Effect Of Local Niche-related Species On The Invasion Of Pinewood Nematode, Bursaphelenchus Xylophilus

The pinewood nematode (PWN), Bursaphelenchus xylophilus, is the largest forestry invasive species, whose ability of population growth and spread is very strong, causing devastating damage to pine forests and the ecological environment in China, but there is little risk in the North American of the origin. What factors have contributed to its successful invasion and outbreaks in China? Our study has combined field investigation and laboratory experiments and described the invasion mechanism of PWN focusing on mutually beneficial relationships between PWN and the local niche-related species (vector beetles and blue-stain fungi). The main findings are as follows:1. Biological factors in the introduced regions are critical to the successful invasion of PWNLocal vector beetles act as the media, the blue-stain fungi as food, are closely related with the infection cycle of PWN. Through the investigation in different regions, which have the similar physical environment such as latitude and longitude, stand structure, climate conditions, we found that:the infected areas and the number of infected pine trees in the regions of Beilu and Yuyao in Zhejiang Province are both evidently higher than that in Hanxian, Hexian and Chaohuqu in Anhui Province. In addition, the degree of damage caused by pine wilt disease has a significant positive correlation with the abundance and distribution of Monochamus alternatus and blue-stain fungi. This indicates that the local biological factors such as vector beetles and blue-stain are the keys to the invasion of PWN.2. Original PWN can take advantage of the local blue-stain fungi for rapid propagation in a new introduced regionThe cross experiments between PWN from China and U.S. and blue-stain fungi also from the two countries conformed that:S. sp.1, a local blue-stain fungus, can most promote the growth and reproduction of U.S. PWN among the three different strains, thereby illustrating that the exotic PWN can utilize the local biological resources (blue-stain fungi) for rapid reproduction and population growth when arriving a new environment, which is the foundation of successful colonization and spread of PWN.3.4-hydroxy-4-methyl-2-Pentanone is an active substance in the local blue-stain fungiGC-MS analysis of Hexane extracts of three different blue-stain fungi shows that:4-hydroxy-4-methyl-2-Pentanone content in S. sp.1is significantly higher than that in S. sp2, O. ips and control. Bioassay to PWN also conformed that:4-hydroxy-4-methyl-2-Pentanone can effectively promote the growth of PWN population density, spawning and its female body length. This indicates that chemical communication or some specific chemical substances are conducive to maintaining the mutually beneficial relationship between the alien species and the local organisms.4. PWN-Vector beetles-Blue-stain fungi symbiontLocal blue-stain fungi can significantly promote the increase in the number of PWN-local vector beetles symbiont. One hand, the local blue-stain fungi can be used as a food by PWN, and can significantly promote the growth and reproduction of PWN, and optimize its population age structure; the other hand, the local blue-stain fungi are also the food of M. alternates, and S. spl can significantly improve the growth of M. alternatus, thereby increasing the number of dispersal stage larva (J4) carried by M. alternatus, leading to the wide spread of PWN. Therefore, In order to effectively adapt the new environments, PWN, local vector beetles and blue-stain fungi combined together as PWN-vector beetles-blue-stain fungi symbiont when PWN introduced into China.On this basis, we proposed a new mechanism of biological invasion:collaborative interaction, namely, the alien species PWN and the local vector beetles/blue-stain fungi establish collaborative interaction relations to promote successful invasion of PWN, Which revealed the reasons for outbreaks of PWN in China, and enriched the theory of invasion biology, and also provides a new approach for the forecast and integrative management of the pine wilt disease.