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

在广岛县较冷的地区，由于种内竞争和降雪，无毒线虫，拟松材线虫，开始在垂死的赤松中繁殖。该线虫主要由Monochamus saltuarius传播。研究区还发现了松材线虫（Bursaphelenchus xylophilus）。其空间分布随海拔高度逐年上下波动。在弱毒线虫占优势的地区，对29株赤松进行了强毒线虫接种。因此，一些树木被杀死和攻击的Monochamus saltuarius。当这些甲虫成年后从枯树上爬出来时，它们携带着无毒的线虫。大部分无毒线虫在甲虫羽化后15-40天离开甲虫。线虫对松树的传播主要发生在甲虫羽化后20-40天。以松墨天牛为试虫，通过室内毒力试验表明，随着环境温度从25 ℃降低到16℃，媒介昆虫寿命缩短，线虫传播效率降低，线虫传播高峰期推迟，高峰高度降低。因此，低温可以抑制强毒线虫的传播过程，这种抑制可能是部分负责防止松树枯萎病破坏松树林在寒冷地区。本文介绍了一种简便的松墨天牛成虫感染方法。该方法可用于松材线虫毒力与传播关系的研究。

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.