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

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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.