Disentangling the drivers of invasion spread in a vector-borne tree disease.

Pine wilt disease (PWD) invaded southern Japan in the early 1900s and has gradually expanded its range to northern Honshu (Japanese mainland). The disease is caused by a pathogenic North American nematode, which is transmitted by native pine sawyer beetles. Recently, the disease has invaded other portions of East Asia and Europe where extensive mortality of host pines is anticipated to resemble historical patterns seen in Japan. There is a critical need to identify the main drivers of PWD invasion spread so as to predict the future spread and evaluate containment strategies in newly invaded world regions. But the coupling of pathogen and vector population dynamics introduces considerable complexity that is important for understanding this and other plant disease invasions. In this study, we analysed historical (1980-2011) records of PWD infection and vector abundance, which were spatially extensive but recorded at coarse categorical levels (none, low and high) across 403 municipalities in northern Honshu. We employed a multistate occupancy model that accounted both for demographic stochasticity and observation errors in categorical data. Analysis revealed that sparse sawyer populations had lower probabilities of transition to high abundance than did more abundant populations even when regional abundance stayed the same, suggesting the existence of positive density dependence, that is an Allee effect, in sawyer dynamics. Climatic conditions (average accumulated degree days) substantially limited invasion spread in northern regions, but this climatic influence on sawyer dynamics was generally weaker than the Allee effect. Our results suggest that tactics (eg sanitation logging of infected pines) which strengthen Allee effects in sawyer dynamics may be effective strategies for slowing the spread of PWD.

Genetic Differences among Established Populations of Aromia bungii (Faldermann, 1835) (Coleoptera: Cerambycidae) in Japan: Suggestion of Multiple Introductions.

Aromia bungii (Faldermann) (Coleoptera: Cerambycidae) is an invasive pest, damaging Rosaceae trees (particularly Prunus) in Japan and Europe. The establishment of this beetle in Japan was first detected in 2012, and subsequently, it has rapidly expanded its distribution. Currently, Japanese populations of A. bungii are widely distributed in six non-contiguous regions. In this study, we compared the nucleotide sequences of mitochondrial cytochrome oxidase subunit 1 of the populations in these six regions in Japan to examine whether multiple introductions or human-mediated long-distance dispersal have contributed to the non-contiguous distribution of A. bungii. Seven haplotypes were detected from Japanese populations, and one of these was identical to a sequence deposited from China. One to two haplotypes were detected in each region, suggesting a genetic bottleneck. Detected haplotypes differed between introduced regions, although two regions shared a single haplotype. These results suggest that multiple independent introductions of A. bungii have contributed to its non-contiguous distribution in Japan. Quarantine measures for wood-packing materials in trade need to be strengthened to prevent the establishment of further populations of A. bungii.

Comparison of the Ecological Traits and Boring Densities of Aromia bungii (Faldermann, 1835) (Coleoptera: Cerambycidae) in Two Host Tree Species.

We investigated the ecological traits of emerging adults and the boring density in Aromia bungii-infested flowering cherry (Cerasus × yedoensis 'Somei-yoshino') and peach (Prunus persica) trees to evaluate their suitability as food resources for A. bungii, and their vulnerability to infestation. The number of adults per m3 that emerged from P. persica was 10-times larger than from C. × yedoensis, and the numbers of emergence holes, entrance holes, and pupal chambers were also larger in P. persica logs. The lifetime fecundity of adults that emerged from P. persica was also higher. Elytral length, sex ratios, and adult lifespans did not differ between the two host trees. Our results indicate that peach trees provide more suitable conditions than do flowering cherry trees for A. bungii larvae. Although flowering cherry trees, primarily C. × yedoensis, which are currently grown as street or ornamental trees in Japan, have been more severely affected by A. bungii to date, the greater risk in the long term is to P. persica, an agricultural species in the main producing areas surrounding the Kanto region.

Efficacy of Two Neonicotinoid Insecticides against Invasive Wood Borer Aromia bungii Larvae in Dietary Toxicity Test.

