Study on inhibitory mechanism of pine wilt disease infestation in cooler area

在广岛地区较冷的地区，由于种内竞争和降雪的原因，无毒性线虫（Bursaphelenchus mucronatus）开始在濒临死亡的密松中繁殖。该线虫的主要媒介是沙鼠。毒力极强的线虫，嗜木线虫，也存在于研究区域。其空间分布在海拔上呈逐年上下波动。在无毒线虫比强毒线虫占优势的区域，接种了29棵松。结果，一些树木被沙鼠杀死和攻击。当甲虫成年后从死树上爬出来时，它们携带着无毒的线虫。大部分无毒线虫在甲虫出壳后15-40天离开。线虫在松树上的传播主要发生在甲虫出壳后20 ~ 40天。结果表明，当环境温度从25℃降至16℃时，媒介寿命缩短，线虫传播效率降低，传播高峰期延迟，传播高峰高度降低。因此，低温可能抑制了毒线虫的传播过程，这种抑制可能部分负责防止松树枯萎病在寒冷地区的破坏性松林。提出了一种简单的给成虫装毒线虫的方法。该方法有助于研究木耳菌毒力与传播的关系。

The avirulent nematode, Bursaphelenchus mucronatus, began to reproduce in dying Pinus densiflora caused by intraspecific competition and snowfall in a cooler area in Hiroshima Prefecture. The nematode was vectored primarily by Monochamus saltuarius. The virulent nematode, Bursaphelenchus xylophilus, is also present in the study area. Its spatial distribution fluctuated up and down in altitude year by year. At the area where the avirulent nematode predominated over the virulent one, twenty-nine Pinus denstflora trees were inoculated with virulent nematode. Consequently, some trees were killed and attacked by Monochamus saltuarius. When the beetles emerged as adults from dead trees, they carried avirulent nematodes. Large proportion of avirulent nematodes left beetles 15-40 days after beetle emergence. Nematode transmission to pine trees occurred mainly 20-40 days after beetle emergence. An experiment using Monochamus alternatus beetles with virulent nematodes showed that longevity of vectors decreased, nematode transmission efficiency decreased, and the peak period of nematode transmission was delayed and its peak height decreased, as the ambient temperature decreased from 25 to 16℃. Thus, cool temperature may inhibit the virulent nematode transmission process and this inhibition could be partially responsible for preventing pine wilt disease from devastating pine forests in cool regions. A simple method for loading adult Monochamus alternatus with virulent nematodes was developed. The method may help the study of the relationship between virulence and transmission in Bursaphelenchus xylophilus.