Reproductive success of the ambrosia beetle Platypus quercivorus on Quercus laurifolia planted in Japan

Abstract

Mortality of Quercus laurifolia trees that had been planted in Japan occurred in 2006. This was caused by an attack by the ambrosia beetle Platypus quercivorus, which causes a disease called Japanese oak wilt and results in mass mortality of Fagaceae trees in Japan. In order to study the infestation of Q. laurifolia by P. quercivorus, dead trees were cut into bolts, and the density of holes bored by P. quercivorus was examined for each bolt. Tube traps were attached to each hole in the bolts of the cut trees, and the number of offspring that emerged from the hole during 2007 (the year following the infestation) was counted. Similar to other Japanese Fagaceae species, the density of holes bored by P. quercivorus decreased with increase in the height of Q. laurifolia. The sex ratio of the emerged beetle was biased towards males, and the number of offspring per hole ranged from 0 to 41. The effects of the height and the hole density on the number of offspring were analyzed with zero-altered negative binomial models with a hurdle component and a truncated count component. The results of the hurdle component in the model for zero counts showed that the probability that one or more beetles emerged from holes was negatively affected by the height above the ground. According to the results of the count component of the model for positive counts, the number of offspring showed a unimodal distribution, with a peak at around one in the density of holes per 100cm[2].

京都に植栽されていたローレルオーク (Quercus laurifolia)が, 日本各地でブナ科樹木の集団枯死被害 (ナラ枯れ)を引き起こしているカシノナガキクイムシ (Platypus quercivorus)によって2006年に枯死した. 枯死したローレルオークは伐採して玉切りし, 丸太毎に穿孔密度を調査した後, 各穿孔穴に羽化トラップを取り付け, 2007年に脱出してきた次世代のカシノナガキクイムシの頭数を調査した. 日本の他のブナ科樹木の場合と同様に, カシノナガキクイムシの穿孔密度は地上高が高くなるに従い低くなっていた. 脱出虫の性比は雄に偏っており, 1穿孔穴あたりの脱出頭数は0頭から41頭だった. 地上高と穿孔密度が脱出頭数に及ぼす影響を zero-altered negative binomial model を用いて解析した. その結果, 1頭以上の次世代虫が脱出する確率には地上高が負の影響を及ぼしていた. また, 1頭以上の脱出頭数は穿孔密度に対して一山型の分布を示し, 樹幹上の穿孔密度が100cm[2] あたり1個程度の時に最大となっていた.