In recent years, insecticide trunk injection was put into practical use for controlling wood boring pests. However, few studies have investigated the dose-response relationships between insecticides and wood-boring pests in detail. This study used two commercial formulations of the neonicotinoid insecticides thiamethoxam and dinotefuran and investigated their dose-response relationships with invasive wood borer Aromia bungii (Coleoptera: Cerambycidae) larvae. Neonates and late instar larvae were reared with an artificial diet containing different insecticide concentrations (0.01-100 ppm) in the laboratory, and their diet excavation activity, survival rate, and weight change were recorded. Diet excavation immediately dropped in larvae exposed to high concentrations of thiamethoxam or dinotefuran (≥1 ppm in neonates and ≥10 ppm in late instar larvae). The weight and survival rate gradually declined over 12 weeks in late instar larvae. These results suggest that the two neonicotinoids intoxicate and debilitate A. bungii larvae gradually to death. In practical use, rapid suppression of A. bungii wood boring damage can be expected by trunk injection of neonicotinoid insecticides. However, a relatively long-term retention of the insecticides may be required to kill large larvae. Neonates may be controlled with lower insecticide dosage and shorter exposure than larger larvae.

Genetic structure of the oak wilt vector beetle Platypus quercivorus: inferences toward the process of damaged area expansion.

BACKGROUND: The ambrosia beetle, Platypus quercivorus, is the vector of oak wilt, one of the most serious forest diseases in Japan. Population genetics approaches have made great progress toward studying the population dynamics of pests, especially for estimating dispersal. Knowledge of the genetic structuring of the beetle populations should reveal their population history. Using five highly polymorphic microsatellite loci, 605 individuals from 14 sampling sites were assessed to infer the ongoing gene flow among populations as well as the processes of expansion of damaged areas. RESULTS: Population differentiation (FST = 0.047, G'ST = 0.167) was moderate and two major clusters were detected by several methods, dividing the samples into north-eastern and south-western populations, a similar genetic divergence was reported in host oak trees. Within the north-eastern populations, the subgroups mostly corresponded to differences in the collection period. The genetic characteristics of the population might have changed after 2 years due to the mixing of individuals between populations with enhanced migration related to population outbreaks. Because isolation by distance was detected for whole populations and also within the north-eastern populations, migration was considered to be limited between neighbouring populations, and most populations were suggested to be in genetic equilibrium of genetic drift and gene flow. Recent bottlenecks were found in some populations with no geographical bias; however, they were all from newly emerged oak wilt forests. The emergence of oak wilt should have induced intense fluctuations in the beetle population size. CONCLUSIONS: Because the genetic boundaries coincide, we suggest that the geographical structuring of the beetle was formed by co-evolution with the host species. Our findings indicate the oak wilt expansion process.

Evolution of a key trait greatly affects underground community assembly process through habitat adaptation in earthworms.

Underground community assemblies have not been studied well compared with aboveground communities, despite their importance for our understanding of whole ecosystems. To investigate underground community assembly over evolutionary timescales, we examined terrestrial earthworm communities (Oligochaeta: Haplotaxida) in conserved mountainous primary forests in Japan as a model system. We collected 553 earthworms mostly from two dominant families, the Megascolecidae and the Lumbricidae, from 12 sites. We constructed a molecular taxonomic unit tree based on the analysis of three genes to examine the effects of a biogeographic factor (dispersal ability) and an evolutionary factor (habitat adaptation) on the earthworm community assembly process. The phylogenetic distance of the earthworm communities among sites was positively correlated with geographic distance when intraspecific variation was included, indicating that the divergence within species was affected by biogeographic factors. The community assembly process in the Megascolecidae has also been affected by environmental conditions in relation to an evolutionary relationship between habitat environment and intestinal cecum type, a trait closely related to habitat depth and diet, whereas that in the Lumbricidae has not been affected as such. Intestinal cecum type showed a pattern of niche conservatism in the Megascolecidae lineage. Our results suggest that investigating the evolution of a key trait related to life history can lead to the clear description of community assembly process over a long timescale and that the community assembly process can differ greatly among related lineages even though they live sympatrically.