Ambrosia beetle guild attacking a deciduous oak tree Quercus serrata in the Central Japan and species risk assessment in relation to potential invasiveness and aggresiveness based on niche analysis

Abstract

For decades, secondary ambrosia beetle species have been increasingly observed to attack apparently healthy trees. Some of them cause mortality to living trees. In Japan, Platypus quercivorus that is widely distributed in Asia has caused mass mortality of trees belonging to the family Fagaceae by vectoring Raffaelea quercivora. Similar disease has been in epidemic in Korea by attacks of Platypus koryoensis. On the other hand, unexpected introduction of ambrosia beetles to non-indigenous area has been increasing because of globalization. Some has become major components of ambrosia beetle fauna in the non-indigenous area. The worst example among invasive ambrosia beetles is Xyleborus glabratus that causes laurel wilt in US by vectoring Raffaelea lauricola. The laurel wilt has caused enormous economic damage to avocado farmers in US. However, ecological backgrounds of these phenomena are still unclear. Therefore, ecological information on ambrosia beetles is needed from a view point of biodiversity conservation as well as economic importance. The purpose of this study was to determine ambrosia beetle guild, factors influencing the guild, and niche of each species attacking on an oak tree Quercus serrata. Risks of invasion and of vectoring tree-killing disease were evaluated for each species. The study was conducted in three locations of the University of Tokyo Forests in the Central Japan. Timing of cutting trees (= TC) was controlled by preparing bolts monthly from March to September in Chichibu. They were covered by metal mesh to protect from insect attacks and left on the forest floor in the three locations: Aichi (LOC-A), Chiba (LOC-B), and Chichibu (LOC-C) (= LOC). The LOC-A had Japanese oak wilt (JOW). However, the LOC-B and the LOC-C had no historical records of JOW. Timing of exposure (= TE) was controlled by removing the metal mesh so that wood oldness (= WO) at the timing of exposure after cutting tree was also experimentally controlled. Bait bolts were exposed to insect attacks for one month in a Normal experiment, while they were left until the end of September in a Conditioning experiment. Twenty eight regimes, each consisted of four bolts, were set in each of a Normal and a Conditioning experiment so that the total number of bolts on each location was 224. Ambrosia beetles were collected by dissecting the bolts after the exposure and identified into species. The number of entry holes was used as an indicator for abundance of each species. In the Normal experiment, twenty morphospecies of Scolytinae and six species of Platipodinae were collected. In the Conditioning experiment, six Scolytid and two Platypodid species disappeared, whereas one Scolytid species recruited so that fifteen species of Scolytinae and four species of Platypodinae were collected. In total, twenty one morphospecies of Scolytinae and six species of Platipodinae were collected. Species richness and abundance peaked on bolts prepared in April–May, on bolts exposed in July, and on 2–3-month-old bolts. Eliminating greatest influences of LOC on abundance, results of hierarchical partitioning showed that TC had a strong influence on both species richness and abundance. LOC-A (Aichi), in which Japanese oak wilt disease (JOW) incidence occurred, showed the greatest species richness and the smallest value of Pielou’s evenness. Abundance of the most abundant (“the major”) species was more than twice that of the second major species, which was a likely cause of the smallest evenness in LOC-A. Trees killed by JOW may have increased the abundance of the major species. On the contrary, in LOC-C (Chichibu), Pielou’s evenness and alpha and gamma diversity and the Shannon index were greatest among the three locations although species richness was smallest. High similarity between guilds in LOC-A and LOC-B (Chiba) was probably caused by similarity in vegetation. The LOC had the greatest effect on determining guild structure. Effect of TE was greater than TC. The effect of WO was negligible. A hierarchical structure among the three factors was a likely cause of their relative importance determining guild structure. Niche center and niche breadth were estimated for the three niche contexts. Colonizing ability of ambrosia beetles for bolts that had been exploited by other species. Abundance was compared between the Normal and the Conditioning experiments before each species stated to attack in Normal experiment. If the number in a regime No. 1 was smaller than 3 and no galleries were found in a regime No.2 and 8, the species was included in the analysis. A paired t-test was employed to test if the difference was significant. Species that have been reported as an alien species tended to have broader niche. All Scolytid species collected in my study have a habit of haplodiploidy and sibling mating so that Scolytid have a higher risk of invasion than Platypodid. Platypodid species tended to attack fresh bolts compared to Scolytid so that Platypodid have higher risk to vector tree-killing disease than Scolytid. Among Platypodinae, Crossotarsus niponicus and Platypus calamus significantly increased in the Conditioning experiment so that they are good competitors and high risk species of invasion. However, they did not show strong preference to fresh bolts compared to other Platypodid species. Xylosandrus germanus and Xylosandrus crassiusculus, which attack living trees in non-indigenous area, also did not show a strong preference to fresh bolts so that there are no guarantees that C. niponicus and P. calamus are safe enough in non-indigenous area. Xylosandrus germanus and of X. crassiusculus supported that they became invasive alien species. Euwallacea validus significantly decreased in the Conditioning experiment compared to the Normal experiment so that this species seemed a poor competitor, whereas this species tended to attack fresh bolts. Xyleborus ganshoensis was considered as the highest potential to establish population in non-indigenous area because of its broad niche. Xyleborus ganshoensis was also likely to attack living trees since many individuals attacked fresh bolts but unlikely an invasive alien species. Platypus quercivorus, a vector of the Japanese oak wilt, showed the highest mean abundance on 0-month-old bolts. On the other hand, Xyleborus seiryorensis had the smallest value of WO niche center, indicating that this species could be aggressive enough to attack living trees. The method developed in this study would be applicable to similar risk assessment of ambrosia beetles on other host species and in other countries.報告番号: ; 学位授与年月日: 2012-05-14 ; 学位の種別: 課程博士 ; 学位の種類: 博士（農学） ; 学位記番号: 博農第3850号 ; 研究科・専攻: 農学生命科学研究科生圏システム学専