The important role of semiochemicals in the interactions between plants and insects has been extensively investigated. The volatiles produced by oak trees are thought to attract the ambrosia beetle Platypus quercivorus (Murayama), which causes wilt disease in Quercus trees, resulting in widespread damage. In the present study, we hypothesized that (a) P. quercivorus is attracted to the leaf volatiles emitted by host trees and (b) the response of P. quercivorus to leaf volatiles is affected by flight. An experiment was performed to survey the preferences of both sexes of P. quercivorus for the leaf volatiles of Quercus crispula Blume at various stages of leaf deterioration, represented by different number of days after cutting. Additionally, the effect of flight on both sexes was evaluated by testing the beetle flight on a flight mill. The results showed that P. quercivorus was attracted to the volatiles emitted from fresh leaves and was not attracted to those emitted from dry leaves. This suggests that leaf volatiles from healthy hosts are primary attractants for P. quercivorus. Further, males exhibited increased olfactory responses to leaf volatiles as their flight duration increased, whereas the opposite pattern was observed in females. These results suggest that the different ecological roles of the sexes contribute to differences in olfactory responses. K E Y W O R D S flight mill, host selection, olfactory response, Quercus crispula, volatile, Y-tube olfactometer
Although chemical volatiles emitted from host and non-host trees have been suggested as important cues for bark and ambrosia beetles, their responses to leaf volatiles is poorly understood. The oak ambrosia beetle, Platypus quercivorus (Murayama) (Coleoptera: Curculionidae), is a vector for the fungus that causes Japanese oak wilt. Using a Y-tube olfactometer, we tested the behavioral response of P. quercivorus to leaf volatiles emitted from four host trees-Quercus crispula Blume, Quercus serrata Murray, Quercus salicina Blume, and Castanea crenata Sieb. & Zucc. (all Fagaceae)and two non-host trees, Fagus crenata Blume (Fagaceae) and Cryptomeria japonica D. Don (Cupressaceae). A flight mill was used to evaluate the effect of flight on the behavioral response to leaf volatiles. The bioassays were repeated 109 before and 109 after flight in the flight mill for each of the 54 individual beetles. Leaf volatile components were analyzed using gas chromatography-mass spectrometry. The bioassay results supported our hypothesis: P. quercivorus was attracted by the leaf volatiles of hosts and was deterred by the leaf volatiles of non-hosts. The behavioral response of P. quercivorus to the leaf volatiles was stronger after flight. Males had a stronger behavioral response than females to leaf volatiles. The leaf volatile chemical profile of the non-host C. japonica differed from the profile of the host plants. However, the chemical profile of the non-host F. crenata was similar to the profile of the hosts. Our findings provide insight into the functions of leaf chemical volatiles in the interaction of P. quercivorus with its hosts and non-hosts and may help improve the control of P. quercivorus and Japanese oak wilt.
Bark and ambrosia beetles sometimes kill trees by attacking them en masse; however, their attack is not necessarily successful. Less than half of the fagaceous trees attacked by the ambrosia beetle Platypus quercivorus (Murayama) die, and the factors affecting this mortality are still unknown. To examine this issue, the survival of all stems of fagaceous trees attacked by the ambrosia beetle was investigated in a secondary forest from 2008 to 2010. In an area of 93 ha, 2130 stems (1278 genets) of fagaceous trees were attacked by P. quercivorus during the study period, and 813 of these stems died. A generalized additive mixed model was constructed to predict the probability of mortality of the attacked stems. A best-fit model showed that the probability of mortality was higher in Quercus crispula Blume than in Castanea crenata Sieb. & Zucc. A positive correlation was determined between the density of the attacked trees and the probability of mortality, suggesting that mass attack of P. quercivorus occurs not only on individual trees, but also on groups of trees. Assuming that trees attacked earlier in the season have a higher probability of mortality, the observed negative effects of altitude suggest that P. quercivorus initially seeks hosts at lower elevations. Résumé :Les scolytes tuent parfois des arbres avec des attaques en masse mais cela ne réussit pas à tout coup. Moins de la moitié des arbres appartenant à la famille des fagacées qui sont attaqués par le scolyte Platypus quercivorus (Murayama) meurent et les facteurs associés à la mortalité sont à ce jour inconnus. Pour étudier cette question, la survie de tous les arbres de la famille des fagacées attaqués par ce scolyte a été étudiée dans une forêt de seconde venue de 2008 à 2010. Dans une zone de 93 ha, 2130 tiges (1278 génotypes) de la famille des fagacées ont été attaquées par P. quercivorus durant la période d'étude et 813 de ces tiges sont mortes. Un modèle mixte additif généralisé a été élaboré pour prédire la probabilité qu'un arbre attaqué meure. Le meilleur modèle a montré que la probabilité qu'un arbre meure était plus élevée pour Quercus crispula Blume que pour Castanea crenata Sieb. & Zucc. Une corrélation positive a été trouvée entre la densité des arbres attaqués et la probabilité qu'un arbre meure, indiquant que les attaques en masse de P. quercivorus surviennent non seulement sur des individus mais aussi sur des groupes d'arbres. En assumant que les arbres attaqués plus tôt durant la saison ont une plus forte probabilité de mourir, les effets négatifs de l'altitude que nous avons observés indiquent que P. quercivorus cherche d'abord des hôtes à plus basse altitude. [Traduit par la Rédaction] Mots-clés : Castanea crenata, densité des arbres attaqués, dépérissement du chêne japonais, probabilité de mortalité, Quercus crispula.
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