexanol+ trans-conophthorin+ geranylacetone+ verbenone showed strong repellent to both sexes of I. subelongatus. As to the single component NHV, verbenone has the strongest repellent bioactivity to I. subelongatus, then geranylacetone and trans-conophthorin were also active repellent. The results suggest that I. subelongatus may select the suitable host trees by rejecting non-host trees.As to Phloeosinus aubei, both analysis on hindguts extraction and behavorial test didn't indicate any signals of aggregation pheromone production by the beetles. However, both sexes of adult P. aubei showed strong attractive response to cypress logs. Volatiles of Platycladus orientalis logs aeration analyzed by GC-MS mainly detected 15 chemicals with a-pinene,△-3-carene, sabinene, widdrene as the major components. GC—EAD detected the electrophysiological responses of P. aubei to the host monoterpene including a-pinene, a-felendre, A-3-carene, a-terpene, DL-limonene. In the field bioassay, the two treatments of turpentine+ethanol and Hostowit showed significant attraction to P. aubei. Whereas another sympatric bark beetle, P. hopehi, was strongly attracted by (95%-)-alpha-pinene.Meanwhile, antenna of I. Subelongatus and P. aubei were observed by scanning electron microscope (SEM). It showed that antenna of the two species of beetles both present the shape of geniculation and was separated as three parts of scape, pedicel and flagellum. The type and distribution of the sensilla of the two beetles were greatly different from that of the known Coleoptera and Hymenoptera. Five types of sensilla were observed on P. aubei antenna which includes sensilla trichoid, sensilla ctenoid, sensilla hackle, sensilla chaetica, sensilla cavity. The main sensilla on I. Subelongatus observed were sensilla hackle, sensilla trichoid, sensilla cavity and sensilla gemmiformium. There was no obvious difference on structure and morphology between the male and female antenna.