BackgroundIntensification of world trade is responsible for an increase in the number of alien species introductions. Human-mediated dispersal promotes not only introductions but also expansion of the species distribution via long-distance dispersal. Thus, understanding the role of anthropogenic pathways in the spread of invading species has become one of the most important challenges nowadays.Methodology/Principal FindingsWe analysed the invasion pattern of the pinewood nematode in China based on invasion data from 1982 to 2005 and monitoring data on 7 locations over 15 years. Short distance spread mediated by long-horned beetles was estimated at 7.5 km per year. Infested sites located further away represented more than 90% of observations and the mean long distance spread was estimated at 111–339 km. Railways, river ports, and lakes had significant effects on the spread pattern. Human population density levels explained 87% of the variation in the invasion probability (P<0.05). Since 2001, the number of new records of the nematode was multiplied by a factor of 5 and the spread distance by a factor of 2. We combined a diffusion model to describe the short distance spread with a stochastic, individual based model to describe the long distance jumps. This combined model generated an error of only 13% when used to predict the presence of the nematode. Under two climate scenarios (stable climate or moderate warming), projections of the invasion probability suggest that this pest could expand its distribution 40–55% by 2025.Conclusions/SignificanceThis study provides evidence that human-induced dispersal plays a fundamental role in the spread of the pinewood nematode, and appropriate control measures should be taken to stop or slow its expansion. This model can be applied to Europe, where the nematode had been introduced later, and is currently expanding its distribution. Similar models could also be derived for other species that could be accidentally transported by humans.

Abstract. Invasive pathogen-insect symbioses have been extensively studied in many different ecological niches. Whether the damage of symbioses in different introduced regions might be influenced by other microorganisms has, however, received little attention. Eight years of field data showed that the varied levels of the nematode and beetle populations and infested trees of the invasive Bursaphelenchus xylophilus-Monochamus alternatus symbiosis were correlated with patterns in the isolation frequencies of ophiostomatoid fungi at six sites, while the laboratory experiments showed that the nematode produced greater numbers of offspring with a female-biased sex ratio and developed faster in the presence of one native symbiotic ophiostomatoid fungus, Sporothrix sp. 1. Diacetone alcohol (DAA) from xylem inoculated with Sporothrix sp. 1 induced B. xylophilus to produce greater numbers of offspring. Its presence also significantly increased the growth and survival rate of M. alternatus, and possibly explains the prevalence of the nematode-vector symbiosis when Sporothrix sp. 1 was dominant in the fungal communities. Studying the means by which multispecies interactions contributed to biogeographical dynamics allowed us to better understand the varied levels of damage caused by biological invasion across the invaded range.

The occurrence and prevalence of pine wood nematode disease has had a great impact on China’s forestry production and ecological environment, but the quantitative evaluation of the economic losses of this biological disease is lacking from a macro-geographic scale and long-term series perspective. This study establishes a pine wood nematode disease economic loss evaluation index system, covering the loss of natural ecology, landscape aesthetics and economic production, and quantitatively evaluates the economic losses of pine wood nematode disease in mainland China at a provincial level from 1998 to 2017. The results show that the economic losses of the pine wood nematode disease in China showed a fluctuant rising trend from 1998, since 2013, it has entered a full-scale outbreak phase, and since 2015, the economic loss has increased significantly, with a growth rate higher than 40%. The average annual total economic loss is CNY 7.17 billion, of which the direct economic losses is CNY 1.53 billion, and the indirect economic losses is CNY 5.64 billion. The loss of forest material resources, the expenditure of ineffective forest management expenditure and prevention and control expenditure were CNY 817 million, CNY 649 million, and CNY 67 million, respectively. Regulate service value loss, support service value loss and cultural service loss are CNY 3.95 billion, CNY 1.41 billion, CNY 276 million, respectively. East China and South China suffered the most due to pine wood nematode disease, and the economic loss of these regions accounted for 79.9% of the total national economic losses. Among them, Zhejiang Province, Guangdong Province and Jiangsu Province lost CNY 2.14 billion, CNY 1.81 billion, and CNY 1.22 billion, respectively, accounting for 26.8%, 22.7% and 15.3% of the total national economic loss.

Larch caterpillars are widely distributed in the Great Xing’an boreal forests; however, the relationship between caterpillar defoliation dynamics and climatic factors is poorly understood. The aims of this study are to investigate the primary weather conditions that might influence forest defoliation and to identify the most important life stage of the larch caterpillar at which forest defoliation might be mitigated by incorporating more inhibitory influences from climatic factors. The life cycle of the larch caterpillar was partitioned into four stages and multiple linear regression and mixed effect models were combined with a relative weight analysis approach to evaluate the importance and influence of meteorological variables on defoliation dynamics. The results show that warmer temperatures in growing seasons and overwintering periods can increase the defoliation area, while rainy and humid growing seasons decrease the defoliation area. Total precipitation during the early instar larval period had the greatest power to explain the variance in defoliation dynamics and had a very strong inhibitory effect, followed by the accumulative temperatures of the late instar larval period which had a positive impact, and precipitation during the middle instar larval period which had a negative impact. Weather conditions during the early instar larval period had the greatest influence on the area defoliated and accounted for 40% of the explained variance. This study demonstrates that climatic warming and drying will increase the risk of larch caterpillar outbreaks in the Great Xing’an Mountains.

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