マツの材線虫病の感染源に関する生態学的研究 (I) : 枯損アカマツ樹体内における病原線虫の動態

Abstract

マツの材線虫病の感染源である枯死アカマツ樹体内に於ける病原線虫 Bursaphelenchus xylophi1us の個体群の動態を, その個体群密度, 分布様式, 定着度, 特殊ステージである分散型第3期幼虫 (以下L_III 幼虫とする) の出現率などを通して調査した。調査にあたっては供試丸太の半数に殺虫剤処理を施し, その影響下にある丸太内の線虫の動態を無処理区 (対照区) との間で比較するという方法を採った。また材の含水率や同じ丸太内に生息する自由生活性線虫 (以下自活性線虫) との関係にも留意した。調査結果の概要は次のようなものであった。(1) マツノザイセンチュウの丸太内密度は11月以降に次第に減少し, 5月には例外なくきわめて低いものとなったが, その密度は調査期聞を通じて材の合水率とは弱い負の, そして自活性線虫の密度とは弱い正の相関を示した。また, マツノザイセンチュウは11 - 1月には丸太内で集中分布していたが, 3月には一たんランダム分布に傾き, 5月には再たび著しい集中分布を示した。このような密度や分布傾向の推移に殺虫剤はほとんど影響しなかった。(2) マツノザイセンチュウ個体群の丸太内での定着度は秋から春にかけて次第に低下し, 伝播者マツノマダラカミキリ (以下カミキリ) の蛹室への移動・集中を伺わせた。(3) マツノザイセンチュウ個体群に於けるL_III幼虫出現率は, 11月には母集団であるマツノザイセンチュウ密度とは負の, 材の合水率とは正の相関を示し, 3月には自活性線虫密度と正の相関関係を示した。(4) 材内のマツノザイセンチュウの密度や分布状態とそとから脱出したカミキリの保持線虫数の間には因果関係は見い出せなかった。(5) 枯損状態の異なる3群の異常 - 枯死木樹体内での線虫類の分布状態を比較したところ, マツノザイセンチュウ個体群の12月から5月にかけての変動性は, 異常発現が遅れ材の変性が抑制されている枯死木に於いて高く, 異常発現が早く材の変性が進んだ枯死木に於いては定常的であった。一方自活性線虫はこれとは全く逆の傾向を示し, 両種線虫の生活形態の違いを示唆した。

Population dynamics of the pine wood nematode, Bursaphelenchus xylophilus, a pathogen of pine wilt in the stems of withered Japanese red pine (Pinus densiflora) was investigated from the aspects of population density, distributional pattern, stationary degree and ratio of dispersal 3rd stage larvae to that of all stages of B. xylophilus. Half of the logs examined were treated with insecticide after the first sampling, and the population dynamics of B. xylophilus in the logs treated with the insecticide were compared with that in non-treated logs. Water content at the drillings from which nematodes were extracted, and the density of free-living nematodes were also examined to determine whether or not such factors influenced the population density of B. xylophilus. After the investigation of the nematode population dynamics, the logs examined were preserved in separate cage, and the number of B. xylophilus carried by the sawyer was examined. The population dymamics of B. xylophilus and free-living nematodes in the trees showing different degrees of withering symptoms were also studied. Generally, the water content in the log treated with the insecticide was higher than that not treated, and the condition of dampness was maintained more stably in the former than in the latter. The population density of B. xylophilus in both treated and untreated logs decreased with time and became the lowest in May. The population density of B. xylophilus was correlated with the water content of the log rather negatively, and with the density of free-living nematodes rather positively. Nematodes of B. xylophilus distributed aggregatively from November to the following January, and randomized in distribution in March then showed clearly aggregated distribution pattern in May. There was no distinct difference in the distribution pattern between the logs treated with the insecticide and those not treated. The stationary degree of B. xylophilus in withered logs decreased, this tendency may be attributed to the movement of nematodes to the pupal chamber of the pine sawyer, a vector of the nematodes. Because the population of B. xylophilus in the log treated with insecticide, which killed the pine sawyer, did not show such tendency of aggregation. The ratio of the dispersal 3rd stage larvae (LIII) to whole stages of B. xylophilus population decreased in March, though it was constant in Nov., Jan., and May. In November, the ratio of LIII correlated with the density of whole stages of B. xylophilus negatively, and with the water content at the drillings positively. In March the ratio positively correlated with the density of free-living nematodes. From 5 of 10 pine logs treated without the insecticide, more than 10 pine sawyers emerged. Most of the nematodes were carried on by the sawyers from two certain logs. There was no relationship between the density or distribution pattern of B. xylophilus in the logs and the number of the nematodes carried on the sawyer emerged from the corresponding logs. The distribution patterns of B. xylophilus, that of dispersal 3rd stage larvae (LIII), and that of free-living nematode were clearly different in all the trees at any degree of withering symptoms. Judging from the water content, the change in the wood structure from December to May was most obvious in the trees which had withered early (group A), and was ambiguous in those showing delayed symptoms (group B, C). The population of B. xylophilus and LIII larvae were constant in the trees of group A, but the population of free-living nematodes in the trees of groups B and C was constant. This difference may be related to the difference in their life style.